Differential expression of virulence genes and role of gyrA mutations in quinolone resistant and susceptible strains of Salmonella Weltevreden and Newport isolated from seafood

AIMS: To investigate the differential expression of virulence genes and role of gyrA mutations in quinolone resistant and susceptible strains of Salmonella isolated from seafood. METHODS AND RESULTS: Forty Salmonella isolates from seafood were tested for antibiotic sensitivity. Minimal inhibitory concentration (MIC) was determined and two nalidixic acid‐resistant isolates, viz Salmonella Weltevreden (SW9) and Salmonella Newport (SN36) were selected for identifying the mechanism of resistance. SW9 showed mutation in the gyrA gene at codon 83 (Ser to Tyr) while SN36 presented at codon 87 (Asp to Asn). Experimental induction of resistance to a sensitive Salm. Newport (SN71) showed point mutation at codon 87 (Asp to Gly) in the gyrA gene, and was designated SN71R. All the isolates resistant to nalidixic acid had a single mutation at different positions in the gyrA gene. However, induction of resistance to a sensitive Salm. Weltevreden (SW30) was exceptional in that it did not show any mutation in the gyrA region. Use of Phe‐Arg‐β‐naphthylamide (PAβN) also could not reduce MIC below the Clinical and Laboratory Standards Institute guidelines revealing the absence of efflux mediated resistance. Thus, the resistance mechanism in SW30R is unknown. The growth rate of quinolone resistant isolates was slower than the susceptible ones. The resistant isolates showed decreased epithelial cell invasion and intracellular replication. The mRNA expression levels of some of the genes were significantly (P < 0·005) reduced in SN71R compared to the sensitive strain (SN71). CONCLUSIONS: Nalidixic acid‐resistant Salmonella strains are associated with lower virulence and pathogenicity than the sensitive strains. SIGNIFICANCE AND IMPACT OF THE STUDY: This study provided valuable information on the difference in the growth, cytotoxicity, infectivity and expression of virulence genes in resistant and susceptible strains. Furthermore, the gyrA mutation was shown to be the main mechanism of quinolone resistance in Salmonella other than the overexpression of efflux pumps or the presence of plasmid mediated quinolone resistance genes.