The Epidemiological Characteristics Of Plasmid-mediated Quinolone Resistance In Chinese Escherichia Coli Isolates

Quinolone resistance is emerging in gram-negative pathogens worldwide. The traditional understanding that quinolone resistance is acquired only through mutation and transmitted only vertically does not entirely account for the relative ease with which resistance develops in susceptible organisms, or for the very strong association between resistance to quinolones and to other agents. The recent discovery of plasmid-mediated horizontally transferable genes encoding quinolone resistance might shed light on these phenomena. Plasmid-mediated quinolone resistance has so far managed to achieve global distribution in a variety of plasmid environments and bacterial genera, with three different resistance mechanisms reported as implicated. The first was Qnr (standing for quinolone resistance), and there are currently several named and described alleles:QnrA, QnrS, QnrB, QnrC and QnrD; the second, quinolone extrusion, a prevalent chromosomally encoded resistance mechanism, has also been found to be plasmid borne and mediated by QepA or OqxAB; and finally, Aac(6’)-Ib-cr, is a variant aminoglycoside acetyltransferase, capable of reducing ciprofloxacin activity. These mechanisms provide the low-level quinolone resistance shown in vitro to facilitate the emergence of higherlevel resistance in the presence of quinolones at therapeutic levels.In this study, it was investigated that the prevalence and disturbution of PMQR determinants in different sources of E. coli strains. We also determined the quinolones minimum inhibitory concentration of the transconjugants and the resistance to other kind of antimicrobial agents to evaluate the effect of PMQR genes and the cotransfer of the other kinds of antibiotical resistance. We tracked and monitored qnrS gene on a farm to understand the distribution and the fluctuation of this gene. And it was further analyzed the genetic relationship of the qnrS positive strains, and also determined the size of the differnet qnrS plasmids to understand the gene’s transmission mode.1. Prevalence of the PMQR genes in Escherichia coli isolates from humans, animals and the environmentThis study evaluated the prevalence of PMQR genes(qnrA, qnrB, qnrC, qnrD, qnrS, qepA, oqxAB and aac(6’)-Ib-cr) in E. coli isolates from different sources (humans, animals and the environment) in20provinces of China within the period of1993-2010by PCR amplification and DNA sequencing. PMQR determinants were present in281(27.5%) isolates; qnr, aac(6’)-Ib-cr, qepA, and oqxAB were detected alone or in combination in58(5.7%),50(4.9%),27(2.6%), and206(20.2%) strains, respectively. None of the isolates carried qnrC or qnrD. The qnr genes detected included one qnrAl gene, three qnrA3genes, one qnrB2gene, one qnrB4gene, seven qnrB9genes, two qnrB10genes,35qnrSl genes, and eight qnrS2genes. We detected the occurrence of oqxAB in isolates from ducks and geese for the first time. Prevalence of PMQR genes was significantly higher in animals intestinal E. coli isolates (45.6%,129/283) than in extraintestinal isolates (23.7%)(P<0.005). The positive rate of oqxAB in strains separated from animals was27.0%(181/671), which was significantly higher than the prevalence of oqxAB in humans (5.2%,16/307)(P<0.005). In PMQR-positive strains,68.8%(11/16) of strains from environment were positive for more than one PMQR determinant, significantly higher than strains from organisms. For strains separated from animals, the positive rate of double or triple PMQR genes in intestinal strains was24.8%(32/129), which was significantly higher than that of extraintestinal strains (6.5%, 6/92)(P<0.005). This demonstrates that the environment and the animals gut play an important role in strain acquisition of PMQR determinants. Phylogenetic analysis showed that the281PMQR-positive isolates mainly belonged to group A and B1(73%).Donor strains were resistance to all nine classes of antimicrobial agents, except meropenem, which is a carbapenem. Seven classes of antimicrobial agents were able to cotransfer with PMQR genes together with resistance except fosfomycin and nitrofurantoin. Varying increases in quinolone MICs were observed in the presence of PMQR genes in the transconjugants. Findings indicate that qnr genes have the greatest effect on ciprofloxacin MIC than other determinants, and the different types of PMQR genes have an additive effect. The accumulation of QRDR mutations is the main reason for the resistance of quinolones and fluoroquinolones, the number of QRDR mutations do not affect olaquindox MIC, oqxAB gene plays an important role in olaquindox resistance.2. The prevalence and dissemination features of qnrS gene in the isolates379strains were separated from different sources of samples (chickens, ducks, geese, water, soil, dust and fodder) which collected from a farm and the prevalence and dynamic changes of qnrS gene was further surveyed. The qnrS positive strains can detecte in all sources of strains and two alleles of qnrS were prevalence on this farm. The positive rate of qnrS gene in environment strains was29.2%, significantly higher than the avian strains (13.4%). The chicken can quickly acquire the qnrS gene after they live on the farm. The xba Ⅰ PFGE analysis of the qnrS positive strains separated on the farm illustrate that the dissemination of qnrS was not mainly due to the clonal dissemination of positive strains. However, qnrS positive strains with indistinguishable PFGE patterns were found in ducks and environment. The similar results also obtained from the strains separated from the different parts of China. The sizes of qnrS plasmids were varying from30kb to300kb, suggesting that the genetic diversity. In conclusion, the qnrS gene can clonally disseminate through contacting or food chain, but it mainly dissenminated by horizontal transmission which may be spread by plasmid distribution or different kind of integrons.