lactis isolates. Although housekeeping genes evolve slowly, however, we still consider that the eight housekeeping genes selected provide sufficient discriminatory power for typing. In our genetic analysis, the eight housekeeping loci had a very low d N/d S ratio (<1), implying strong purifying selection. This was particularly the case in groEL where five substitutions were synonymous and the amino acid composition did not change. The d N /d S ratio was close to zero and similar to that seen by Madslien et al., which supports our estimation that the housekeeping loci are all under stabilizing selection . Leuconostoc lactis isolates
are important industrially in the production of fermented foods. However, Y-27632 concentration their population GSK2126458 structure has not been investigated fully before. We used linkage disequilibrium expressed as the index of association (I A) by the equation, I A = VO/VE - 1 (VO, observed variance; VE, expected variance) to investigate population
structure. This index of association is a generalised measure of linkage disequilibrium; does not rely on the number of loci analysed; has an expected value of zero if there is no association between loci, i.e. alleles are in linkage equilibrium (free recombination) [34, 35]; and was pioneered to describe population structure in Hordeum spontaneum. In our study, the value of I A and I A S for eight loci were calculated as 1.8 and 0.4264 (p = 0.000), respectively. These high values are indicative of a strong clonal population and similar to reports for other bacteria. For example, in Lb. plantarum, where seven selected housekeeping genes were analysed and assigned to 17 different STs, the I A S value was 0.444 . In Bacillus licheniformis, where six housekeeping loci were analysed from 53 diverse isolates, the value of I A S was 0.4365 . These results are similar to our study on L. lactis and, therefore, support our hypothesis that these are clonal populations and that allelic selection is close
to linkage disequilibrium. In general, Leuconostoc species are used as starter cultures for dairy fermentations. All isolates initiate lactose fermentation and lactic acid production and here we have shown that some essential housekeeping genes are highly conserved. However, the value of I A S and stiripentol the number of unique STs reflect the genetic diversity amongst isolates that have each adapted to specialised environments during their evolution. Similar results have been reported for other LAB isolated from dairy products; for example 197 isolates of Lactococcus lactis isolated from homemade yogurt were assigned to 72 different STs and their I A S value was 0.3038 . Uniformly, a clonal structure was also found in Streptococcus thermophilus, where eight housekeeping loci were analysed from 26 isolates from different dairy products .