Variability in cadmium tolerance of closely related <i>Listeria monocytogenes</i> isolates originating from dairy processing environments

Mobile genetic elements in <i>Listeria monocytogenes</i> contribute to its survival in natural and food processing environments. This study focused on how different genetic variants of the efflux pump gene <i>cadAC</i> and group of closely related <i>cadA1C1</i> strains respond to cadmium exposure. When exposed to two cadmium salts, cadmium chloride and cadmium sulfate, we observed varying growth patterns, with a significantly longer lag phase in cadmium sulfate compared to cadmium chloride. Strains with <i>cadA1</i> to <i>cadA3</i> had similar growth trends, whereas a strain with the <i>cadA4</i> variant had the highest minimum inhibitory concentration value. Among 88 strains from dairy processing facilities, significant phenotypic differences were observed despite core genome similarities, indicating other underlying genetic and physiological factors contribute to cadmium tolerance. Since cadmium tolerance studies in <i>L. monocytogenes</i> are limited, with rare phenotypic comparisons between closely related strains, our study makes an important observation and contribution to understanding of <i>L. monocytogenes</i> tolerance to cadmium by providing phenotypic comparisons between numerous strains within the same clonal group (<16 single nucleotide polymorphisms).