Lactobacillus delbrueckii subsp. bulgaricus

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Lactobacillus delbrueckii subsp. bulgaricus
Gram-positive rod bacterium, presumably Lactobacillus delbrueckii subsp. bulgaricus, from a drinking yoghurt (light microscope image, image width 2 µm)

Gram-positive rod bacterium,
presumably Lactobacillus delbrueckii subsp. bulgaricus ,
from a drinking yoghurt (light microscope image, image width 2 µm)

Systematics
Class : Bacilli
Order : Lactic acid bacteria (Lactobacillales)
Family : Lactobacillaceae
Genus : Lactobacillus
Type : Lactobacillus delbrueckii
Subspecies : Lactobacillus delbrueckii subsp. bulgaricus
Scientific name
Lactobacillus delbrueckii subsp. bulgaricus
( Orla-Jensen 1919) Weiss et al. 1984

Lactobacillus delbrueckii subsp. bulgaricus (written out Lactobacillus delbrueckii subspecies bulgaricus , common abbreviations are L. delbrueckii subsp. bulgaricus and L. delbrueckii ssp. bulgaricus ) is a gram-positive , rod-shaped bacterium that is used to make yogurt . Here metabolized the bacteria to the milk existing milk sugar ( lactose ) into lactic acid ( lactic acid fermentation). This leads to the acidification and coagulation of the milk. Lactobacillus delbrueckii subsp. bulgaricus is one of the lactic acid bacteria that carry out homofermentative lactic acid fermentation. It produces the so-called "levorotatory lactic acid". In dairies it is usually used in combination with Streptococcus salivarius subsp. thermophilus used as a starter culture . The flavoring substances typical of yoghurt are also produced by the two lactic acid bacteria.

Lactobacillus delbrueckii subsp. bulgaricus is a subspecies of Lactobacillus delbrueckii . Before 1984 the bacterium was known as Lactobacillus bulgaricus . The name goes back to the discovery of the bacterium in Bulgarian yogurt. This discovery was attributed to the Bulgarian scientist Stamen Grigorov by some authors . However, it is not clear whether in 1905 he actually described the bacterium that is now known as Lactobacillus delbrueckii subsp. bulgaricus is understood. According to current opinion, the bacterium was not until 1919 by Sigurd Orla-Jensen under the name Thermobacterium bulgaricum first described . The bacterial strain examined and described by Orla-Jensen at the time is known as Lactobacillus delbrueckii subsp. bulgaricus ATCC 11842 is still available for investigation purposes today. Its genome was completely sequenced in 2006 . The subspecies also includes other bacterial strains, some of which have long been used in industrial yogurt production. Lactobacillus delbrueckii subsp. bulgaricus is not pathogenic ("pathogenic"). To what extent it contributes to the health of people who regularly consume it in yoghurt has not yet been conclusively clarified , as is the case with many probiotics .

etymology

The generic name Lactobacillus (the grammatical gender is masculine) goes back to the occurrence and appearance of the bacterial cells: lac is Latin and means "milk", while bacillus (also Latin) indicates the rod-shaped shape. The specific epithet delbrueckii was chosen in honor of the German bacteriologist Max Delbrück , while bulgaricus , the epithet of the subspecies, refers to the origin of the bacteria from Bulgarian yoghurt (see also taxonomic history ).

characteristics

Appearance

Colonies of Lactobacillus delbrueckii subspecies bulgaricus on China blue lactose agar ; the bacterial colonies are stained blue as a result of the formation of lactic acid.

Lactobacillus delbrueckii subspecies bulgaricus is a gram-positive , medium-length to long, rod-shaped bacterium. A single cell is 2.0–9.0  micrometers (µm) long and 0.5–0.8 µm wide. In the light microscope image, there are individual cells, but also short chains of cells one behind the other. The bacterium has no flagella for active movement and cannot form persistent forms such as endospores .

In pure culture , L. delbrueckii subsp. bulgaricus on solid, glucose -containing culture media circular, colorless to opaque appearing colonies . When viewed from the side, they appear convexly raised. After incubation at 37 ° C for three days, the colonies reach a diameter of 2 to 5  mm .

Growth and metabolism

As typical representatives of the lactic acid bacteria , lactobacilli grow anaerobically , but aerotolerantly, i. that is, they grow in the presence of oxygen in the air, but do not need oxygen for their metabolism . They are catalase negative and oxidase negative. However, they are able to form cytochromes when cultured on media containing heme or blood . In this case they show a positive reaction in the oxidase test. In the presence of oxygen, the electrons released during the breakdown of carbohydrates in an oxidation are transferred to the oxygen (O 2 ). One possible reaction product is hydrogen peroxide (H 2 O 2 ), which acts as a cell poison . Since the enzyme catalase, which is often used to break down hydrogen peroxide, is not derived from Lactobacillus delbrueckii subsp. bulgaricus is produced, it uses the enzyme NADH peroxidase to break down H 2 O 2 into non-toxic O 2 and water (H 2 O). Another characteristic of lactic acid bacteria is the need for complex growth factors and amino acids during cultivation. So requires L. delbrueckii subsp. bulgaricus include riboflavin (vitamin B 2 ), niacin (vitamin B 3 ) and pantothenic acid (vitamin B 5 ).

The temperatures usually used for cultivation are in the range of 30-40 ° C, at 15 ° C or below there is no longer any growth . The bacterium also grows well at 45 ° C, the optimum temperature of an examined strain is 42 ° C. Thus, the bacterium is one of the mesophilic organisms, with a tendency to thermophilicity . The optimal pH value for growth is in a slightly acidic environment, at pH values ​​5.2 to 6.2, whereby pH values ​​up to pH 4.0 are also tolerated. The lactic acid produced lowers the pH value of the nutrient medium, provided it does not contain any buffering additives.

With the help of the " colorful series ", specific enzymes are detected that are used to break down certain substrates . A certain combination of enzymes is typical for a certain bacterial taxon. L. delbrueckii subsp. bulgaricus is characterized by the fact that it does not produce many of these enzymes. He has neither catalase nor oxidase. Nitrate reduction by the enzyme nitrate reductase (NADH) ( EC  1.7.1.1 ) is not possible, nor is the breakdown of urea by the enzyme urease . In addition, the enzyme arginine dihydrolase ( EC  3.5.3.6 ) is not present, so that no ammonia is split off from the amino acid arginine . The enzyme β-galactosidase is present and enables it to grow in milk using the milk sugar ( lactose ) it contains. It is unable to break down gelatin through hydrolysis , but it can utilize casein , a protein in milk. The proteinases and peptidases required for this are not released into the environment as exoenzymes , but are bound to the cell wall.

L. delbrueckii subsp. As a typical lactic acid bacterium, bulgaricus can utilize various carbohydrates for energy production through fermentation . It is able to utilize glucose and lactose, while the closely related subspecies L. delbrueckii subsp. delbrueckii can not utilize lactose. Other monosaccharides that can be used are fructose , galactose and mannose , these results being variable; In other words, they do not always lead to a positive test result or different working groups come to different results. Some sources state that galactose and mannose cannot be used.

Because of these contradicting statements, some of which are based on studies that were carried out in the 1970s, a new study was carried out in 2012 to show which carbohydrates are used under which conditions. On the one hand, a miniaturized test system (API 50 CHL) was used, which is based on the oxidation-fermentation test , in which the breakdown of a carbohydrate through acid formation and the associated change in color of a pH indicator is recognized. As a further method, a method with microtiter plates (AN MicroPlate) was used, with which cell respiration is measured colorimetrically . For this purpose, the redox indicator tetrazolium is used, which is itself reduced when the substrates oxidize and appears red. The investigation comprised 300 bacterial strains in the test for acid formation and 29 strains in the test for cellular respiration. The incubation was carried out anaerobically in each case. However, the different examination methods sometimes come to different results. There is no doubt that glucose, lactose and fructose are recycled. For galactose, on the other hand, the acid formation test shows a positive result in 25% of the strains examined, while the test with the redox indicator gives a negative result in the strains examined. For mannose, the acid formation test shows a positive result in 25% of the strains examined, while all strains examined give a positive result in the test with the redox indicator. As a possible explanation it is stated that the reduction of the tetrazolium directly indicates the metabolic process in which a substrate is oxidized, while possible end products, but also intermediate products , are indicated during acid formation .

Lactic acid fermentation

Structural formula of the D- lactate ion
Structural formula of D- lactic acid

A characteristic of fermentation ( fermentation ) is that the bacteria do not need oxygen to break down the substrates ( anaerobia ). The end product of fermentation that is typical and named for lactic acid bacteria is lactic acid . Accordingly, this special form of fermentation is called lactic acid fermentation . Since Lactobacillus delbrueckii subsp. bulgaricus almost exclusively produces lactic acid or lactate (the anion of lactic acid), it is counted among the homofermentative Lactobacillaceae .

Homofermentative lactic acid fermentation as an overview scheme, based on glucose.

The first cascade of chemical reactions in lactic acid fermentation is glycolysis . It involves the conversion of sugar into energy and pyruvate . The reactions are catalyzed by enzymes. Which types of sugar can be metabolized therefore depends on which enzymes a bacterium has at its disposal. That Lactobacillus delbrueckii subsp. bulgaricus can produce the enzyme aldolase , which enables it to convert fructose. The disaccharide lactose is split into glucose and galactose with the help of the enzyme β-galactosidase. The galactose can then be converted into glucose with the help of the enzyme UDP-glucose-4-epimerase . L. delbrueckii subsp. bulgaricus has UDP-glucose-4-epimerase, but studies show that galactose is not always used.

During the actual glycolysis, one molecule of glucose or fructose is broken down into two molecules of pyruvate in several intermediate steps. The pyruvate is then reduced to lactate by the enzyme lactate dehydrogenase with the coenzyme NADH formed during the glycolysis during the oxidation of glyceraldehyde phosphate . The NADH is oxidized to NAD + . Two molecules of lactic acid are thus formed from one molecule of glucose. Accordingly, the energy gain is two molecules of ATP per molecule of glucose (see figure).

The L. delbrueckii subsp. bulgaricus present lactate dehydrogenase is stereospecific , so that in this reaction between 96.0 and 99.4% D - (-) - lactate ( syn .: ( R ) -lactate) is formed. This enantiomer is also known as levorotatory lactic acid , in contrast to dextrorotatory lactic acid , L - (+) - lactate (syn .: ( S ) -lactate). The statements “with dextrorotatory lactic acid” or “with dextrorotatory cultures” occasionally given for yoghurt products relate to this distinction.

In addition to the main product lactate, small amounts of acetate , carbon dioxide , ethanol and acetoin are produced . In total, they make up less than 10%. The acetaldehyde contained in small quantities as a flavoring substance in yoghurt is also caused by L. delbrueckii subsp. bulgaricus , but the Streptococcus salivarius subsp. thermophilus is more involved in its production. Acetone and diacetyl are other flavorings that are important for yogurt taste and that are produced in low concentrations as by-products of lactic acid fermentation .

Chemotaxonomy

The murein layer in the cell wall contains the diamino acid L - lysine as a diagnostically important amino acid in position 3 of the peptide bridge. The peptidoglycan type is A4α, lysine is bound to the amino acid D - aspartic acid , which is bound to the next peptide bridge with a second carboxy group . The fatty acids occurring in the membrane lipids are mainly molecules with an even or odd number of carbon atoms (C 16 to C 19 ) and no or one double bond. These are the fatty acids cis - vaccenic acid (abbreviated as C18: 1 cis -11) and lactobacillic acid (cyclopropane-19: 0 cis -11), their proportion is 35.8 and 25.5%, measured in the stationary Growth phase . Since lactobacillic acid is formed from cis -vaccenic acid, its proportion decreases in the course of cultivation. In addition, saturated hexadecanoic acid (C16: 0, palmitic acid ) and unsaturated hexadecenoic acid (described as C16: 1) account for 18.8 and 13.8%, respectively. Branched-chain fatty acids , which are typical of other gram-positive bacteria, occur only in small amounts, for example anteiso - pentadecanoic acid ( anteiso -C15: 0) at 0.7%.

genetics

The genome of numerous strains of the bacterium has already been completely sequenced . The first sequencing took place in 2006 on Lactobacillus delbrueckii subsp. bulgaricus ATCC 11842, which is considered the type strain for the subspecies and can be traced back to a bacterial strain that was used in Bulgarian yogurt in 1919. The genome has a size of 1865 kilobase pairs (kb), which is only 40% of the genome size of Escherichia coli . 1529 proteins are annotated . The small genome size is a further indication of the adaptation to the milk habitat , because there are many complex growth factors, such as amino acids and vitamins , so that the bacterium has lost the ability to synthesize numerous metabolites over time .

The scientists who carried out the genetic test conclude that the genome is in a phase of reductive evolution. Symptoms and signs include a large number of genes of ribosomal RNA (rRNA) and transfer RNA (tRNA). In the Firmicutes, their number normally correlates with the genome size, so that in this case a size of 3000 to 4000 kb would be expected for the genome. The actual value found shows that the genome size decreased last. Another sign is the presence of numerous pseudogenes . These are altered genes that no longer serve as a template for a functional protein . 227 pseudogenes were discovered in the genome, making up a large proportion of 12% of the total number of genes. For some of the pseudogenes there are still functional, paralogous genes in the genome , but not for others. In these pseudogenes, the function of the associated proteins has been lost. Among other things, the carbohydrate metabolism and the biosynthesis of amino acids are affected. This is interpreted as an ongoing process of specialization. As a result, L. delbrueckii subsp. bulgaricus adapted from a habitat originally associated with plants to the protein and lactose-rich milk environment and lost superfluous genes in the process.

In 2011 the genome of the L. delbrueckii subsp. bulgaricus 2038, which has long been used in industrial yogurt production. In addition, the differences to the previously investigated strains of the subspecies and other representatives of the genus Lactobacillus were examined more closely. Compared to the strain examined first, the genome size is slightly larger at 1873 kb. L. delbrueckii subsp. bulgaricus 2038 is e.g. B. in contrast to the other strains able to synthesize the amino acid lysine .

The results of the sequencing show a GC content (the proportion of the nucleobases guanine and cytosine ) in the bacterial DNA of about 50  mol percent . This is considerably more than other Lactobacillus - species such. B. L. acidophilus , the GC content of which in the DNA is 34–37 mol%. The genetic analysis from 2006 shows that this high GC content is also shown in the third position of the codons , 65% of which is occupied by guanine or cytosine. Since molecular evolution proceeds faster at the third position of a codon, this is interpreted as the result of a more recent evolutionary change.

Pathogenicity

Lactobacillus delbrueckii including its subspecies is not pathogenic ("pathogenic"), it has been used in the food industry for a long time. Due to the Biological Agents Ordinance in conjunction with TRBA 466, it is  assigned to risk group 1.

proof

The bacterium is well in MRS broth cultured, maintained at a temperature of 37 ° C microaerophilic to anaerobically incubated is. A selective medium is not available, but selective enrichment can take place if the medium has a low pH value of up to pH 4.0 and contains carbohydrates. For example, a tomato juice - peptone agar is used for this. For the analysis of milk and milk products, the SL medium with pH 5.4 is recommended, in which the growth of the Lactococcus species, which are often also contained in the starter culture, is suppressed. However, Pediococcus and Leuconostoc species can grow. M16 agar, which has a pH value of 5.6, is also suitable. If the incubation takes place at 45 ° C for 3 days, the MRS agar with the addition of fructose or the MRS agar with a pH value of 5.2 are also suitable for treating Lactobacillus delbrueckii subsp. bulgaricus , while other lactic acid bacteria do not grow. China blue lactose agar (CLA) is often used for the general detection of lactic acid bacteria , whereby the bacterial colonies are colored blue as a result of lactic acid formation.

Biochemical tests, such as e.g. B. the oxidation-fermentation test , with different carbohydrates as well as the detection of enzymes in the metabolism can be used. In addition, a microscopic examination can provide clues. However, an exact identification down to the level of the species or subspecies is not guaranteed in this way. Therefore, it is also investigated which enantiomer of lactic acid is formed. A differentiation of the subspecies is possible by separating the cell proteins with the help of electrophoresis ( SDS-PAGE ). With the introduction of genetic testing methods, the most reliable results for an identification can be achieved with these methods. For phylogenetic studies, the nucleotides of 16S and 23S rRNA, the typical representatives of ribosomal RNA for prokaryotes, were determined . These gene sequences can also be used for identification. A PCR process ( polymerase chain reaction ), in which different target sequences from the rRNA and the DNA are combined, enables the species to be identified while at the same time differentiating the subspecies. A PCR method whose primers are aligned with the pepIP gene, which codes for the enzyme proline iminopeptidase , is also suitable for differentiating the subspecies .

Occurrence

Bulgarian yogurt

The natural habitat of Lactobacillus delbrueckii subspecies bulgaricus is milk and products made from it. Sour milk products have been used for a long time because they have a longer shelf life than fresh milk. In the past, sour milk products were not specifically made, but rather arose from the lactic acid bacteria naturally present in milk. The bacterium which was later called Lactobacillus bulgaricus and which, according to the current nomenclature, L. delbrueckii subsp. bulgaricus is. In dairies it is used in combination with Streptococcus salivarius subsp. thermophilus is used as a starter culture in yoghurt production, as well as in the production of quark and sour milk cheese . It can also be detected in the human intestine and is also present in the digestive tract of mice and rats . In humans, however, it is not one of the bacteria that permanently colonize the intestine, the so-called autochthonous species.

Systematics and taxonomy

Systematics

Lactobacillus delbrueckii , the species of L. delbrueckii subsp. bulgaricus is a type of the genus Lactobacillus . This genus belongs tothe Lactobacillaceae family , is placed in the order of Lactobacillales (lactic acid bacteria) in the class of Bacilli , which in turn is assigned to the phylum of Firmicutes .

The approximately 200 species of the genus are genetically closely related, but with regard to the lactic acid fermentation carried out, they are divided into homofermentative and heterofermentative types. Also with regard to other features, e.g. B. the GC content in the bacterial DNA, the representatives of the genus differ considerably from each other. Despite the high GC content compared to the related Lactobacillus species, L. delbrueckii subsp. bulgaricus together with L. acidophilus , L. johnsonii and others. to the Lactobacillus delbrueckii group.

Taxonomic history

The early taxonomic history of the bacterium is varied and marked by errors. The Bulgarian natural and medical scientist Stamen Grigorow isolated a bacterium from Bulgarian yoghurt (кисело мляко, kisselo mljako , contemporary transcribed "kissélo mléko") in Geneva at the beginning of the 20th century , which he calls "Bacille A" and which he is responsible for Fermentation of milk into yogurt is considered. In the following (1907) the German researchers Arthur Luerssen and M. Kühn publish the description of a yoghurt bacterium, which they assume is identical to Grigorov's “Bacille A”, and call it “ Bacillus bulgaricus ”. In a US work on lactic acid fermentation from 1915 the name " Bacterium bulgaricum " appears, which apparently also refers to Grigorow's or Luerssen & Kühn's descriptions. In 1919 the Dane Sigurd Orla-Jensen described a lactic acid bacterium that he had isolated from Bulgarian yoghurt under the name " Thermobacterium bulgaricum ". In a "list of the most common types of bacteria" compiled by the American Dorothy Holland in 1920, the name " Lactobacillus bulgaricus " was used for this taxon for the first time and its original name as " Bacillus bulgaricus " was incorrectly attributed to Grigorov's teacher in Geneva, Léon Massol . In the first editions of the Bergey's Manual of Determinative Bacteriology published by David Hendricks Bergey (1st edition 1923) this name is adopted, but Grigorow ("Grigoroff") is named as the first author. At the same time, further articles appear on the alleged Grigorow or Luerssen & Kühn yogurt bacterium, in which this is sometimes assigned new name combinations , such as " Acidobacterium bulgaricum ", " Plocamobacterium bulgaricum " and " Lactobacterium bulgaricum ".

After more than three quarters of a century of research into lactic acid bacteria, the American bacteriologists Morrison Rogosa and P. Arne Hansen published a revision of the taxonomy of various Lactobacillus species in 1971 . In it they state that neither Grigorow nor his teacher Massol are the first authors of " Bacillus bulgaricus ". They also find that both Grigorow's and Luerssen & Kühn's descriptions are incorrect or do not meet modern standards for establishing a species. According to Rogosa & Hansen, Grigorov's description only shows that his discovery was a gram-positive rod bacterium. On the basis of Luerssen & Kühn's description, the affiliation of their " Bacillus bulgaricus " to the genus Lactobacillus could be recognized, but not the affiliation to a specific species. Much more serious, however, was the fact that none of the strains isolated by Luerssen & Kühn at the time of the revision still existed, so that no determination of these organisms could be made afterwards. Since the Lactobacillus species L. helveticus (“ L. jugurti ”) and L. lactis also occur in yoghurt , it is unclear which of these species was actually the subject of Luerssen & Kühn's investigations. Accordingly, Rogosa & Hansen finally note: “ […] and we shall never know what this organism really was. ”(German:“ […] and we will never find out what kind of organism that was in reality. ”) Bacillus bulgaricus Luerssen & Kühn 1907 is therefore a de facto noun dubium . Most of the other above Names cannot be used as synonyms of L. delbrueckii subsp. bulgaricus , because it is also unclear which bacteria were actually involved at the time. The imprecise, inadequate descriptions from the early 20th century ultimately led to errors also appearing in subsequent works. The description of “ Lactobacillus bulgaricus (Luerssen & Kühn 1907) Holland 1920” in Bergey's Manual of Determinative Bacteriology until the 1970s is actually a mixture of the descriptions of two different bacterial taxa, namely Lactobacillus delbruecki subsp. bulgaricus and Lactobacillus helveticus var. jugurti .

Only the description by Orla-Jensen is, according to Rogosa & Hansen, detailed enough to be able to clearly identify the Bulgarian yoghurt bacterium. Above all, however, the corresponding bacterial strain (Orla-Jensens strain No. 14) was available to Rogosa & Hansen. Therefore, the Dane is now the official first author of the taxon. Its strain no. 14 has since acted under the designation ATCC 11842 as a type for the taxon and meanwhile several offshoots of this strain have been deposited in various collections of microorganisms . As early as the 1950s and 60s, papers appeared on bacterial isolates from Bulgarian, Russian, Greek, Syrian and Armenian yoghurts whose properties matched those of the type strain of “ Thermobacterium bulgaricum ”. Since, according to Rogosa & Hansen, this strain was clearly a representative of the genus Lactobacillus , they proposed the new combination “ Lactobacillus bulgaricus (Orla-Jensen)”. This was initially accepted and the taxon was listed in the Approved Lists of Bacterial Names published in 1980 as " Lactobacillus bulgaricus (Orla-Jensen 1919) Rogosa & Hansen 1971".

In 1983, the German bacteriologist Norbert Weiss and colleagues published the results of a molecular genetic analysis of the relationships between the type strains of the Lactobacillus species L. delbrueckii , L. lactis , L. leichmannii and L. bulgaricus using DNA-DNA hybridization . The matching of the DNA of these bacterial strains was between 90 and 100%, from which the authors concluded that all strains examined should be regarded as representatives of one and the same species. Since Lactobacillus delbrueckii was described as early as 1896 (as " Bacillus delbrueckii ") and thus earlier than the other taxa examined, this name had priority over the other three according to the International Rules of Bacteriological Nomenclature . Lactobacillus bulgaricus (Orla-Jensen 1919) Rogosa & Hansen 1971 was thus a younger synonym of Lactobacillus delbrueckii (Leichmann 1896) Beijerinck 1901. The genetic differences and the special way of life justified the classification of the Orla-Jensen yogurt bacterium as a subspecies. Since the results of Weiss and colleagues subsequently found general acceptance, this yoghurt bacterium is now known as the subspecies Lactobacillus delbrueckii subsp. bulgaricus (Orla-Jensen 1919) Weiss et al. 1984 run.

In Bulgaria, Grigorov's country of origin, where yoghurt has been known for much longer than in Central and Western Europe, the more recent developments in the systematics and taxonomy of the Lactobacillus species are sometimes ignored. At an international symposium on the topic of On Original Bulgarian Yogurt (English for “About the real Bulgarian yoghurt”) held in 2005, there was still talk of a “ Lactobacillus bulgaricus Grigoroff”. But in Germany, too, dairies often use the old combination of Lactobacillus bulgaricus to describe the cultures in the yoghurt products they make . It is also used sporadically in specialist literature.

Ecological importance

It has been observed that Lactobacillus species are antagonists of other types of bacteria and prevent them from growing. In the 1950s, the bacteriocins class of substances was made responsible for this. At least with Lactobacillus delbrueckii subsp. bulgaricus does not appear to be the case; instead, the production of lactic acid and hydrogen peroxide is cited as the cause. The antibacterial effect of L. delbrueckii subsp. bulgaricus affects Staphylococcus aureus and Pseudomonas species.

The combination of L. delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus is based on a symbiosis with advantages for the manufacturing process. The Lactobacillus has proteolytic enzymes with which it breaks down casein into dipeptides and amino acids that promote the growth of the Streptococcus. Its growth lowers the redox potential , which is noticeable in a reduced content of dissolved oxygen. It also produces formate , which in turn stimulates the growth of lactobacillus. The substances typical for the yoghurt aroma are also optimally formed by the symbiotic community of the two lactic acid bacteria.

The ecological importance for humans is linked to the question of whether L. delbrueckii subsp. bulgaricus has a positive effect on its health. Metschnikow is considered to be the founder of the probiotic diet . In 1908 he attributed the high life expectancy of Bulgarian farmers to their consumption of Bulgarian yoghurt and the bacteria in it. For a probiotic effect, the bacteria must survive the gastric passage in significant quantities and be detectable in the intestine. Although some Lactobacillus species belong to the autochthonous intestinal bacteria, L. delbrueckii subsp. bulgaricus does not. Nevertheless, it has been detected in the human intestine. It is therefore assumed that it is one of the allochthonous bacteria that enter the intestine, for example through the consumption of foods that contain them, and can be detected there for a certain period of time. Whether it survives the passage through the acidic environment of the stomach in yoghurt has been the subject of several studies, with contradicting results. In a 1989 study on people with lactose intolerance , bacteria from yogurt that were able to reproduce were found to be able to reproduce in the duodenum , the first section of the small intestine , after consuming yogurt . A study from 1995 came to a different conclusion, in which none of the eleven healthy test persons had consumed L. delbrueckii subsp. bulgaricus or Streptococcus salivarius subsp. thermophilus could be detected in the stomach or in the small intestine.

In a 2013 study, 100 isolates of L. delbrueckii subsp. bulgaricus made from Bulgarian yogurt tested for its ability to survive passage through the digestive tract . In an in vitro study, the K98 bacterial strain showed the greatest resistance to bile salts and a low pH value. Therefore yoghurt was made with it and tested on 20 healthy volunteers. They consumed 500 g of yogurt per day over a period of 15 days. Before and after this period, stool samples were taken from the subjects and examined. With the help of denaturing gradient gel electrophoresis (DGGE) in connection with the polymerase chain reaction, the PCR products (gene segments of the DNA of the bacterial strain) could be detected in all 20 samples after consumption. Viable cells of L. delbrueckii subsp. bulgaricus K98, on the other hand, could only be detected in the sample of one participant. In research on probiotic bacteria, L. delbrueckii subsp. bulgaricus does not play a significant role. For example, it is not mentioned in a detection method for probiotic lactobacilli presented by the specialist group Microbiota, Probiota and Host of the German Society for Hygiene and Microbiology . However, regardless of a proven probiotic effect, the consumption of yogurt or other dairy products containing L. delbrueckii subsp. bulgaricus is recommended. One of the reasons for this is that these products are better tolerated by people with lactose intolerance and that they lead to faster recovery in patients with diarrhea of different etiologies .

Industrial importance

Yogurt production

Lactobacillus delbrueckii subspecies bulgaricus is used in the production of traditional yoghurt . In dairies it is usually used in combination with Streptococcus salivarius subsp. thermophilus ( Streptococcus thermophilus ) specifically added as a starter culture to pasteurized and homogenized milk. The batch is incubated at 43–45 ° C, and the lactic acid fermentation causes the milk to acidify and curdle within a few hours. Due to the lactic acid production of the starter cultures, the pH value in the finished yoghurt drops to 4.0 to 4.2 and the food contains around 0.7–1.1% lactic acid after fermentation.

This classic yogurt is characterized by a sour taste. One with other Lactobacillus species, v. a. Acid milk product made from L. acidophilus is characterized by a milder taste. It became increasingly popular with German consumers under the names "Acidophilus milk" or "Bioghurt". According to the regulation on dairy products , this product is called "mild yoghurt". Due to the changed starter culture, this product contains predominantly dextrorotatory lactic acid , i.e. L - (+) - lactic acid (Syn .: ( S ) -lactic acid). This enantiomer is the more suitable physiological form for humans, since L -lactic acid is formed in the human metabolism and this is broken down more quickly by a specific enzyme, L - (+) - lactate dehydrogenase. The traditional yogurt with L. delbrueckii subsp. bulgaricus is still widespread in southern European countries.

More dairy products

Lactobacillus delbrueckii subsp. bulgaricus is also used in the production of curd and sour milk cheese . The beginning of the process is similar to that of yogurt production. The protein casein contained in the milk is precipitated - it coagulates, which is achieved in the production of sour milk cheese with the help of lactic acid bacteria. In the thermophilic starter cultures, Streptococcus salivarius subsp. thermophilus used in combination with L. delbrueckii subsp. bulgaricus or the subspecies L. delbrueckii subsp. lactis , in individual cases with other Lactobacillus species. The batch is incubated at 32-45 ° C.

A drink used in Asian cuisine is Kumys , another spelling is Kumiss. This sour milk product made from mare's milk also contains L. delbrueckii subsp. bulgaricus , together with certain yeasts that carry out alcoholic fermentation , which is why the drink contains, in addition to lactic acid, ethanol and carbon dioxide.

Web links

Commons : Lactobacillus  - collection of images, videos and audio files

sources

literature

  • Walter P. Hammes, Christian Hertel: The Genera Lactobacillus and Carnobacterium (Chapter 1.2.10) . In: Martin Dworkin, Stanley Falkow, Eugene Rosenberg, Karl-Heinz Schleifer, Erko Stackebrandt (eds.): The Prokaryotes. A Handbook on the Biology of Bacteria, Volume 4: Bacteria: Firmicutes, Cyanobacteria . 3. Edition. Springer-Verlag, New York 2006, ISBN 0-387-25494-3 , pp. 320-403 , doi : 10.1007 / 0-387-30744-3_10 .
  • Michael T. Madigan, John M. Martinko, Jack Parker: Brock Microbiology. German translation edited by Werner Goebel, 1st edition. Spectrum Akademischer Verlag GmbH, Heidelberg / Berlin 2000, ISBN 3-8274-0566-1 .
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  • Microorganisms in the classroom . In: Horst Bayrhuber, Eckhard R. Lucius (Hrsg.): Handbook of practical microbiology and biotechnology . 1st edition. tape 3 . Metzler-Schulbuchverlag, Hanover 1992, ISBN 3-8156-3351-6 .

Individual evidence

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  2. ↑ a b c d e f g h i j Microorganisms in the classroom . In: Horst Bayrhuber, Eckhard R. Lucius (Hrsg.): Handbook of practical microbiology and biotechnology . 1st edition. tape 3 . Metzler-Schulbuchverlag, Hanover 1992, ISBN 3-8156-3351-6 , p. 54-56 .
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  4. ^ WP Hammes, C. Hertel: The Genera Lactobacillus and Carnobacterium. (see literature ) p. 365.
  5. ↑ a b c d e f g h i j k M. Rogosa, PA Hansen: Nomenclatural Considerations of Certain Species of Lactobacillus Beijerinck: Request for an Opinion. In: International Journal of Systematic Bacteriology. Volume 21, No. 2, 1971, pp. 177-186, ISSN  1932-6203 , doi: 10.1099 / 00207713-21-2-177 .
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