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

Abstract

ABSTRACTWe studied a collection of 105 clinical enterobacteria with unusual phenotypes of quinolone susceptibility to analyze the occurrence of plasmid-mediated quinolone resistance (PMQR) andoqxgenes and their implications for quinolone susceptibility. TheoqxAandoqxBgenes were found in 31/34 (91%)Klebsiella pneumoniaeand 1/3Klebsiella oxytocaisolates. However, theoqxA- andoqxB-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, oraac(6′)-Ib-cr)], belong to 8 enterobacterial species, and were disseminated throughout the country, and most of them were categorized as susceptible by the current clinical quinolone susceptibility breakpoints. We developed a disk diffusion-based method to improve the phenotypic detection ofaac(6′)-Ib-cr. The most common PMQR genes in our collection [qnrB19,qnrB10, andaac(6′)-Ib-cr] were differentially distributed among enterobacterial species, and two different epidemiological settings were evident. First, the species associated with community-acquired infections (Salmonellaspp. andEscherichia coli) mainly harboredqnrB19(a unique PMQR gene) located in small ColE1-type plasmids that might constitute its natural reservoirs.qnrB19was not associated with an extended-spectrum β-lactamase phenotype. Second, the species associated with hospital-acquired infections (Enterobacterspp.,Klebsiellaspp., andSerratia marcescens) mainly harboredqnrB10in ISCR1-containing class 1 integrons that may also haveaac(6′)-Ib-cras a cassette within the variable region. These two PMQR genes were strongly associated with an extended-spectrum β-lactamase phenotype. Therefore, this differential distribution of PMQR genes is strongly influenced by their linkage or lack of linkage to integrons.