Lactococcus
Lactococcus lactis ( Gram stain)
Lactococcus is the name of a genus of gram-positive, spherical bacteria from the family of Streptococcaceae. His name is " Germanized " as Laktokokkus (plural: lactococci ) written. Lactococcus belongs together with other species of bacteria to the order of the lactic acid bacteria, they all produce lactic acid by fermentation.
Lactococcus species are important for the food industry. They are used for the preparation of acidified milk products. The people themselves, they usually add to any harm, they are non-pathogenic.
The species of the genus Lactococcus were attributed to 1986 of the genus Streptococcus. The division of the genus Streptococcus in the genera Streptococcus ( more pathogenic species) and Lactococcus ( more " useful " species) was carried out in 1986. Often present species of the genus Lactococcus genus Streptococcus are still allocated. .
- 3.1 Outer systematics
- 3.2 Internal systematics
- 3.3 Etymology
- 5.1 Literature
- 5.2 Notes and references
Features
Appearance
The members of the genus are Gram-positive bacteria, they do not form endospores and are not Überdauerungsformen as the active movement in most cases capable. The cells of Lactococcus are round ( cocci ), and less than 2 micrometers ( microns) in size. The cells are arranged into chains. Since such an arrangement is referred to as streptococci, it was also applied to lactococci.
On fixed -carbohydrate media containing the cells grow into colonies approach, these are usually colorless to white in Lactococcus and typically quite small, its diameter is less than 1 mm.
Growth and metabolism
As a representative of lactic acid bacteria grow anaerobically lactococci, and they are aero- tolerant, that is, they grow in the presence of oxygen, but do not require oxygen for their metabolism. They are catalase - negative and oxidase - negative. However, they are able to form cytochromes when cultured on nutrient media containing hemins or blood. In this case, they then exhibit a positive reaction in the oxidase test. Further, a typical indicator for lactococci, the need for complex growth factors and amino acids in the culture. They can be cultured on MRS- agar, a nutrient medium, which allows the growth of lactic acid bacteria demanding.
The temperature optima of growth and reproduction are for most species in the range of 25-30 ° C, thus Lactococcus is one of the mesophilic organisms. It grows even at 10 ° C but not at 45 ° C, it is used as a discriminating criterion to the more thermophilic strains of Streptococcus. Another distinguishing feature is that Lactococcus species contains on a agar nutrient medium, the blood - a so-called blood agar - show no hemolysis as can be observed in Streptococcus species alpha or beta hemolysis. Lactococcus species survive 30 minutes heating at 60 ° C and tolerate a high concentration of bile ( usually ox bile ) in the culture medium.
Lactic acid fermentation
Can lactococci in a fermentation utilize various carbohydrates for energy. Features of the fermentation ( fermentation ), in that the substrates are degraded without oxygen. Typical of lactic acid bacteria in the fermentation product, the lactic acid or lactate ( the anion of the lactic acid ), hence this pathway is referred to as lactic acid fermentation. A distinction is made between homofermentative and heterofermentative species. Homofermentative species produce from glucose by fermentation almost exclusively lactic acid, while heterofermentative species besides lactic acid produce a significant proportion of other end products, mostly ethanol and carbon dioxide, and sometimes acetic acid. The heterofermentative usually lack the enzyme aldolase.
The members of the genus Lactococcus have the enzyme aldolase and all belong to the homofermentative species. The homofermentative lactic acid bacteria Lactobacillus delbrueckii subspecies bulgaricus also heard, the typical pathway is explained there. Also present in Lactococcus species enzyme lactate dehydrogenase is stereospecific, unlike Lactobacillus delbrueckii subsp. bulgaricus is formed in this reaction almost exclusively L ( ) -lactate ( Syn: (S) -lactate ). The proportion of L- ( )- lactic acid is 92 to 99 %. This enantiomer is known as dextrorotatory lactic acid.
In addition to glucose also other carbohydrates may be utilized, including ribose, lactose (milk sugar), maltose, sucrose and the sugar alcohol mannitol. However, this does not apply to all species of the genus.
Genetics
The genome of several strains of bacteria from the genus Lactococcus has been completely sequenced, this is especially true of the way to Lactococcus lactis. The size of the genome of all previously studied representative is 2370-2810 kilobase pairs (kb ), which is only 55 % of the genome size of Escherichia coli. There are about 2300-2600 proteins annotated. The small genome size is a further indication of their adaptation to the habitat milk. There are many complex growth factors such as amino acids and vitamins are present, that the bacteria had lost the ability to synthesize a number of metabolites over time.
Lactococcus is counted among the gram-positive bacteria with low GC content ( the proportion of nucleobases guanine and cytosine) in the bacterial DNA. The GC content is from 38 to 41 mole percent, which is comparable to that of related genera Streptococcus or Leuconostoc.
Pathogenicity
Lactococcus is considered non-pathogenic, most of Lactococcus species by the Biological Agents in connection with the TRBA (Technical Rules for Biological Agents ) assigned 466 Risk Group 1. An exception is Lactococcus garvieae, which is assigned to risk group 2. However, we find here the restriction that he was only detected in single cases as pathogens and this in immunocompromised people to continue the identification of the species is not often reliable.
Evidence
A more general assessment of whether it is lactococci, is possible by the light- microscopic appearance after Gram staining. Through an oxidation - fermentation test ( OF test ) can be checked whether they form both aerobically and anaerobically acid from glucose, gas formation is not observed. The catalase and oxidase test is negative, further biochemical tests to distinguish the Lactococcus species among themselves contain typical detection reactions from a "colorful series". It is checked that carbohydrates and other substrates they can utilize and which enzymes they have, here is the proof of the arginine dihydrolase (ADH ) is a distinguishing feature. The growth in a culture medium in a proportion of 4% sodium chloride (NaCl) is checked. Likewise, the influence of various antibiotics is examined to see if they react to it are sensitive or resistant. To distinguish the species Lactococcus thereby inter alia, the antibiotic cefuroxime, tetracycline, erythromycin and polymyxin B are used.
Occurrence
The types of Lactococcus occur mainly in milk and dairy products. They are used as starter cultures in the production of many dairy products, due to the lactic acid produced by them are also referred to as lactic acid. Furthermore, they are located in or on plants, and are involved in the production of silage. This is also reflected in the name of the species Lactococcus plantarum, planta from Latin means "plant" plantarum means "the plant " ( genitive plural).
Since the genus was established in 1986, species and subspecies are still added ( subspecies ) that occur in other habitats: So in 2011 from the intestinal mucosa of the brown trout and rainbow trout Lactococcus lactis subsp. tructae isolated. Already in 1990, also from the family Salmonidae ( salmonids ), a new lactic acid bacterium isolated and designated Lactococcus piscium, piscis from Latin means "fish", piscium means " fish " ( genitive plural). Lactococcus lactis subsp. hordniae is named after the leafhopper Hordnia circellata from which it was isolated and Lactococcus chungangensis was found in 2008 in the foam of the activated sludge of a sewage plant.
Lactococcus fujiensis turn is of plant origin and in 2011 in Japan isolated from the outer leaves of Chinese cabbage, while Lactococcus taiwanensis was found in 2013 in Taiwan, one of the main ingredients for Pobuzihi, a traditional fermented food in Taiwan.
System
Outer systematics
Lactococcus is closely related to the genus Streptococcus, both of which belong to the family Streptococcaceae. Previously they were - to the genus Streptococcus - together with the enterococci of faecal origin. To distinguish the common pathogenic species of the types used for human -hazardous, and partially also in the food industry, first carried out in a formal separation of the genera Streptococcus, Lactococcus, and Enterococcus. Enterococcus belongs to the family of Enterococcaceae, the order Lactobacillales is with other families are also referred to as lactic acid bacteria.
The separation of the genera Streptococcus and Lactococcus appears in part arbitrary. The investigations of Karl -Heinz Schleifer et al 1985 chemotaxonomic characteristics were used. In this case, some Streptococcus and Lactobacillus species have been studied and the relationship shown by the similarity of the composition of the bacterial cell wall, particularly anchored there teichoic acids. The existing lipids and fatty acids and menaquinones show a similar pattern. This led to the assignment of the studied bacteria in the genus Lactococcus newly described. However, even the old names still find use and have been a so-called Basonym (comparable to the basionym in the taxonomy of plants), such as Streptococcus lactis is the Basonym for Lactococcus lactis, as it was originally assigned to the genus Streptococcus.
Inside systematics
More than 200 tribes are counted in the genus Lactococcus, some are clearly assigned to the species contained therein, some are not. Currently (2013 ) 12 Lactococcus species and subspecies from the Leibniz Institute DSMZ - German Collection of Microorganisms listed and Cell Cultures GmbH in the Prokaryotic Nomenclature up-to -date ( " Prokaryotic Nomenclature up to date "). This compilation includes all according to the Bacteriological Code validly published names and takes into account the validation list of the International Journal of Systematic and Evolutionary Microbiology. Lactococcus lactis is the type species.
- Lactococcus chungangensis Cho et al. 2008, sp. nov.
- Lactococcus fujiensis Cai et al. 2011, sp. nov.
- Lactococcus garvieae ( Collins et al., 1984) Schleifer et al. 1986 comb. nov. (previously Streptococcus garvieae )
- Lactococcus lactis (Lister 1873) Schleifer et al. 1986 comb. nov. (formerly Streptococcus lactis ) Lactococcus lactis subsp. cremoris ( Orla -Jensen 1919) Schleifer et al. 1986 comb. nov. (previously Streptococcus cremoris )
- Lactococcus lactis subsp. hordniae ( Latorre - Guzman et al., 1977) Schleifer et al. 1986 comb. nov. (previously Lactobacillus hordniae )
- Lactococcus lactis subsp. lactis (Lister 1873) Schleifer et al. 1986 comb. nov. (formerly Streptococcus lactis )
- Lactococcus lactis subsp. tructae Pérez et al. 2011 subsp. nov.
The bacterial strains, formerly Streptococcus lactis subsp. (also known as Streptococcus or Streptococcus diacetylactis diacetilactis ) diacetilactis as well as the bacterial strains that were previously assigned to Lactobacillus xylosus, now belong to the subspecies Lactococcus lactis subsp. lactis. The assignment of Lactococcus garvieae is not yet completely clarified. He does not seem to quite match the other lactococci, because it may trigger diseases among others. 1996, the species associated strains were isolated from water buffalo, which were suffering from mastitis, without showing symptoms. They were Genotypic and phenotypic studies, however, the type Enterococcus seriolicida Kusuda et al. Allocated in 1991.
In the taxonomic classification of new bacterial strains increasingly genetic studies are performed by examining the DNA sequences in the genome by means of DNA-DNA hybridization, and additionally, the 16S rRNA gene in bacteria, a typical representative for prokaryotes, the ribosomal RNA. The analysis of the 16S rRNA sequences have at Lactococcus taiwanensis a match of 98.22 to 98.82 % of the Lactococcus lactis subspecies on the comparison of the DNA sequences is a compliance of 9.7 to 15.2 %, these differences lead means that it is referred to as a separate species. For Lactococcus fujiensis results from 16S rRNA sequences, a match of 94.4 % to Lactococcus lactis and Lactococcus garvieae, during the comparison of the DNA sequences is a match of less than 20.2% for the type strains of other species is Lactococcus.
Etymology
The genus name refers to the occurrence and appearance of the cells lactis from the Latin stands for " milk " and κόκκος kókkos is ancient Greek for " berry ", Lactococcus is therefore a kokkenförmiges bacteria in the milk.
Industrial importance
The lactic acid fermentation is used in the food industry, especially in the production of milk products such as cheese and yogurt. Without lactobacilli, there would be virtually no dairy products. In the dairy are pure cultures of Lactococcus species in the production of sour milk, sour cream products such as fresh cream, cultured butter, buttermilk, cottage cheese and other cheese products used by being added as starter cultures. Curdled milk and cream contain 0.5-0.9 % lactic acid by the processing performed by Lactococcus lactic acid fermentation. In the sour cream butter is the formation of diacetyl by the Lactococcus species is important. Diacetyl is desirable as a flavoring agent with characteristic odor and taste of butter.
In cheese production Lactococcus species are used as so-called mesophilic lactic acid and then incubated the milk at 20-30 ° C. For the Coagulation an addition of rennet can be made. In addition to the Lactococcus species are mostly other lactic acid bacteria mitbeteiligt.