The genus Streptococcus contains many diverse species with a wide array of habitats. Included in this genus are human and animal pathogens, oral commensals, intestinal commensals, and one (and only one) species, Streptococcus thermophilus, that is used in the manufacture of fermented foods. In general, streptococci are non-motile, facultative anaerobes, with an obligate homofermentative metabolism.
Since the mid-1980s,there have been several major taxonomical revisions within this genus. Some of these changes were especially relevant for food microbiologists. For the previous fifty years, the streptococci were divided into four main groups: pyogenic, enterococcus, viridans, and lactic.These groupings, which were based on the so-called Sherman scheme (published in 1937, as described in Chapter 3), were revised in subsequent years, but generally served as the primary means for organizing streptococcal species. Starting in 1984, however, two major revisions were proposed and adopted. First, the enterococcus (or enteric streptococci), which included Streptococcus faecalis, Streptococcus faecium, and Streptococcus durans, were moved to a new genus, Enterococcus. Then, in 1985, two species that had been referred to as "lactic streptococci" (Streptococcus lactis and Streptococcus cremoris) were also assigned to a new genus, Lactococcus (see above). Thus, S. thermophilus is now the only member of this
Box 2—2. Lactococcus lactis—Differences Between the Subspecies
As might be expected, Lactococcus lactis subsp. lactis and Lactococcus lactis subsp. cremoris share many phenotypic properties.In fact,these organisms are so similar that in the 1986 Bergey's Manual of Systematic Bacteriology, the lactis and cremoris strains were considered as belonging to a single species (Mundt, 1986). However, several phenotypic differences do exist, as described earlier, and these differences have traditionally been used to distinguish between the two subspecies (Table 1).As noted previously, differences also exist at the molecular level. Currently, assignment of a strain to the appropriate subspecies is based on the sequence of a 9 to 10 base pair region of the VI segment in the 16S rRNA gene. Moreover, the genomes of both organisms have now been sequenced, and other differences are likely to become apparent as comparative analyses of these genome are completed (Klaenhammer et al., 2002; Makarova et al., 2006).
Table 1. Distinguishing characteristics of dairy Lactococcus1.
Property Lactococcus lactis Lactococcus lactis subsp. lactis subsp. cremoris
Arginine hydrolysis + —
1Adapted from Teuber, 1992
Interestingly, it now appears that classification of L. lactis by both phenotypic and genotypic criteria is not so clear-cut. In fact, one research group recently proposed that L. lactis strains can be organized into one of five different groups (Kelly and Ward, 2002). Group 1 consists of strains having a L. lactis subsp. lactis genotype and a "lactis" phenotype.These strains may be considered as the common or typical L. lactis subsp. lactis. Group 2 strains also have a L. lactis subsp. lactis genotype, but have one particular distinguishing phenotypic property, namely the ability to ferment citrate and to produce diacetyl. (As noted previously, these citrate-citrate-fermenting strains had, for a long time, held subspecies status.) Strains in Group 3 have a cremoris pheno-type, but a L. lactis subsp. lactis genotype.They are not commonly found. In contrast, Group 4 strains, having a L. lactis subsp. cremoris genotype but a lactis phenotype, are frequently isolated from milk and plant sources.Their lactic-like phenotype makes it difficult to distinguish them from other lactis strains. Finally, Group 5 strains have a L. lactis subsp. cremoris genotype and phenotype.They are found almost exclusively in milk and dairy environments, and are the most common and preferred strains for manufacture of cheese.
On a practical basis, it is probably easier to establish the genotype of a particular lactococcal strain than it is to determine its phenotype (at least with regard to traits relevant to fermentations). However, it is the phenotypic properties of a given strain that dictate whether that strain will be useful in fermented foods manufacture. In some cases, properties beyond those normally considered as part of a phenotype will be relevant, such as bacteriophage sensitivity or the ability to produce good cheese flavor.A genetic method recently was devised to identify L. lactis strains to the subspecies level; it also provided a basis for determining phenotypes (Nomura et al., 2002).The method was based on the presence of glutamate decarboxylase activity in L. lactis subsp. lactis, which contains a functional gadB gene, and its absence in L. lactis subsp. cremoris, which contains insertions, deletions, and point mutations in gadB. Although the sample set was small, the size of specific DNA fragments from the gadB gene (amplified by
Box 2—2. Lactococcus lactis—Differences Between the Subspecies (Continued)
PCR) from L. lactis subsp. lactis and L. lactis subsp. cremoris strains correlated with both their genotypes and phenotypes. Ultimately, this and other similar procedures could provide a relatively rapid and efficient means to screen lactococcal strains with the desired phenotype.
Bolotin,A., P.Wincker, S. Mauger, O.Jaillon, K. Malarme, J.Weissenbach, S.D. Ehrlich, and A. Sorokin. 2001.The complete genome sequence of the lactic acid bacterium Lactococcus lactis ssp. lactis IL1403. Genome Res. 11:731-753.
Goffeau,A., B. G. Barrell, H. Bussey, R.W. Davis, B. Dujon, H. Feldmann, F. Galibert, J. D. Hoheisel, C.Jacq, M. Johnston, E.J. Louis, H.W. Mewes,Y. Murakami, P. Philippsen, H.Tettelin, and S. G. Oliver. 1996. Life with 6000 genes. Science 274:546-567. Kelly, W., and L. Ward. 2002. Genotypic vs. phenotypic biodiversity in Lactococcus lactis. Microbiol. 148:3332-3333.
Klaenhammer et al. (36 other authors). 2002. Discovering lactic acid bacteria by genomics. Antonie van Leeuwenhoek 82:29-58.
Makarova, K.,Y.Wolf, et al., (and 43 other authors) 2006. Comparative genomics of the lactic acid bacteria.
2006. Proc. Nat.Acad. Sci. In Press. Mundt, J.0.1986. Lactic acid streptococci. In Sneath, P.H.A., N.S. Mair, M.E. Sharpe,J.G. Holt. Bergey's Manual of Systematic Bacteriology, Volume 2.Williams and Wilkins, Baltimore, Maryland. 1065-1071. Nomura,M., M. Kobayashi, and T. Okamoto. 2002. Rapid PCR-based method which can determine both phenotype and genotype of Lactococcus lactis subspecies.Appl. Environ.Microbiol. 68:2209-2213. Teuber, M. 1992.The genus Lactococcus. In B.J.B.Wood and W.H. Holzapfel, ed. The Lactic Acid Bacteria, Volume 2, The genera of lactic acid bacteria. Blackie Academic and Professional pp 173—234.
genus used in food fermentations (mainly yogurt and cheese).
That is not to say that this organism has been exempt from taxonomical considerations. Originally, this species was part of the Sherman viri-dans group (which included oral streptococci), and in the 1986 edition of Bergey's manual, it was listed as an "Other Streptococci" Its physiological, structural, and genetic similarities to Streptococcus salivarius led to the recommendation in 1984 that it be reclassified as a subspecies of S. salivarius. Although this name change was indeed adopted (and "Streptococcus salivarius subsp. thermophilus" is sometimes still seen in the literature), subsequent DNADNA homology studies eventually led to the restoration of its original name.Thus, labels on yogurt products can justifiably claim the presence of Streptococcus thermophilus rather than the less appetizing Streptococcus salivarius.
In several respects, S.thermophilus is not that different from the mesophilic dairy lactococci (L. lactis subsp. lactis and L. lactis subsp. cre-moris), as is evident by their close phylogenetic position (Figure 2-2). Like the lactococci, S. ther-mophilus is highly adapted to a milk environment in that it ferments lactose rapidly and produces lactic acid in homolactic fashion. The route by which lactose is metabolized by S. ther-mophilus, however, is quite different from how L. lactis ferments this sugar (discussed later). Also, S. thermophilus has a higher temperature optima (40°C to 42°C), a higher maximum growth temperature (52°C), and a higher thermal tolerance (above 60°C). Its nutritional requirements are somewhat more demanding than lactococci, in that S. thermophilus is weakly pro-teolytic and, therefore, needs pre-formed amino acids. Salt tolerance, bile sensitivity, and a limited metabolic diversity are also characteristic of S. thermophilus. In fact, the statement made by Sherman in 1937 that S. thermophilus "is marked more by the things which it cannot do than by its positive actions" describes this organism quite well. Finally, in contrast to lactococci, S. thermophilus strains contain few plasmids. When plasmids are found, they are generally small and cryptic (i.e.,having no known function).
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