L Lactococcus lactis subsp hordniae

Figure 2-3. Phylogeny of Lactococcus based on 16S rRNA sequence analysis.

First, they are readily isolated from raw milk; in fact, it is difficult to find L. lactis subsp. cremoris anywhere but milk. Second, both species grow rapidly in milk, producing lactic acid and lowering the pH to below 4.5.Thus, they generally out-compete most potential competitors. Finally, the genes required for growth in milk are located on plasmids (extrachromosomal DNA), indicating they were acquired recently (relatively, speaking).

Specific plasmid-borne genes in lactococci encode for proteins involved in lactose transport and metabolism and casein hydrolysis and utilization. There is considerable selective pressure, therefore, for milk-borne strains to retain these plasmids, since plasmid-cured derivatives grow poorly in milk. Indeed, some lactococcal plasmids are readily exchanged among different strains (via conjugal transfer), and in other cases, plasmid DNA can integrate within the chromosome, resulting in stable, highly adapted derivative strains. Plasmids, however, are common in lactococci, even in strains isolated from non-dairy sources.

Although L. lactis subsp. lactis and L. lactis subsp. cremoris differ in just a few seemingly minor physiological respects, at least two of these differences are significant during milk fermentations. For example, the highest temperature at which most strains of L. lactis subsp. lactis are able to grow is 40°C, whereas most L. lactis subsp. cremoris strains do not grow above 38°C. In addition, L. lactis subsp. lactis has greater tolerance to salt (up to 4%) than does L. lactis subsp. cremoris.

What makes these differences relevant is that both temperature and salt are among the primary means by which the activity of the starter culture is controlled during cheese manufacture. Moreover, L. lactis subsp. cre-moris is generally considered to be, overall, less tolerant to the environmental stresses encountered during both culture production and during dairy fermentations. Thus, if L. lactis subsp. cremoris is used as a culture for cheese making (and it is generally accepted that this organism makes better quality products), then temperature and salt concentrations must be adjusted to account for this organism's particular growth requirements.

Complicating this discussion on the differences between these two organisms, it has recently been noted that there are strains of L. lactis subsp. lactis that have a "lactis" genotype, but a "cremoris-like" phenotype. Likewise, there are strains of L. lactis subsp. cre-moris that have a "cremoris" genotype, but a "lactis-like" phenotype (Box 2-2). The use of genomics (discussed later in the chapter), will help to determine the genetic basis for these industrially relevant differences.

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