Listeria monocytogenes from the British Columbia Food Chain Show Evidence of Co-selection of Cadmium and Benzalkonium Chloride Resistance

Introduction:  Listeria monocytogenes (Lm), a foodborne pathogen, causes rare but severe disease in at-risk populations. This pathogen is well suited to persistence in natural and food processing environments. Transferable genetic determinants that increase environmental fitness in Lm, including resistance to heavy metals and/or food processing sanitizers, are of concern.

Purpose: To evaluate Lm fitness in low cadmium environments and co-selection of cadmium and benzalkonium chloride (BAC) resistance in Lm isolated from the British Columbia food chain.

Methods: Lm sensitive (n = 30) or resistant (n = 17) to 10 µg/ml BAC were evaluated for resistance to 35 µg/ml cadmium chloride (CdCl₂) at 37°C and screened for known cadmium resistance determinants. Lm growth rates at 25°C in the presence of sub-lethal CdCl₂ (1 µg/ml) were determined. The effect of CdCl₂ in combination with low pH on growth rate was examined at 25°C. Transferability of resistance determinants was assessed through conjugation of co-resistant (BAC + cadmium) donor strains and a recipient cadmium sensitive derivative of Lm at 25°C. Transconjugants were confirmed by PCR and characterized for co-resistance.

Results: Forty strains were cadmium resistant, with 16 being co-resistant. Five cadmium resistant strains lacked known cadmium resistance determinants. In the presence of sub-lethal CdCl₂, sensitive strains showed an average 29% reduced growth rate (P = 0.003) compared to resistant strains. CdCl₂ and low pH significantly reduced the growth rate of cadmium sensitive strains (P < 0.001). Two of 16 co-resistant donor strains produced transconjugants. Transferred BAC and cadmium resistance determinants showed no increase in BAC resistance when combined with the BAC efflux pump emrE but restored growth in the presence of sub-lethal CdCl₂.

Significance: These findings suggest cadmium resistance benefits Lm fitness and environmental heavy metals may act as a selective pressure influencing co-resistance to common food processing sanitizers. Horizontal transfer of this co-resistance between Lm strains has not previously been reported.