Plasmid-borne cadmium resistant determinants are associated with the susceptibility of Listeria monocytogenes to bacteriophage

Listeria monocytogenes is an intracellular pathogen causing gastroenteritis, central nervous system infections and abortions. Chromosomal virulence determinants have been extensively investigated. However, the function of genes encoded by plasmids in L. monocytogenes has not been fully understood. In this study, we determined the prevalence and molecular profile of plasmids in food isolates of L. monocytogenes and examined the contribution of four plasmid-borne cadmium-resistant genes to the susceptibility of L. monocytogenes to bacteriophage infection. The results showed that plasmids were isolated from 55% (11/20) of the isolates and the plasmids exhibited 10 molecular types as determined by restriction enzyme digestion. Furthermore, 65% and 15% of the isolates were tolerant to cadmium and benzalkonium chloride (BC), respectively. All the BC-resistant isolates were resistant to cadmium. The prevalence of predicted cadmium resistance determinants (cadA1, cadA2, cadA3 and cadC) was determined and the results showed that cadA1 (35%) in isolates of serotypes 1/2a and 1/2b was much more prevalent than cadC (15%). As expected, both cadA and cadC mutants had reduced resistance to cadmium, while the resistance to BC was not significantly affected. Interestingly, both cadA and cadC mutants showed significantly higher susceptibility against L. monocytogenes phage LipG2-5 and FWLLm3 compared with the wide-type strain. Based on these results, we concluded that plasmids from L. monocytogenes encoded important functional determinants that are not only associated with cadmium resistance, but also phage susceptibility.