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UBC Theses and Dissertations

Resistance to food processing sanitizers and heavy metals in Listeria monocytogenes from British Columbia, Canada and antibiogram profiles of clinically relevant Listeria monocytogenes from British Columbia and Alberta, Canada Milillo, Michael Steven


Listeria monocytogenes (Lm), a foodborne pathogen, causes the rare but severe disease listeriosis in at-risk populations. Resistance that increases environmental fitness in Lm, including resistance to heavy metals and/or food processing sanitizers, is of concern as it may increase survival in natural and food processing environments that leads to increased potential for product contamination. Lm (n=46) from the British Columbia food chain were evaluated for resistance to food processing sanitizers and heavy metals. All isolates were found to be sensitive to triclosan (16 µg/ml) and peroxyacetic acid (150 ppm exposure for 60 seconds). Seventeen isolates were resistant to quaternary ammonium compounds (QUATs; 10 µg/ml), with all positive for one known resistance determinant (bcrABC, n=16; emrE, n=1). Resistance to cadmium (Cd) and arsenic was found in 89% and 24% of the isolates, respectively. Under sub-lethal concentrations of Cd, all Cd sensitive isolates showed reduced growth. Decreasing pH and increasing Cd resulted in a minimum inhibitory concentration for a representative Cd sensitive isolate at 1.5 µg/ml CdCl₂ at pH 6 and 0.5 µg/ml CdCl₂ at pH 5.5. All bcrABC positive isolates were Cd resistant, and two were capable of co-transferring QUAT and Cd resistance to L. monocytogenes 08-5578. When bcrABC and emrE were combined through conjugation, there was no observable increase in resistance to QUATs. While L. monocytogenes from British Columbia remain sensitive to common sanitizers, a sub-population is resistant to low QUAT concentrations and co-resistant to Cd. Further work is needed to explore if co-selection between heavy metal and sanitizer resistance is a concern in the food chain. As Lm continue to cause disease, surveillance of antibiotic resistance within the population of Lm is necessary to aid in effective early treatment. A collection of sequence type 120 Lm (n=21) were all found to be sensitive to the primary drugs of choice for treatment of listeriosis - ampicillin, gentamicin, and co-trimoxazole. Four isolates were resistant to an ampicillin alternative, linezolid. This data supports the continued use of the primary drugs of choice in treating listeriosis in Canada from sequence type 120 Lm, but suggests the possibility of decreased effectiveness of alternative antibiotics.

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