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.
Weimer, Utah State University
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 (912).
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 (1314). 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 (1516). 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 strains genetic secrets
has been made (1923), 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.
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J.-J. Godon, D. Ehrlich, and P. Renault. 1994. Microbiology 140:3053-3060.
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P., Daveran-Mingot, M. L., and Ritzenthaler, P. 2000. J. Bacteriol.
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P., M. Lautier, L. van den Berghe, M. J. Gasson, and P. Ritzenthaler.
1995. J. Bacteriol. 177(10):2840-2850.
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W. Ludwig, and K.-H. Schleifer. 1997. System. Appl. Microbiol. 20:216-221.
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J. A. E. Farrow, and B. A. Phillips. 1981. Zbl. Bakt. Hyg., I Abt.
Orig. C 2:151-165.
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E. 1988. Biochemie 70:519-522.
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W. E. Sandine, and S. Giovannoni. 1991. Appl. Environ. Microbiol.
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Delorme, S. D. Ehrlich, and P. Renault. 1992. Appl. Environ. Microbiol.
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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.
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P. Vos, H. de Haard, and I. Boerrigter. 1989. Gene 85:169-176.
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M., J. P. Petzel, E. Pasalodos, K. A. Baldwin, and L. L. McKay. 1991.
Environ. Microbiol. 57:539-548.
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K. M. Polzin, and L. L. McKay. 1991. J. Bacteriol. 173:7573-7581.
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S. Mauger, K. Malarme, S. D. Ehrlich, and A. Sorokin. 1999. Antonie