Purpose: The objective of this study was to determine the antimicrobial susceptibility of Listeria strains isolated from ready-to-eat foods and examine the presence of mutations in their gyrase A and B (gyrA and gyrB) genes.
Methods: We determined the Gradient Minimum Inhibitory Concentration (GMIC) and End Concentration (EC) of seven antimicrobial agents for 35 strains of Listeria isolated from various foods (chicken breast, soft cheeses and produce) using a spiral gradient endpoint method.
Results: Results of this experiment showed that 79%, 72%, 70% and 85%, 12%, 58% and 42% of the strains were resistant to ciprofloxacin (GMIC = 3.33 µg/ml; EC = 2.42 µg/ml) enrofloxacin (GMIC = 2.7 µg/ml; 1.74 µg/ml), norfloxacin (GMIC = 10.91 µg/ml; EC = 4.71 µg/ml) and ofloxacin (GMIC = 7.1 µg/ml; EC = 4.83 µg/ml), tetracycline (GMIC = 4.1 µg/ml; EC = 5.08 µg/ml), trimethoprim (GMIC = 61.61 µg/ml; EC = 31.14 µg/ml) and ampicillin (GMIC = 17.7 µg/ml; EC = 12.8 µg/ml), respectively. To determine the mechanism of quinolone resistance of Listeria, 10 food isolates were randomly selected and the sequences were analyzed. The analysis of quinolone resistance determining regions (QRDRs) of both the gyrA and B genes were determined. Amino acid substitutions were found in several strains for both gyrA and B gene. All ten strains exhibited distinct PFGE patterns when compared to each other.
Significance: Our study suggested that resistance to quinolone can be attributed to mutations in QRDRs of both the gyrA and B gene and this genetic mutation can be universally found in multiple strains of L. monocytogenes isolated from various foods.