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Lactococcus lactis cremoris SK11
   
   
 

Photo: Bart Weimer, Utah State University
Lactococci are mesophilic lactic acid bacteria that were first isolated from green plants. However, today they are used extensively in food fermentations, which represent about 20% of the total economic value of fermented foods produced throughout the world. In 1998, the economic value of American type cheeses alone was over $3.3 billion (1). This group of bacteria, previously designated the lactic streptococci (Streptococcus lactis subsp. lactis or S. lactis subsp. cremoris) was placed in this new taxon in 1985 by Schleifer (2). Lactococci gained notable interest because many of their functions important for successful fermentations are linked to plasmid DNA (3), which are commonly exchanged between strains via conjugation (3, 4) and with the chromosome by IS elements (5). Presumably, these exchanges and rearrangements mediate rapid strain adaptation and evolution but add to the instability of important metabolic functions.

These bacteria are selected for use in fermentations based on their metabolic stability, their resistance to bacteriophage, and their ability to produce unique compounds – often from amino acid catabolism. The study of their physiology in adverse conditions such as low pH and high NaCl indicates that they adapt to these environments quickly and change their metabolism based on carbohydrate starvation (6). Recent genome studies and physical maps indicate that bacterial genomes are very dynamic (5). These rearrangements are mediated by IS elements and result in gene duplication, translocation, inversion, deletion and horizontal transfer events. For example, an inversion encompassing approximately one-half of the chromosome of L. lactis ML3 occurred by homologous recombination between two copies of IS905 (7). The response to these stresses, particularly to exposure to bacteriophage (8), highlights the plasticity of the genome (9–12). Establishing the links between environmental conditions, genome organization, and cellular physiology in lactococci will provide new and exciting information about the molecular mechanisms of these important bacteria. Advances that define the fundamental knowledge of the genetics, molecular biology, physiology, and biochemistry of lactococci will provide new insights and applications for these bacteria.

The importance of lactococci, specifically L. lactis subsp. cremoris, is demonstrated by its continual use in food fermentations (13–14). L. lactis subsp. cremoris strains are preferred over L. lactis subsp. lactis strains because of their superior contribution to product flavor via unique metabolic mechanisms (15–16). The DNA sequence divergence between the subspecies is estimated to be between 20 and 30% (17). Of the many lactococcal strains used, L. lactis subsp. cremoris SK11 is recognized for the beneficial flavor compounds it produces (18). Although some progress in unlocking this strain’s genetic secrets has been made (19–23), much more can be accomplished by using a genomics/proteomics approach. With this genome sequence, it will be possible to confirm the metabolic and evolutionary differences between subspecies of lactococci in order to identify the important characteristics that define this genus.

References:

  1. Cheese Facts. 1999. National Cheese Institute. Washington D.C.
  2. Schleifer, K.-H. 1987. FEMS Microbiol. Rev. 46:201-203.
  3. McKay, L.L. 1985. In S.E. Gilliland (ed.). p. 159-174. Bacterial Starter Cultures for Food. CRC Press, Inc., Boca Raton, Florida.
  4. Dunny, G., and L. L. McKay. 1999. Antonie van Leeuwenhoek 76:77–88.
  5. Hughes, D. 2000. Genome Biology 1:reviews0006.1–0006.8.
  6. Stuart, M., L.–S. Chou, and B. C. Weimer. 1998. Appl. Environ. Microbiol. 65:665–673.
  7. Daveran-Mingot M. L., N. Campo, P. Ritzenthaler, and P. Le Bourgeois. 1998. J Bacteriol 180:4834.
  8. Forde, A., and D. Fitzgerald. 1999. Antonie van Leeuwenhoek 76:89–113.
  9. Davidson, B., N. Kordis, M. Dobos, and A. Hillier. 1996. Antonie von Leeuwenhoek 70:161–183.
  10. Delorme, C., J.-J. Godon, D. Ehrlich, and P. Renault. 1994. Microbiology 140:3053-3060.
  11. Le Bourgeois, P., Daveran-Mingot, M. L., and Ritzenthaler, P. 2000. J. Bacteriol. 182: 2481-2491.
  12. Le Bourgeois, P., M. Lautier, L. van den Berghe, M. J. Gasson, and P. Ritzenthaler. 1995. J. Bacteriol. 177(10):2840-2850.
  13. Beimfohr, C., W. Ludwig, and K.-H. Schleifer. 1997. System. Appl. Microbiol. 20:216-221.
  14. Garvie, E. I., J. A. E. Farrow, and B. A. Phillips. 1981. Zbl. Bakt. Hyg., I Abt. Orig. C 2:151-165.
  15. Sandine, W. E. 1988. Biochemie 70:519-522.
  16. Salama, M., W. E. Sandine, and S. Giovannoni. 1991. Appl. Environ. Microbiol. 57:1313-
    1318.
  17. Godon, J., C. Delorme, S. D. Ehrlich, and P. Renault. 1992. Appl. Environ. Microbiol.
    58:4045-4047.
  18. Lawrence, R. C., T. D. Thomas, and B. E. Terzaghi. 1976. J. Dairy Res. 43:141-193.
    19. de Vos, W. M, H. M. Underwood, and F. L. Davies. 1984. FEMS Microbiol. Lett. 23:175-
    178.
  19. de Vos, W. M., P. Vos, H. de Haard, and I. Boerrigter. 1989. Gene 85:169-176.
  20. Feirtag, J. M., J. P. Petzel, E. Pasalodos, K. A. Baldwin, and L. L. McKay. 1991. Appl.
    Environ. Microbiol. 57:539-548.
  21. Horng, J. S., K. M. Polzin, and L. L. McKay. 1991. J. Bacteriol. 173:7573-7581.
  22. Bolotin, A., S. Mauger, K. Malarme, S. D. Ehrlich, and A. Sorokin. 1999. Antonie van
    Leeuwenhoek 76:27-76.