Lactic acid bacteria

Lactic acid bacteria. Lactic acid bacteria (BAL) are microorganisms that have various applications, one of the main ones being the fermentation of foods such as milk, meat and vegetables to obtain products such as yogurt , cheese, pickles, sausages, silages, butter , cream of milk , kefir and koumiss, etc., As is known for around 4 millennia. Likewise, BALs are very useful in the production of wines and beer.

Summary

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  • 1 What are lactic bacteria?
  • 2 History
  • 3 Dairy crops
    • 1 Bacteria triggering
    • 2 Characteristics of lactic acid bacteria
      • 2.1 Lactobacillus
      • 2.2 Genus Lactobacillus
      • 2.3 Nutrition and growth conditions
      • 2.4 Ecological conditions
      • 2.5 Metabolism
      • 2.6 Plasmids
      • 2.7 Phages
    • 4 See also
    • 5 Reference
    • 6 Sources

What are lactic bacteria?

Lactic bacteria (BAL) are a group of microorganisms represented by various genera with common morphological, physiological, and metabolic characteristics. In general, BAL are cocci or Gram positive bacilli, not sporulated, non-mobile, anaerobic, microaerophilic or aerotolerant; oxidase, catalase and benzidine negative, they lack cytochromes, they do not reduce nitrate to nitrite and produce lactic acid as the only or main product of carbohydrate fermentation(Carr et al., 2002; Vázquez et al., 2009). Furthermore, BALs are acid tolerant and some can grow at pH values ​​as low as 3.2, others at values ​​as high as 9.6, and most grow at pH between 4 and 4.5, allowing them to survive naturally in environments where other bacteria would not tolerate the increased activity produced by organic acids (Carr et al., 2002).

History

It originated in the 18th century , when farmers in Asia, Africa and Europe observed the behavior of raw milk in the hot months. The milk coagulated and under these conditions had a different flavor, sometimes pleasant, the farmers selected the best-tasting to inoculate the milk the next day. Live BALs can be contained in a group of microorganisms called dairy cultures or starters, being used in the dairy industry for various applications. [one]

Dairy crops

Lactic acid bacteria constitute a vast group of benign microorganisms , endowed with similar properties, that manufacture lactic acid as the final product of the fermentation process . They are found in large quantities in nature, as well as in our digestive system . Although they are mainly known for their fermentation of dairy products, they are also used to pickle vegetables in the oven, in the baking of wine , and to cure fish , meat and sausages .

Although without understanding the scientific basis that explained their action, many peoples used these bacteria thousands of years ago to make modified foods, which could be kept much longer, and were endowed with characteristic textures and flavors, different from those of the original product. . These illustrious microbial allies are also being used to good effect today in the manufacture of a wide range of fermented dairy products, whether liquid, such as kefir, or dense and semi-solid, such as cheese or yogurt.

Bacteria Power

The action of these bacteria triggers a microbial process by which lactose (milk sugar) is transformed into lactic acid. As the acid accumulates, the structure of the milk proteins changes (curdles), and the same happens with the texture of the product. There are other variables, such as the temperature and composition of the milk, that influence the particular qualities of the different resulting products.

Lactic acid is also what gives fermented milk that slightly acidic flavor. Elements derived from lactic acid bacteria often produce other characteristic flavors or aromas. The acetaldehyde , for example, gives yogurt its characteristic aroma, while the diacetyl butter gives a taste of the fermented milk. They can also be added to the culture of microorganisms, such as yeasts, in order to obtain particular flavors.

Dairy products fermented by the microbial process

The Alcohol and carbon dioxide produced by the yeast, for example, give kefir, koumiss and leben (varieties liquid yogurt) freshness and fluffiness characteristics. Other techniques used include those consisting of removing whey or adding flavors, which allow creating a wide range of products.

Characteristics of lactic acid bacteria

Due to their characteristics, when processed and multiplied for use as a group, they comprise a broth of fermenting bacteria and producers of lactic acid, a function for which they are used in industry to give certain qualities to food and protect them against the action of other organisms. harmful. One of them may be the lactobacillios which provide the product with good care.

They presently present an immense biotechnological potential, given their presence in a multitude of fermentative processes for food intended for human consumption (dairy, vegetable, meat and bakery products, as well as alcoholic beverages) and animal (silage). These bacteria not only contribute to the development of the organoleptic and rheological characteristics of food, but also generate in the same environments unfavorable for the development of pathogenic microorganisms due to their marked antagonistic capacity, which favors their proliferation in food, to the detriment of any other microbial group present in the raw material (raw food) or that subsequently contaminates the product. In addition to this important role in bioconservation processes, it has been found that some strains of lactic bacteria,

Lactobacillus

They are beneficial to health , both human and animal (probiotics). Both beneficial effects, caused by its antagonistic capacity, are based on the production of organic acids and other inhibitory metabolites, among which it is worth mentioning hydrogen peroxide (H2O2) and other derivatives of oxygen metabolism, as well as aromatic compounds (diacetyl, acetaldehyde), dehydrated glycerol derivatives (reuterine), enzymesbacteriolytic, bacteriocins and others. Lactic bacteria can be used in the prevention and control of certain diseases, as well as in the improvement of the quality of preservation of certain foods, so its value lies in having available substances from microorganisms that serve as a starting point for obtaining biotechnological products applicable to solving human and animal health problems.

Lactic acid bacteria are located in the Lactobacillaceae family which is characterized by its members being long or short bacilli, but also cocci that divide like bacilli, only in one plane, producing occasional chains or tetrads and filaments, falsely called branching. These bacteria are normally non-motile, although they can also be. Motile species present perimeter flagellation. They are Gram positive, with rare production of pigments, although a few species produce them yellow, orange, red or brown.

Microaerophilic species rarely liquefy gelatin, however strict anaerobes do it more commonly. They present poor or no superficial growth in any medium. Carbohydrates are essential for their proper development, since they are fermented to give lactic acid (sometimes with volatile acids), alcohol and carbon dioxide (CO2) as by-products. They do not produce nitrites from nitrates, but among the strict anaerobes there are some species that reduce nitrates and others that have not been tested with this reaction. They are microaerophilic towards anaerobiosis. They are found regularly in the mouth and intestinal tract of man and other animals, in foods and dairy products, and in fermented vegetable juices. A few species are highly pathogenic

Genus Lactobacillus
  • Morphological characters: The genus Lactobacillus (lactis-milk; bacillus-small bacilli) is characterized by presenting cells in the form of long and extended bacilli, although frequently short bacilli or coco-bacilli can be observed, which makes them confuse with isolated genera. usually from clinical materials These bacilli commonly appear forming chains and are generally nomotile, but when they have motility it is due to the presence of perimeter flagellation. They are Gram positive and only dead cells can give Gram staining variable results. Furthermore, they do not sporulate and some strains have bipolar bodies that probably contain polyphosphate. Large homofermentative bacilli have internal granules revealed by Gram staining or by methylene blue staining.
  • Cell wall and ultrastructure: The cell wall of lactobacilli, observed under the electron microscope, is typically Gram positive and contains peptidoglycans (mureins) of various chemotypes, hence the peptidoglycan of the Lysine-D-Asparagine type is the most widely distributed. This wall also contains polysaccharides linked to peptidoglycan by phosphodiester bonds, but it only has related teicoic acids in some species. Large mesosomes that characterize this genus can also be seen under the electron microscope.
  • Cultural and colony characters: Lactobacillus colonies on solid media are small (2-5 mm), convex, smooth, with full margins, opaque and without pigments. Only in some cases do they appear yellowish or reddish.

 

Fermentative processes of alcoholic beverages intended for human consumption

Some species form rough colonies. Others, such as lactobacillus confusus, have viscous colonies by exception. They generally do not have proteolytic or lipolytic activity that can be appreciated by clear halos formed in solid media that contain proteins or fats. However, many strains exhibit slight proteolytic activity due to proteases and peptidases linked to or released from the cell wall, as well as weak lipolytic activity due to the action of intracellular lipases.

They do not normally reduce nitrates, but this reaction can occur in some cases, when the pH is above 6.0. Lactobacilli do not liquefy gelatin or digest casein, although many strains produce small amounts of soluble Nitrogen . They also do not produce indole or sulfhydric (H2S). They are catalase negative, but some strains produce the enzyme pseudocatalase that breaks down hydrogen peroxide. They are cytochrome negative, due to the absence of porphyrins; they have a benzidine-negative reaction.

Pigment production by these bacteria is rare and when it occurs, they can be yellow or orange to a ferrous or reddish hue. Its growth in liquid medium occurs through it, although its cells precipitate rapidly after growth ceases; giving rise to a smooth and homogeneous sediment, without film formation. On rare occasions this sediment is granular or viscous. Lactobacilli do not develop typical lords when growing in common media, but they contribute to modifying the flavor of fermented foods, producing volatile compounds such as diacetyl and its derivatives, and even hydrogen sulfide (H2S).

Nutrition and growth conditions

Lactobacilli have particularities for each species regarding complex nutritional requirements for amino acids, peptides, nucleic acid derivatives, vitamins, salts, fatty acids or esters of fatty acids and fermentable carbohydrates. They require not only carbohydrates as sources of Carbon and energy, but also: amino acids, vitamins and nucleotides. Generally these varied requirements are usually met when the lactobacilli culture medium contains fermentable carbohydrates, peptone, meat extract and yeast extract, although a supplementation with tomato juice , manganese, acetate and esters of oleic acid, especially Tween 80, are stimulating and even essential for many species. Therefore, these compounds are included in the MRS medium.

There are species that adapt to very particular substrates and need special growth factors. Because lactic acid bacteria (LAB) have similar nutritional and growth requirements; its classification has been made difficult by traditional microbiological methods. The use of molecular tests, based on ribosomal DNA sequences, to identify bacteria isolated from their natural environment. Due to the high variability of this region between species, an efficient method for the specific identification and detection of probiotic lactic acid bacteria has been used for some years, which is useful for a better characterization of them, called PCR (polymerase chain reaction (polymerase chain reaction)

Ecological conditions

♦ PH: Lactobacilli grow well in slightly acidic media, with an initial pH of 6.4-4.5 and an optimal growth rate between 5.5 and 6.2. Its growth ceases when the pH reaches values ​​from 4 to 3.6 depending on species and strains and decreases markedly in neutral or slightly alkaline media. Lactobacilli are able to lower the pH of the substrate where they are below 4.0 by the formation of lactic acid. In this way, they prevent or at least considerably decrease the growth of almost all the other competing microorganisms, except that of other lactic bacteria and that of yeasts. ♦ Oxygen Needs: Most Lactobacillus strains are mainly aerotolerant; its optimal growth is achieved under microaerophilic or anaerobic conditions and it is known that an increase in the CO2 concentration (of approximately 5% or up to 10%) can stimulate growth, especially in the case of surface growth on solid media ♦ Temperature of growth: Most of the lactobacilli are mesophilic (30 -40 ° C), with an upper limit of 40ºC. Although their temperature range for growth ranges from 2 to 53 ° C, some grow below 15ºC and there are strains that grow below 5ºC. Others grow at low temperatures near the freezing point (for example, those that live in frozen meat and fish). So-called “thermophilic” lactobacilli can have an upper temperature limit of 55ºC and do not grow below 15ºC. The true thermophilic lactobacilli growing above 55 ° C are not yet known.

Metabolism

In your metabolism, lactobacilli go from anaerobic to aerobic life. These microorganisms lack cytochrome systems to carry out oxidative phosphorylation and do not have superoxide dismutase or catalase enzymes. Members of this genus transform glucose and similar aldehyde hexoses, the carbohydrates produced by these simple sugars, and polyhydric alcohols into lactic acid by homofermentation, or into lactic acid and other additional end products such as acetic acid, ethanol, carbon dioxide , formic acid and succinic acid by heterofermentation constituting at least 50% of the final products lactic acid, which is usually not fermented.

The main fermentation routes for hexoses are: that of Embden Meyerhof, where 1 mol of hexose is converted into 2 moles of lactic acid by homolactic fermentation and the 6-phosphogluconate route, which results in 1 mol of CO2, 1 mol of ethanol (or acetic acid) and 1 mole of lactic acid, by heterolactic fermentation. Under aerobic conditions, most strains re-oxidize NADH2 using O2 as the final electron acceptor, so Acetyl-CoA is not, or at least is not completely reduced to, ethanol.

In this way, additional ATP is formed by phosphorylation at the substrate level, as well as varying proportions of acetic acid and ethanol, depending on the oxygen supply. In terms of enzyme levels, heterofermentative lactobacilli possess phosphoketolases, but not FDP aldolases, whereas homofermentatives possess FDP aldolases, but not phosphoketolases.

Plasmids

In lactobacilli there are frequently plasmids related to drug resistance (drugs) or lactose metabolism. But the only known case of natural genetic exchange of plasmids in Lactobacillus is the one that occurs by conjugation of the plasmid that determines the metabolism of lactose in Lactobacillus casei.

Phages

The morphology of numerous double-stranded DNA phages that are virulent for many Lactobacillus species has been described, and the chemical-physiological parameters of seven of these phages have become known. Virulent phages have hexagonal heads and generally have long contractile tails or not, so they all belong to groups A or B, with the exception of a virulent phage for Lactobacillus plantarum. In general, these phages are very similar to those that attack other groups of bacteria. For these reasons, lysogeny is a widely distributed process within the Lactobacillus genus.

 

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