Purpose: The objectives of this study were to compare the genes found on different Lm plasmids and determine if plasmid harborage is associated with enhanced resistance to food-related stresses.
Methods: Whole genome sequencing was performed on 166 Lm strains and the assemblies were screened for the plasmid replicon gene, repA. All sequences of repA positive strains were aligned to the closed genome of Lm EDG-e using Mauve. Contigs not aligning to EGD-e were excluded as plasmid-associated if they contained chromosomal DNA elements or did not align to any previously published plasmids. The resulting concatenated plasmid sequences were divided into two groups (G1, G2) using a repA phylogeny. Additionally, plasmid-positive and -negative strains were compared on their ability to tolerate cold (4°C), salt (6% NaCl), acid (pH 5), or desiccation stress (33% RH).
Results: Plasmids were observed in 55% of strains and plasmid-harborage was associated (p=0.013) with enhanced acid tolerance. G1 plasmids were significantly smaller (26-88 kb) than G2 plasmids (p<0.0005, 55-100 kb). The most prevalent plasmid (56,553 bp – G1) was observed in 26 strains from both Switzerland and Canada. G1 plasmids uniquely contained genes encoding alcohol dehydrogenases, and specific lipoproteins, while G2 plasmids contained genes for DNA topoisomerase III, DEAD box helicases, thermonucleases, and multidrug resistance proteins (ebrAB). Noteworthy, one strain, which exhibited the highest level of acid tolerance, contained two plasmids (i.e., one from each repA group).
Significance: This work highlights the widespread abundance of specific Lm plasmids, the unique properties associated with G1 and G2 plasmids, and the association between plasmid harborage and enhanced acid tolerance.