Differential Distribution of Plasmid-Mediated Quinolone Resistance Genes in Clinical Enterobacteria with Unusual Phenotypes of

We studied a collection of 105 clinical enterobacteria with unusual phenotypes of quinolone susceptibility to analyze the occur-rence of plasmid-mediated quinolone resistance (PMQR) and oqx genes and their implications for quinolone susceptibility. The oqxA and oqxB genes were found in 31/34 (91%) Klebsiella pneumoniae and 1/3 Klebsiella oxytoca isolates. However, the oqxA-and oqxB-harboring isolates lacking other known quinolone resistance determinants showed wide ranges of susceptibility to nalidixic acid and ciprofloxacin. Sixty of the 105 isolates (57%) harbored at least one PMQR gene [qnrB19, qnrB10, qnrB2, qnrB1, qnrS1, or aac(6=)-Ib-cr)], belong to 8 enterobacterial species, and were disseminated throughout the country, andmost of themwere categorized as susceptible by the current clinical quinolone susceptibility breakpoints. We developed a disk diffusion-based method to improve the phenotypic detection of aac(6=)-Ib-cr. The most common PMQR genes in our collection [qnrB19, qnrB10, and aac(6=)-Ib-cr] were differentially distributed among enterobacterial species, and two different epidemiological set-tings were evident. First, the species associated with community-acquired infections (Salmonella spp. and Escherichia coli) mainly harbored qnrB19 (a unique PMQR gene) located in small ColE1-type plasmids that might constitute its natural reser-voirs. qnrB19was not associated with an extended-spectrum -lactamase phenotype. Second, the species associated with hospi-tal-acquired infections (Enterobacter spp., Klebsiella spp., and Serratia marcescens) mainly harbored qnrB10 in ISCR1-contain