Lactic acid bacteria are known for their ability to convert sugar into lactic acid. This process has long been used by people to preserve food, prepare feed, make a variety of dairy products, and wine.

Characterization of bacteria

Lactic acid bacteria are Gram-positive anaerobes. This means that they do not require oxygen for oxidative processes and metabolism. Lactic acid bacteria belong to the group (family) Lactobacillaceae, which includes:

The first two kinds are the most significant for a person and his economic activity. Despite their close relationship, representatives of the order Lactobacillales, like pneumoniae streptococci, are usually not classified as lactic acid bacteria. And beneficial bifidobacteria or microbes from the genus Bacillus, which are spore-forming aerobes, are sometimes referred to as a group of lactobacilli due to similarities in carbohydrate metabolism and their role in the food industry.

Classification

The classification of lactic acid bacteria is not well developed. According to the nature of the released fermentation products, they are divided into two groups.

• Homofermentative. As a result of the fermentation of carbohydrates, mainly lactic acid is released. In small quantities, the process is accompanied by succinic and fumaric acids, carbon dioxide and ethanol.
• heterofermentative also form lactic acid as a result of the decomposition of carbohydrates. Along with this, they use about half of the sugars for the production of acetic acid, carbon dioxide and ethanol.

Classification by shape is difficult because lactic acid bacteria belong to a group of mutable microorganisms. The shape of a microbial cell depends on the age of the bacterium, the chemical environment, and environmental conditions. To determine the type of lactobacilli, specific conditions are created, a standard medium is used, and a culture is studied at a certain age. The type of carbohydrate fermentation, the need for food sources, and the optical rotation of the lactic acid molecule are also evaluated.

streptococci

Species of the genus Streptococcus, according to the type of fermentation, are homofermentative. During fermentation, more than 90% of the initial sugars are converted into lactic acid and only a small amount of them into acetic acid and alcohol. The most famous representatives are cultures:

• Str. lactis.
• Str. citrovorus.
• Str. diacetilactis.
• Str. paracitrovorus.
• Str. thermophilus.
• Str. cremoris.
• Str. liquefaciens.

lactobacilli

In the dairy industry, lactobacilli are called lactic acid sticks. They ferment milk much faster than cocci, reaching lower pH values ​​(about 3.5). The optimal development of lactobacilli is shown in an acidic environment with a reduced oxygen content. In nature, these bacteria live on the surface of plants, are excreted from the saliva and digestive tract of humans and animals.

It is noted that sterile milk does not contain lactic acid sticks - they enter it from the external environment. Lactobacilli survive short-term pasteurization, but die at high sterilization temperatures. Therefore, in pasteurized milk, lactic acid bacteria are significantly reduced, but still present. The most common representatives of the genus Lactobacterium:

• L. bulgaricum.
• L. casei.
• L. plantarum.
• L. acidophilus.
• L. brevis.

Basic properties

Coccal forms of lactobacilli have a diameter of 0.6 - 1.1 microns. In culture, cocci are arranged singly, in pairs, or in chains of various lengths. The rods are very variable in shape: from spherical to filamentous forms with a length of 0.7 to 8.0 µm, single or in chains. Cell morphology is significantly influenced by the chemical composition of the habitat. Lactic acid bacteria, the photo of which is presented below, are isolated from yogurt starter.

Lactobacilli reproduce mainly by division; cases of cell ligation and reproduction with the help of gonidia are described. The presence of filterable forms and the process of spore formation have been proved.

Where do lactic acid bacteria live?

Lactobacilli cannot independently synthesize amino acids and some vitamins. For this reason, they are neither in the soil nor in the water. Under natural conditions, they are isolated from the contents of the intestines of humans and animals, from the surface of plants. The optimal environment for the vital activity of lactic acid bacteria is milk and dairy products.

Food sources for lactobacilli are mono- and disaccharides. Some varieties ferment polysaccharides such as dextrose. Also, under certain conditions, these microorganisms use organic acids as an energy source: malic, acetic, pyruvic, formic, fumaric and citric acids. In the absence of carbon-containing substrates for nutrition, amino acids can be processed.

Lactic acid bacteria are not capable of synthesizing organic nitrogen, therefore they are demanding on its content in the nutrient medium. They also need vitamins, especially purine bases: biotin, thiamine, pantothenic, folic acids. All forms of lactobacilli are resistant to high concentrations of alcohol. At the same time, they multiply more slowly, but live longer. So, in clarified wines, lactic acid bacteria persist for up to 7 months.

The microbe is mesophilic, rarely thermophilic. The optimum temperature for vital activity is + 25 °С... + 30 °С. At + 15 °C, fermentation slows down significantly, and at + 45 °C, lactobacilli stop multiplying. The habitat of lactic acid bacteria can be either oxygen or without air. They do not need oxygen, in most cases it inhibits the development of microbes and interferes with the normal fermentation process.

lactic acid fermentation

Lactic acid fermentation is the process of anaerobic oxidation of carbohydrates, in which lactic acid is released. As a result of lactic acid fermentation, bacteria receive energy, which is realized for growth and reproduction in anoxic conditions. At the same time, lactobacilli lower the pH to values ​​below 5, inhibiting the growth of other microorganisms.

Heterofermentative fermentation is a more complex process. Depending on the conditions and microbial culture, various combinations of lactic and acetic acids are formed from carbohydrates with the release of carbon dioxide and ethanol.

Lactic acid fermentation in its pure form is used in the chemical industry to produce lactic acid. It is widely used for leather dressing, in the dyeing industry, in pharmaceuticals, in the manufacture of plastics and washing powders. In the food industry, lactic acid is required for the production of confectionery and soft drinks.

Not always lactic acid fermentation is useful for human activity. A spontaneous process starting in milk, wine, soft drinks leads to food spoilage. Organoleptically, this is expressed in souring, turbidity and sliming of the substrate.

Food industry and lactobacilli

Lactic acid bacteria are widely used for the production and preservation of various products. Their value is especially great in the dairy business.

• Dairy industry.

To obtain lactic acid products, sterilized milk or cream is fermented by introducing pure cultures. They are called "starter cultures". Depending on the type of sourdough, different products are obtained.

For the production of kefir and koumiss, cultures are used that, in addition to lactic acid, also provide alcoholic fermentation. The sourdough is prepared on the basis of kefir grains, which are the source of an extensive community of microorganisms that have not yet been fully studied (lactic acid bacilli and streptococci, micrococci and yeasts).

In the process of making cheese, lactic acid bacteria work at the first stage, ensuring the folding of casein, then they are replaced by propionic acid microorganisms.

To obtain fermented milk butter, the culture of Str. lactis, Str. cremoris and Leuconostoc cremoris. When added to homogenized milk L. bulgaricus and Str. thermophilus get yogurt.

In the production of cottage cheese and cheeses of the German group, starter cultures containing Str. lactis or L. bulgaricus and Str. thermophilus. And for the manufacture of hard cheeses at the ripening stage, the culture of L. casei and Str. lactis.

• Winemaking.

Three genera of lactobacilli are widely used in the production of wines: Lactobacillus, Pediococcus, Leuconostoc. These are mainly heterofermentative cocci that provide malolactic fermentation in highly acidic wines. At the same time, they ferment malic acid and do not affect other chemical components of the wine. Lactobacilli can spoil the drink by causing lactic acid fermentation. As a result, such wine defects as rancidity, obesity, decomposition of tartaric acid appear.

• Bakery.

About 70 flavoring and aromatic substances are found in bread, among them 28 acids, 11 alcohols, 28 carbonyl compounds, 6 esters, methyl mercaptan and ammonia. Lactic acid bacteria take part in the formation of most of them. Lactobacilli are of the greatest importance for the production of rye bread. Sourdough gives the dough elasticity, loosens it and promotes rise. The acidity of the dough is an important indicator of quality. In the production of wheat bread, lactobacilli play a minor role, mainly the process depends on yeast cultures. The main constituents of lactic acid starters for dough preparation are L. brevis, L. plantarum and L. fermenti.

• Preservation of meat and fish.

Lactic acid bacteria are used in the manufacture of salami and cervelat, other sausages, and in the maturation of lightly salted fish. Lactic acid speeds up the preservation process and gives the products valuable taste.

• Biological preservation of fruits and vegetables.

Harvesting is carried out according to the same principle as forage ensiling. Plant carbohydrates under the influence of lactic acid bacteria are converted into lactic and acetic acids, which are excellent preservatives.

Sauerkraut and cucumbers, pickled apples, tomatoes and watermelons are examples of simple preparations for the winter. So, in chopped and well-packed cabbage with a small addition of salt, a spontaneous fermentation process begins, in which first Leuconostoc and later L. plantarum take part.

Role in agriculture

Ensiling forage is the best way to harvest and preserve green mass. To create the necessary conditions, the feedstock (grass, green mass of corn, tops) is placed in special silo pits, carefully compacted and covered with a layer of earth. This creates conditions in which the bulk of microbes die, and lactic acid bacteria process plant carbohydrates until the concentration of lactic acid is 60% or more, and the acidity of the silage reaches pH 4.5. In addition to lactic acid, acetic acid also accumulates in the silo. It takes about one month to complete the process.

Microflora in the human intestine

The human intestinal tract is inhabited by many lactic acid bacteria called lactobacteria and bifidobacteria. The product of their metabolism - lactic acid - has a number of positive aspects.

• Stimulates intestinal peristalsis.
• Reduces gas formation.
• Stimulates the secretion of digestive juices.
• Improves the absorption of calcium, phosphorus and iron.

In addition, lactobacilli have the ability to resist various pathogenic microbes. Due to the production of biologically active substances (organic acids, hydrogen peroxide, antibiotics and bacteriocins), microorganisms dangerous for the activity of the intestine are replaced. If in the content of chyme lactic acid bacteria are reduced in number, then their place is occupied by opportunistic microflora. Based on strains isolated from human and animal intestines, drugs have been developed that improve the patient's condition in many infections.

What are probiotics

At the beginning of the 20th century, the famous Russian scientist Ilya Ilyich Mechnikov conducted a series of experiments to restore the microflora of the human intestinal tract using the culture of lactic acid bacillus. L. bulgaricus. As a result of research, Mechnikov developed the first probiotic - "Mechnikov yogurt", which he himself used for many years, prescribed to patients and recommended drinking to all his friends.

Currently, probiotics are a class of drugs aimed at restoring the body's natural environment. Long-term studies have proven the effectiveness of the use of probiotics (including lactobacilli) in various clinical cases.

Lactic acid probiotic bacteria are widely used in medicine for the prevention and treatment of acute and chronic diseases of the intestines and respiratory tract, to restore the intestinal microflora and stimulate the immune system. Probiotic cultures can be taken both in the form of tablets and powders, and in their natural form (kefir, yogurt, acidophilus milk, yogurt and other dairy products).

lactic acid fermentation bacterium homofermentative

Lactic acid bacteria, microbes that cause lactic acid fermentation in milk (dairy products), which is expressed in the fermentation of milk sugar into lactic acid; milk coagulation occurs due to the formation of acid. Lactic acid bacteria include rods and cocci. The former belong to the acidophilic bacilli and possess their properties; according to the classification of Lehman and Neumann, the sticks, together with other acidophilic bacilli, form the group "Plocamobacteria", and according to Heim and Schlirf (Heim, Schlirf) - the group of acidobacteria. According to the American classification (Bergey), all lactic acid bacilli form a special species, Lactobacillaceae. The most important representatives of the group are you. bul-garicus, Vas. caucasic and others. Mechnikov you. bulgaricus has been proposed to replace the "wild" intestinal flora in humans; Bact. mazun, 2.7-21 m long and 1-1.1 m wide, non-motile, Gram-positive bacillus; does not grow on ordinary nutrient media; on agar with whey forms colonies with a jagged edge and hairy offspring into the environment. Contained in matsun, a dairy product made in Armenia. Apparently identical to LactobaC. caucasicus.-L actobac. lactis acidi Leichmann. In addition to those listed, lactic acid bacteria include Lactobacillus Boas-Op p 1 e g i, found in gastric contents, mainly in gastric cancer; Lactobacillus helvetieus (syn. Vas. caseiFreudenreich "a), isolated from sour milk and cheeses; Lactobacillus bu-sae asiaticus (Bact. busae asiaticae Tschekan), isolated from buza, and others. - A microbe that most often causes milk to turn sour in the cold, is Streptococcus acidi lactici (Grotenfeldt) or, according to the American classification, Streptococcus lacticus (Lister) Lonis.To obtain the best growth of all types of lactic acid microbes, Omelyansky recommends Kohendy agar: boil 1 liter of milk for 5 minutes, add 1.5 cm3 of hydrochloric acid and filtered through a canvas.The resulting whey is slightly alkalized and 300 cmg of water, 3 g of gelatin, 15 g of peptone and 20 loons are added per 1 liter. The mixture is heated in an autoclave, filtered and sterilized. Lactic acid bacteria are of great importance in the dairy industry, as they participate in the formation of various lactic acid products (yogurt, lactobacilli, sour cream, cottage cheese, cheeses, kefir, koumiss, etc.).

In nature, lactic acid bacteria are found on the surface of plants (for example, on leaves, fruits, vegetables, grains), in milk, in the external and internal epithelial integuments of humans, animals, birds, fish (for example, in the intestines, vagina, on the skin, in the mouth, nose and eyes). Thus, in addition to their role in the production of food and feed, lactic acid bacteria play an important role in wildlife, agriculture and normal human life. The impact of the accelerated industrialization of the production of lactic acid bacteria based on a small number of strains adapted for plants, but the natural diversity of these bacteria and human health remains unexplored.

Our knowledge in microbiology, genetics and plasmid biology of lactic acid bacteria is advancing rapidly. Research is unraveling the molecular mechanisms of gene transmission, control, and expression.

Lactococcus lactis and Lactococcus cremoris primarily lactic acid bacteria of the genus Streptococcus. These organisms develop naturally in milk and were among the first genera of lactic acid bacteria studied by microbiologists. They are able to have an N antigenic structure in the cell wall.

Their role in lactic acid fermentation and utility as a starter culture has renewed interest in microbiology and genetics, especially with the latest development of gene transfer and cloning systems. These discoveries provide an opportunity to guide genetic improvements in existing starter cultures used in the dairy industry. This approach complements classical culture improvement programs for increasing bacteriophage resistance, establishing culture robustness and viability while eliminating undesirable traits. The use of food grade microorganisms as the main bacteria to create genetically engineered proteins provides an additional incentive for more detailed genetic analysis.

micrococci

The Micrococcaceae family includes the genera: Micrococcus, Staphylococcus, Sarcina.

Cocci of this family usually have the shape of an almost regular ball. Division coccus in three perpendicular planes leads to the formation of cuboid clusters (genus Sarcina). Sometimes there are cells connected in pairs.

Majority Microcossaceae - aerobes and facultative anaerobes. There are a small number of species related to obligate anaerobes. Along with saprophytic species, there are pathogenic species (Staphylococcus aureus, etc.), which can cause various pathological processes in the human and animal body, as well as be the cause of food poisoning. Saprophytic microorganisms of the Micrococcaceae family are widely distributed in nature, when they get on food products, they can cause various defects: bitter taste of milk and cheese, thickening of milk, rancid taste of butter, the formation of brown spots on cheese, etc.

The genus Micrococcus is a strict aerobe unlike the genus Staphylococcus. When growing on MPA, they form medium-sized, round, white, yellow or pink colonies. There are also various shades from red to orange, especially common among saprophytic micrococci M. roseus. M. flavus. Pigments formed by micrococci are insoluble in water. The optimum development temperature is 20-25°C. Many types of micrococci can develop with 5 -- 8°C. In contrast, toxigenic staphylococci are mesophilic. Certain types of micrococci can withstand heating to 63-65 ° C for 30 minutes and short-term pasteurization at high temperature.

Micrococci are highly resistant to salt and sugar. Some varieties of micrococci (M. radiodurans) are exceptionally resistant to ionizing radiation.

Micrococci are peptonizing microorganisms. When developing in milk, they ferment lactose, increasing the acidity of milk to 40-45 ° T, and at the same time secrete rennet; this leads to the formation of a clot in the milk and the appearance of defects in milk and dairy products (premature clotting, thickening of condensed milk with sugar, bitter taste). Some types of micrococci decompose fat and cause a rancid taste in the product.

lactic acid bacteria

Lactic acid bacteria are widely distributed in nature. Under certain conditions, they can cause spoilage of many foods.

According to morphological features, they are divided into streptococci and rods. In each group there are homo- and heterofermentative bacteria.

Lactic acid streptococci. Lactic acid streptococci belong to the family Streptococceae, genera Lactococcus and Leuconostoc. Homofermentative include milk (Laс. lacnis) and creamy (Lac. cremoris) streptococci. Aroma-forming streptococci, or citrovorus, are heterofermentative, capable of producing aromatic substances (diacetyl, acetoin) and assimilating citric acid salts - citrates. This group includes Lac. diacetylactis, Leu. cremoris, Leu. dextranicum

An intermediate position between homo- and heterofermentative streptococci is occupied by thermophilic streptococcus Str. thermophilus, which is why it is sometimes called a medium heterogeneous species.

Lactic acid streptococci are spherical or oval cells up to 1--2 microns in diameter, located in the form of short chains or in pairs; are immobile, do not form spores or capsules, stain positive according to Gram. In young cultures, some strains of creamy streptococcus form a mucous capsule. Cells of aroma-forming streptococci are somewhat smaller than Lac cells. lactis and Lac. cremoris, and the cells of thermophilic streptococcus are larger than creamy.

Lactic acid streptococci, like rods, are facultative anaerobes in relation to oxygen, that is, they grow not only under anaerobic conditions, but also with the access of molecular oxygen. However, in the presence of oxygen, their type of respiration does not change, since aerobic respiration does not appear, but the fermentation process continues.

Therefore, lactic acid bacteria can be classified as aerotolerant (air-tolerant) anaerobes.

The temperature limits of the vital activity of these microorganisms are quite wide. For mesophilic species, the optimum temperature is 25–30 °C, but there are also thermophiles growing at 38–43 °C. The minimum development temperature for mesophilic lactic acid bacteria is 10 ° C, for thermophiles - 20-22 ° C. There is evidence that some lactic acid bacteria are able to grow at very low positive temperatures (up to 3 °C).

According to the need for nutrients, lactic acid bacteria are among the most complex microorganisms. They can use mono- and disaccharides, organic acids as a carbon source.

They do not develop on ordinary nutrient media, but grow on media with the addition of amino acids, meat protein hydrolysates, lactalbumin, casein, and various types of flour. Most types of lactic acid bacteria need amino acids: arginine, cysteine, glutamic acid, leucine, phenylalanine, tryptophan, tyrosine, valine. Only certain types of lactic streptococci can grow on media containing ammonium salts as the only source of nitrogen,

Most lactic acid bacteria need vitamins - riboflavin ( IN 2), thiamine (B1), pantothenic (Bz), nicotinic (PP), folic ( Sun) acids, pyridoxine ( AT 6) and others. This explains the positive effect on the growth of microorganisms of additives to nutrient media of various nutrient extracts (corn, carrots, potatoes), yeast autolysate and other vitamin-containing compounds. The growth of lactic acid bacteria is also stimulated by some peptides, purines (adenine, guanine, hypoxanthine), pyrimidines (uracil, thymine, etc.), fatty acids (acetic, oleic), and citric acid.

Lactic acid bacteria are cultivated in sterile skimmed milk or in solid and liquid artificial nutrient media using hydrolyzed milk and other nutrients derived from milk.

With the development of lactic acid streptococci in milk, they cause its coagulation (with the exception of Leu. demons), i.e., the formation of an even, without abundant separation of whey, a dense clot that has a pleasant sour-milk taste and smell. Aroma-forming streptococci form a clot in which carbon dioxide bubbles can be found in a small amount. On a nutrient medium (agar with hydrolyzed milk and chalk), lactic streptococci form small (0.5–1 mm) drop-shaped colonies with a smooth edge, with chalk clearing zones. Colonies in the thickness of the digestion medium (deep colonies) have the shape of a boat or lentil grain. Lac. diacetilactis on 3% agar; can form deep colonies in the form of spiders or cotton balls, resembling colonies of lactic acid sticks.

Lactic acid bacteria grow in environments with low pH values ​​from 5.5 to 8.8, some at pH 2.9-3.2. A characteristic property of lactic acid bacteria is high alcohol resistance. They can develop on nutrient media containing 15-18 % ethyl alcohol, less often - at 24%.

The biochemical properties of lactic acid bacteria are studied by the energy of acid formation, limiting acidity, the ability to ferment citric acid salts, the quality of the clot, the possible proteolytic activity of bacteria, etc.

The energy of acid formation is determined by the time of formation of a milk clot (acidity is about 58--60 ° T) when 0.5 cm of a young (12--20-hour) culture is introduced into 10 cm 3 of sterile skimmed milk and crops are grown at the optimum temperature.

The acidity of milk according to Turner is determined by titration with a decinormal sodium hydroxide solution with the indicator phenolphthalein. For titration, take 10 cm 3 of milk diluted with 20 cm 3 of water (you can take 2 times less). The volume of alkali (in cm 3) used to neutralize the acid is multiplied by 10 (20) and thus the acidity of milk is obtained (1 ° T corresponds to 9 mg of lactic acid in 100 cm 3 of milk).

The proteolytic activity of bacteria is studied on meat-peptone gelatin, milk or determined using special biochemical studies and judged by the total amount of water-soluble protein breakdown products formed, the formation of ammonia, hydrogen sulfide, indole, which characterize the deep breakdown of protein substances.

The ability to ferment salts of citric acid (citrates) is determined by inoculation of bacteria on a dense medium with calcium citrate. The appearance of clear zones around the colonies indicates the formation of water-soluble fermentation products in the presence of the citritase enzyme.

Activity the formation of aromatic substances is determined by the amount of volatile compounds formed (by the sublimation method) and four-carbon compounds (diacetyl and acetoin).

Lactic acid streptococci have different enzymatic activities.

Lac. lactis is an active acid-forming agent. Active strains coagulate milk in 4-7 hours. The maximum acidity during its development reaches 120 °T. Restores and coagulates litmus milk, does not form acetoin, decomposes arginine with the formation of ammonia. Does not develop in an environment containing 6.5 % NaSi, and in an alkaline medium at pH 9.5. Many strains produce the antibiotic nisin, which is a polypeptide with a molecular weight of 3500. It inhibits most streptococci (but not enterococci), staphylococci, micrococci, some types of bacilli, lactobacilli, clostridia, actinomycetes. At the same time, nisin does not have a bactericidal effect against gram-negative bacteria.

Lac. cremoris, unlike lactic streptococcus, does not ferment maltose and dextrin, and lacks the ability to deaminate arginine. Does not grow on media containing 4 % KaS1, as well as at a temperature of 39--40 ° C. At low cultivation temperatures (15--20 °C), some strains form a significant amount of volatile acids, restore and coagulate (sometimes only partially) litmus milk. There are mucus-forming strains that form milk clots. They are used in starters for the production of sour cream.

The energy of acid formation in Lac. cremoris is weaker than Lac. lactis, and is 6--8 hours, and the limiting acidity is 110-115°T.

Aroma-forming streptococci contain the enzyme citritase, which breaks down citrates to form carbon dioxide (CC) and aromatic substances - acetoin and diacetyl. Relatively weak acid former, but forms a significant amount of diacetyl. It has a weak energy of acid formation (more than 16 hours), the limiting acidity in milk reaches 70--100 °T. The milk clot often contains bubbles of gas (CO2). The smell of the clot is specific, due to the accumulation of diacetyl. Restores and coagulates litmus milk, at first it turns pink, then quickly discolors. Many strains decompose arginine with release of ammonia, are resistant to content in the medium 4 % NaCl. Leu. dextranicum is also a weak acid former. It coagulates milk at the optimum temperature after 2-3 days. The limiting acidity is 70 - 80 °T. For the development of Leu. dextranicum and Leu. cremoris, manganese is of great importance, the addition of which to milk stimulates their growth and aroma formation.

Str. thermophilus surpasses all lactic acid streptococci in the energy of acid formation, reaching the level of thermophilic lactobacilli. It ferments milk after 3.5 - 6 hours, the maximum acidity is 1 10 - 1 15 ° T.

Thermophilic streptococcus does not grow on media with penicillin 0.01 IU/cm 3 and streptomycin 5 μg/cm 3 is used as a test culture for the detection of antibiotics in milk. Sensitive to the action of specific bacteriophages. More intensive growth of thermophilic streptococci is observed when basic amino acids - valine, leucine, isoleucine, lysine, arginine, methionine, histidine and proline are added to nutrient media. Str. thermophilus has a relatively high thermal stability. It withstands a temperature of 75 °C for 15 minutes and 65 °C for 30 minutes, as a result of which it makes up a significant part of the residual microflora in milk after pasteurization.

In a liquid medium containing glucose and 4% NaCl, thermophilic streptococcus does not form acid, but at a content of 2 % MAC1 lactic acid is synthesized by individual strains. If present in the environment 0.1 % methylene blue Str. thermophilus does not develop, it does not restore litmus milk. Some strains form diacetyl, synthesize acetoin in a small amount.

Lactic acid sticks. Lactic acid sticks (lactobacilli) belong to the family Lactobacteriaceae, the genus Lactobacterium, which includes three subgenera: Thermobacterium, Streptobacterium and Betabacterium. Thermo- and streptobacteria are homofermentative, and beta-bacteria are heterofermentative lactic acid bacilli.

To thermobacteria there are 8 types of sticks, among which L. helveticum, L. acidophilum, L. bulgaricum, L. lactis are most often used. The subgenus of streptobacteria includes 7 species, among which L. plantarum and L. rhamnosus are used in the dairy industry. The subgenus of beta bacteria includes 11 species of rods, the most studied among them are L. brevis, L. buchneri, L. fermentum, etc.

lactobacilli are sticks, single or connected in pairs, with a size of (4.. .10) x (0.5.. .0.6) microns. They are immobile, do not form spores and capsules, stain positive according to Gram. Streptobacteria cells are smaller than thermobacteria cells and are often arranged in chains. Beta bacteria have the smallest and thinnest cells.

Lactic acid sticks are facultative anaerobes or microaerophiles. In relation to temperature, streptobacteria and beta bacteria are mesophiles, thermobacteria are thermophiles. They do not grow on ordinary media, they are grown on media with milk. When developing in milk, they cause the formation of a homogeneous dense clot with a pleasant sour-milk smell and taste.

On a dense nutrient medium, lactobacilli form small, smooth, shiny colonies with a gray-white spherical surface. Colonies of lactobacilli of different species almost do not differ. However, in some cases, fibrous, growing into the substrate R-form colonies are observed, in contrast to the smooth colonies belonging to the S-forms. Deep colonies of thermobacteria can be dark, yellowish-brown, sometimes with short outgoing filaments. Unlike deep colonies, surface colonies are larger, curl-like or granular. Deep colonies of streptobacteria are boat-shaped, sometimes with an outgrowth.

The temperature limits of growth for thermobacteria are 20 - 55 °С, for mesophiles - 15-38 °С. The optimal development temperature for L. helveticum is 40 °C, for L. bulgaricum, L. lactis - 45 °C, L. acidophilum - 37-38 °C. For mesophiles, the optimum temperature is 30 °C.

Lactobacilli have weak proteolytic activity and therefore do not grow in substrates where the only source of nitrogen is protein, i.e. where various amino acids are absent.

At the same time, there are lactic acid bacteria that can break down proteins.

Lactic acid bacteria do not reduce nitrates to nitrites and do not form pigments. Cytochromes and peroxidase do not form, but some produce catalase, which decomposes hydrogen peroxide (H 2 O2). Lactobacilli have well-defined saccharolytic properties. In addition to glucose and lactose, they ferment other sugars. Thus, many homo- and heterofermentative species (L. plantarum and L. brevis, etc.) intensively use pentoses, sometimes even more actively than glucose.

Heterofermentative lactic acid bacteria ferment fructose because they have mannitol dehydrogenase, which reduces fructose to mannitol. Fructose fermentation products are also lactates, acetates and carbon dioxide.

Thermophilic lactic acid rods are active acid-formers, they ferment milk after 4--5 hours, the maximum acidity reaches 200--350 ° T,

L. helveticum is the most active acid-forming agent, the maximum acidity of milk during its development reaches 350 °T. This stick ferments maltose and dextrin, does not ferment sucrose, raffinose, salicin. Some strains develop in substrates containing up to 5% salt.

Strains of L. helveticum can be isolated from calf abomasum or sour raw milk.

L. bulgaricum brings the maximum acidity of milk to 200-300°T. Strains of Bulgarian sticks form acetaldehyde - an aromatic substance that gives a specific taste and smell, and antibiotic substances that suppress unwanted intestinal microflora. Bulgarian stick is sensitive to many antibiotics, resistant to bacteriophage.

Strains of L. bulgaricum are usually isolated from raw milk. L. acidophyllum is an intestinal microbe that can be isolated from the digestive tract of humans and various animals. Acidophilus bacillus is able, after cultivation in milk, to take root again in the human intestine and suppress the development of pathogenic and unwanted microorganisms there (Salmonella, Shigella, Staphylococcus, Escherichia, etc.). The antagonistic effect of L. acidophylum is due to the produced antibiotics - acidophilus and lactocidin.

Acidophilic bacteria are resistant to alkaline reaction (pH 8.3), the presence of phenol in the medium (0.25-0.4%), bile (20 %), CaCl (2%). The limiting acidity of acidophilus bacillus reaches 200-250 °T. L. acidophylum ferments sucrose, maltose, salicin, often raffinose, dextrin. There are mucus-forming strains of acidophilus bacillus.

L. lactis in its properties and behavior in the sourdough shows great similarity with L. bulgaricum. They ferment glucose, lactose, maltose, sucrose, galactose, raffinose, dextrin and salicin. The limiting acidity of milk fermented by L. lactis reaches 120--180 °T.

As a result of the vital activity of heat-resistant sticks, intense acid formation occurs, causing the defect of cottage cheese, sour cream, ordinary curdled milk - an excessively acidic taste. Can cause viscidity and an impure, unpleasant taste.

Streptobacteria have a less pronounced acid-forming ability. They ferment milk in 2-3 days, the maximum acidity is 180 °T.

Streptobacteria L. plantarum, L. rhamnosus are able to assimilate, in addition to lactose, also salts of lactic acid, i.e. lactates. They grow in hydrolyzed milk containing 6% MaCl and 20-40% bile, reduce and coagulate litmus milk and do not form ammonia from arginine. They have high proteolytic activity (2 times higher than that of mesophilic lactic streptococci), the content of free amino acids in milk is increased from 10 to 60 mg%. L. rhamnosum, in contrast to L. plantarum, forms CO2 of sodium citrate.

L. plantarum produces the antibiotic lactolin, which acts depressingly on the intestinal microflora and butyric acid bacteria.

Streptobacteria have well-defined saccharolytic properties. They ferment fructose, galactose, mannitol, mannose, raffinose, ribose, salicin, sorbitol, trehalose, esculin, etc. Glucose is fermented without the formation of gas.

The human body, especially its intestines, is densely populated by various types of microorganisms, a significant part of which are various species.

They play an important role in digestion, ensuring the normal functioning of the digestive tract, while affecting other internal systems.

What are lactic acid bacteria

Lactic acid bacteria are Gram positive

Lactic acid bacteria are Gram-positive micro-aerophilic bacteria.

They got their name due to the ability to process those entering the human body, forming several products, one of which is lactic acid.

In accordance with the Latin term for "dairy" products, this group is also called lactobacilli.

For a person, the most important role is played by lactic acid bacteria that live on the internal mucous membranes of his body. Most microorganisms of this type are located. Their main purpose in this case is to accelerate the digestion of food and its decomposition into digestible components.

A large number of lactobacilli are found in the female body in the vagina. There they form a normal microflora, counteracting infection by pathogenic microorganisms.

In addition to the human body, lactobacilli inhabit the organisms of fish, birds and animals, where they perform the same functions. Also, these microorganisms can be found in dairy and sour-milk products, which are their natural habitat.

It should be noted that lactic acid strains do not grow on conventional nutrient media. For the growth of colonies, the presence of meat or milk protein, as well as a number of amino acids, is necessary.

In nature, lactobacilli can be found on the surface tissues of plant parts (fruits, leaves).
Microorganisms by type of life activity are anaerobes, that is, they exist in an oxygen-free environment.

But they also remain viable even with little access to oxygen, which ensures their high survival rate. The main factor that has a negative effect on bacteria is high temperatures.

Lactic acid colonies are formed in the human body from the moment of his birth and during the first days of his life reach a significant amount. The normal concentration of lactobacilli in the intestines of an adult ranges from 106 to 108 units. Their number reaches 8% of the total intestinal mass.

The number and composition of microorganisms is influenced by many factors:

• digestive tract infections;
• chronic diseases and pathologies of the digestive tract;
• numerous and frequent stressful situations;
• violation of the diet, unbalanced diet;
• long-term therapy with antibiotics, non-steroidal anti-inflammatory drugs, laxatives.

These events are among the reasons for the decrease in the concentration of lactobacilli in the body, which negatively affects human health in general.

Types of lactic acid bacteria

Lactic acid bacteria - in dairy products

The group of microorganisms producing lactic acid from degradable carbohydrates is quite numerous and diverse.

It is possible to distinguish the main types of lactic acid bacteria, which are the most common and play an important role in digestion:

1. Bulgarian stick - isolated from Bulgarian sour milk. It is not mobile and does not form spores. The length of one bacterium reaches 20 microns, they often form short chains. The Bulgarian stick is activated at temperatures above 40 ° C, very quickly coagulates milk with a high concentration of lactic acid (up to 32 g / l).
2. Lactic acid streptococcus - found in most dairy products. It does not form a dispute and connects in pairs. The optimum development temperature is about +30°C. When infected with streptococcus, milk coagulates within 12 hours. The bacterium is able to resist the reproduction of pathogenic bacteria.
3. Lactococcus - for the first time after formation, it looks like a streptococcus. It does not form spores and is not mobile. Lactococcus develops at a temperature of about + 30 ° C, milk ferments within 25 hours. The content of lactic acid does not exceed 7 g / l, since at higher acidity the bacterium becomes unviable.
4. Acidophilus bacillus was first isolated from children's feces. It develops at a temperature of about +40°C and has antibacterial properties. The content of lactic acid as a result of fermentation by this microorganism reaches 2-3 g/l.
5. Creamy streptococcus - grows best at 25°C, but is also viable at lower temperatures (starting from +10°C). The bacterium forms long chains.

Depending on the type of lactobacilli, during development in nutrient media, they form colonies that differ in appearance:

• dewy;
• in the form of boats;
• rough;
• spider-like;
• lumpy;
• mucous membranes;
• stellate.

From the whole variety of lactic acid bacteria, pathogenic microorganisms (including pneumococci) are distinguished into a separate group and considered outside this classification.

Some spore-forming aerobic microorganisms, on the contrary, are classified as lactic acid bacteria (bifidobacteria, inulin sporolactobacteria) due to similar functions and metabolism.

The benefits of lactic acid bacteria for humans

There are several types of lactic acid bacteria

The beneficial properties of lactic acid bacteria were discovered as early as the beginning of the 20th century. The first discoveries in this area belong to the famous Russian-French biologist I.I. Mechnikov.

The scientist investigated the possibility of restoring a healthy intestinal microflora with the help of lactobacilli.

In the course of long and painstaking experiments, Mechnikov studied the properties of the Bulgarian stick and created a fundamentally new fermented milk product for those times - yogurt, which at first was called Mechnikovskaya.

Ilya Ilyich suggested the positive effect of lactic acid bacteria on human immunity. Despite the venerable "age" of his theory, real research in this area was carried out relatively recently.

During the experiments, it turned out that lactobacilli have pronounced antibacterial properties. In one way or another, they are able to counteract pathogenic microorganisms, reducing the activity of the latter and eliminating harmful bacteria and fungi.

Counteracting microbes is carried out with the help of various substances that are produced during the activity of lactic acid bacteria:

• hydrogen peroxide;
• organic acids;
• bacteriocins, etc.

The immunomodulatory properties of lactobacilli are determined by their ability to produce cytokines, antibodies, increase the activity of phagocytes and stimulate the production of interferon.

Thanks to this, the human body becomes more resistant to ARVI and acute respiratory infections, influenza, fungi and various bacteria of pathogenic etiology.

The ability to positively affect the immune system provides the benefits of lactobacilli in the fight against, especially in childhood. With a sufficient amount of lactic acid microflora, the concentration of allergens in the body decreases and the production of its own antibodies is stimulated.

In addition to increasing immunity, lactobacilli are able to normalize their own microflora of the intestines and other organs of the gastrointestinal tract, while healing the entire body.

They restore the state of the digestive system after food and other kinds of poisoning, prolonged antibiotic therapy, exposure to stressful situations. At the same time, intestinal motility normalizes, constipation and other signs of dyspeptic disorders disappear.

Studies have shown that some lactic acid strains help lower blood cholesterol and improve liver function.

Lactic acid bacteria are of great benefit to humans. But when exceeding the norm in the body, they are able to cause pathogenic processes. Treatment in this case will depend on the age and condition of the patient, the degree of infection with bacteria and other factors.

The use of lactic acid bacteria in industry

Yogurt is a source of lactic acid bacteria

The concentration of lactic acid bacteria in the human body is not constant.

Their number and species composition varies depending on external and internal factors (nutrition, medication, environmental pollution, chronic diseases, etc.).

In this regard, it is necessary to constantly maintain the lactic acid microflora in sufficient quantities. This is helped by fermented milk products containing lactobacilli. They are called probiotics.

Products with lactic acid bacteria are diverse:

1. fermented baked milk;
2. curdled milk;
3. leaven;
4. acidophilus drink, etc.

All these products have a pleasant taste and, provided that milk protein is tolerated, have a positive effect on digestion. In addition to those already present in them, manufacturers add special strains to the composition, the beneficial properties of which have been studied and proven in the course of research.

The use of products containing lactic acid bacteria in childhood contributes to the formation of strong immunity, the normal development of tissues and organs. Children who use such products are more active, attentive, and better absorb information.

In addition to the food industry, lactobacilli are actively used in pharmaceuticals. Currently, there are many preparations of various forms (powders, suspensions, capsules, etc.) containing lactic acid microorganisms.

They, as well as fermented milk products, are designed to stabilize the work of the intestines and restore normal microflora.

Lactobacilli are used not only for therapeutic purposes. Their properties cause wide application in the production of cheeses, preparation of dough, fermentation of vegetables, ensiling of fodder. The lactic acid fermentation provided by these microorganisms has found wide application in the production of soft drinks and canned foods.

Lactic acid bacteria have a set of useful properties and are of high value to the human body. They stimulate digestion and boost immune forces. To maintain the concentration of lactobacilli in the norm, it is necessary to consume fermented milk products containing certain strains of microorganisms.

How lactic acid bacteria look under a microscope will show the video:

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Super popular in our time are lactobacilli and bifidobacteria, these products are written "healthy". However, graduates of my healthy eating course know that a product that says "healthy" is not.) Let's see what are the features of "enriched" factory fermented milk products. Normally, bifidobacteria make up 80-90% of the intestinal flora of breastfed children and young mammals in the lactation period.

The presence of bifidobacteria in the intestines is beneficial for the child and young animals, since bifidobacteria inhibit the development of various putrefactive and pathogenic microbes, and promote the digestion of carbohydrates. At the end of milk feeding, the bifidoflora is replaced by the usual intestinal microflora, characteristic of adult organisms. Thus, the predominance of lactobacilli is natural for children, but not for adults.

The main function of lactobacilli is the conversion of lactose, which enters the body with food, into a simpler element - lactic acid. Without exception, all lactic acid products contain lactobacilli. The names of dairy products in the prefixes "acido-", "bio-", "lacto-", "bifido-" - indicate that they contain live bacteria. Let us pay attention to the fact that lactobacilli absorb a large amount of fiber and compete with bacteria that produce butyric and propionic acids.

We pay special attention to the fact that acetic, propionic and butyric acids dominate in the large intestine only of adults. In breastfed children, there are mainly acetic and lactic acids. In the large intestine of adults (as well as in other parts of the gastrointestinal tract), there should not be an excess of lactic acid! This is due to the fact that the most important SCFA is butyric, and lactic acid sharply reduces its production, they compete with each other!

Artificial enrichment of dairy products with "live bacteria" will not work! It is for this reason that adults who excessively consume sour-milk acidophilic foods enriched with lactobacilli clog their intestines with such an amount of lactic acid that is completely incompatible with normal physiology. Lactic acid not only creates the wrong intestinal environment - it is also extremely easily absorbed and interferes with overall metabolism.

Lack of butyric acid leads to inflammation of the intestinal mucosa (especially characteristic of ulcerative colitis), accompanied by the growth of many pro-inflammatory cytokines. A sufficient amount of butyric acid prevents colon cancer and prevents the development of type 2 diabetes (non-insulin dependent). Butyric acid is not produced during the fermentation of carbohydrates by lactic acid bacteria - therefore, modern acidophilic foods rich in lactobacilli in the modern diet are useless at best, but often harm the body.


In traditional fermented milk products, there is no isolation and enrichment of the product only with lactobacilli, and a certain amount of propionic acid bacteria (which live, for example, on the skin of the hands) gets into the product one way or another. Moreover, the aging time of factory fermented milk products is short and the fermentation process stops at lactic acid fermentation. This allows you to slow down the fermentation process and increase the shelf life.

What is the difference between traditional fermented milk products? The fact that propionic acid bacteria are also present in it, which logically continue lactic acid fermentation. Propionic acid fermentation is the process of converting sugar or lactic acid into propionic and acetic acids. Propionic acid bacteria are close to lactic acid bacteria and often develop together with them. Thus, propionic acid fermentation is a continuation of lactic acid fermentation, lactic acid is fermented, the final product becomes slightly acidic.

Free propionic acid is a product of anaerobic fermentation and is found in foods that undergo enzymatic processing, such as traditional yogurt, kefir, and cheeses.

It turns out that with a sufficiently long natural storage and at the same time fermentation, first with lactobacilli, and then with propionic bacteria, we get an excellent product!

Conclusions.

1. It is acceptable to consume small amounts of traditional fermented milk products with low shelf life.

2. The safe use of fermented milk products is possible only when combined with foods containing a lot of fiber and saturated fat.

3. Factory fermented milk products enriched with lactobacilli are not healthy and should not be included in the regular diet.


4. If you really like factory fermented milk products, then choose traditional recipes. Fermented foods will help you compensate for excess lactic acid.



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