Purpose: Assess genetic diversity amongst food chain-derived Lm using MLST and PFGE with respect to mutability and cold growth phenotypes.
Methods: Lm isolates (n = 54) were analyzed by MLST, PFGE, and serotyping. InlA genotypes were determined by sequencing, mutability assessed by plating on agar containing rifampicin (100 μg/ml), and cold adaption determined at 4°C following a downshift from 37°C in BHI broth.
Results: Thirteen sequence types (STs) and 36 pulsotypes were observed, with one novel ST identified. Strains discriminated by PFGE and serotyping were also found to possess different STs. Distinct STs were observed among lineage I (LI) and LII isolates. ST321 (n = 10) isolates possessed type 3 (a.a. 700) premature stop codon inlA mutations, whilst type 4 (a.a. 8) and 11 (a.a. 685) were seen in 67% and 22% of ST9 (n = 9) isolates, respectively. Ten STs were observed amongst Lm encoding full-length InlA. LI isolates were more mutable than LII (P = 0.002), and no correlation with fast, intermediate, and slow cold adaptors was observed amongst STs.
Significance: MLST revealed diversity amongst food chain strains, generally agreeing with serotyping and PFGE data. MLST data, however, allowed a more comprehensive assessment of phylogenetic relationships amongst strains, including an association of some STs with inlA genotypes. Also, we showed that STs in LI isolates commonly linked to listeriosis more readily acquire point mutations compared to LII. This suggests positive selection plays a role in the maintenance of wild-type inlA sequences in LI strains.