Lactococcus lactis characteristics, morphology, habitat

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Robert Johnston

Lactococcus lactis It is a Gram-positive, non-mobile, catalase-negative, homofermentative bacterium. It belongs to the group of Lactic Acid Bacteria (LAB). The metabolism of bacteria in this group leads to the production of lactic acid.

In appropriate conditions, L. lactis It can rapidly lower the pH and redox potential of the culture medium. Due to this, it is used in the manufacture of fermented dairy products..

Lactococcus lactis. Scanning electron micrograph, Magnification 20000X. Done by: Joseph A. Heintz, University of Wisconsin-Madison. Taken and edited from https://bioinfo.bact.wisc.edu/themicrobialworld/Lactococcus.html

The species Lactococcus lactis it comprises four subspecies and a biovar. Despite its wide use in food production, it is associated with a variety of diseases.

The predisposition of patients with a weakened immune system and long-term exposure to unpasteurized dairy products are the main causes of infection by this bacterium..

Article index

  • 1 Features
  • 2 Taxonomy
  • 3 Morphology
  • 4 Habitat
  • 5 Benefits
  • 6 Diseases
    • 6.1 In humans
    • 6.2 In animals
  • 7 References

Characteristics

Lactococcus lactis it is a homofermentative bacterium that produces only L-lactic acid when fermenting glucose. It does not form spores. Grows at 10 ° C, but not at 45 ° C.

It grows in media with 4% (w / v) NaCl, except L. lactis subsp. cremoris, which only supports salt concentrations of 2% (p / v).

Some of its strains are capable of excreting extracellular polysaccharide substances. All subspecies are facultative anaerobic, non-hemolytic, catalase negative, and contain phosphatidylglycerol and cardiolipin.

Taxonomy

Lactococcus lactis it is the type species of the genus. Previously it was contained within Streptococcus (lactis) of group N of the Lancefield Classification. It belongs to the phylum Firmicutes, order Lactobacillales, family Streptococcaceae.

Four subspecies and a biovar are currently recognized., L. lactis subsp. lactis biovar diacetylactis. This biovar differs from L. lactis subsp. lactis Y cremoris for its ability to use citrate with the production of diacetyl.

However, as this ability is mediated by plasmids, it is an unstable characteristic, for which the bacterium cannot be recognized as a subspecies..

Morphology

Lactococcus lactis it is pleomorphic, has the shape of a coconut or ovoid, it can grow solitary, in pairs or in chains. In the case of being chain-shaped, cells can become rod-shaped. It does not have a flagellum or fimbriae. They have numerous plasmids that can vary in size from 1 kb (Kilobase) to more than 100 kb.

Lactococcus lactis it is characterized by numerous phenotypic variations, it is sometimes difficult to recognize the differences that exist between the subspecies that comprise it.

Lactococcus lactis subsp. lactis biovar diacetylactis, for example, according to some authors, it produces ammonia from arginine. However, other authors argue the opposite, pointing out that this characteristic corresponds to Lactococcus lactis subsp. cremoris.

Habitat

Despite the common association of Lactococcus lactis With dairy products, the bacteria were originally isolated from plants. Some authors believe that in plants it is in a dormant state and is activated when entering the digestive tract of ruminants after ingestion..

In plants it can grow as an epiphyte and as an endophyte. It can grow on different parts of plants, including the stems of Eucalyptus, corn, peas and sugarcane leaves.

Additionally, it has been isolated from animals and from the soil in livestock farms. It has also been found in cheese-producing plants, wastewater from forest industries, and surface marine and swimming pool waters..

Lactococcus lactis. Point colonies, semitransparent. Culture incubated for 48 hours in a micro-aerobic atmosphere. Taken and edited from http://atlas.sund.ku.dk/microatlas/food/bacteria/Lactococcus_lactis/

Profits

Lactococcus lactis It is used in the fermentation of dairy products, such as cheese and yogurt, and of vegetables to obtain sauerkraut and the like. Bacteria impart flavor to food and produce acid that helps preserve it.

It also produces probiotics and bacteriocins. The latter are biologically active peptides or protein complexes..

Among the bacteriocins produced by this bacterium is nisin, which is active against gram-positive bacteria, bacterial spores of clostridia and bacilli, pathogenic streptococci, and staphylococci..

Lactoccocus lactis it has also been genetically modified to produce other compounds of medical and industrial utility.

Diseases

Lactococcus lactis it is considered a low virulence opportunistic pathogen. However, its incidence in humans and animals has been increasing in recent years..

In the case of humans, a weakened immune system and exposures to, or consumption of, unpasteurized dairy products are risk factors.

In humans

Lactococcus lactis subsp. cremoris has been reported as a causative agent of bacteremia, acute diarrhea, bacterial endocarditis, septicemia, liver and brain abscesses, necrotizing pneumonitis, purulent pneumonitis, septic arthritis, deep neck infection, catheter infection of the bloodstream, peritonitis, empyema, ascending cholangitis and canaliculitis.

Lactococcus lactis subsp. lactis It has also been isolated from clinical samples of blood, skin lesions, and urine. There are some reports that indicate the involvement of Lactococcus lactis subsp. lactis in emergency situations such as septic arthritis, peritonitis, and osteomyelitis.

Treatment

There is no defined standard treatment for infections due to Lactococcus lactis subsp. cremoris. Susceptibility tests have been the basis for defining the treatments in each case..

Penicillin, third-generation cephalosporin, cefotaxime, and coamoxiclav have been used based on these criteria. Cefotaxime showed poor results in treating a liver abscess, perhaps due to complications from empyema.

As long as there is no specific guideline, antimicrobial therapy must comply with the susceptibility of the pathogen isolated from the cultures. Vancomycin has been effective in most cases.

An alternative antimicrobial therapy that has also been successful consists of ceftriaxone and gentamicin for 10 days, followed by intravenous ceftriaxone for 6 weeks..

In animals

Lactococcus lactis subsp. lactis has been linked to a case of mass death of waterfowl in Spain. The event, which occurred in 1998, affected more than 3,000 birds (0.6% of the total population of waterfowl in the area).

The most affected species were coots, shovels and wild ducks. The symptoms were: drooping wings, slowness and respiratory distress. Postmortem exams showed mild lung congestion.

This subspecies has also caused mortalities of between 70 and 100% in hybrid sturgeons under cultivation conditions. Sick fish showed anorexia, pale body color, and reddish spots on the abdomen.

Histopathological examinations revealed multiple massive necrotic, hemorrhagic or coagulative foci in the liver and spleen. In the Malay shrimp Macrobrachium rosenbergii has been associated with white muscle disease.

References

  1. S. Hadjisymeou, P. Loizou, P. Kothari (2013). Lactococcus lactis cremoris infection: not rare anymore ?. BMJ Case Reports.
  2. D. Samaržija, N. Antunac, J.L. Havranek (2001). Taxonomy, physiology and growth of Lactococcus lactis: a review. Mljekarstvo.
  3. J. Goyache, A.I. Vela, A. Gibello, M.M. Blanco, V. Briones, S. González, S. Téllez, C. Ballesteros, L. Domínguez, J.F. Fernández-Garayzábal (2001) Lactococcus lactis subsp. lactis Infection in Waterfowl: First Confirmation in Animals. Emerging infectious diseases.
  4. M.Z. Nuryshev, L.G. Stoyanova, A.I. Netrusov (2016). New Probiotic Culture of Lactococcus lactis ssp. lactis: Effective Opportunities and Prospects. Journal of Microbial and Biochemical Technology.
  5. G. Georgountzos, C. Michopoulos, C. Grivokostopoulos, M. Kolosaka, N. Vlassopoulou, A. Lekkou (2018). Infective Endocarditis in a Young Adult due to Lactococcus lactis: A Case Report and Review of the Literature. Case Reports in Medicine.
  6. H.S. Kim, D.W. Park, Y.K. Youn, Y.M. Jo, J.Y. Kim, J.Y. Song, J.-W. Sohn, H.J. Cheong, W.J. Kim, M.J. Kim, W.S. Choi (2010). Liver Abscess and Empyema due to Lactococcus lactis cremoris.  Journal of Korean Medical Science.

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