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Objectives: To investigate the impact of plasmid-borne oqxAB genes on the development of fluoroquinolone resistance, mutations and bacterial fitness in Escherichia coli .Methods: MICs and mutation prevention concentrations were compared among E. coli strain TOP10 and two corresponding transformants harbouring the OqxAB-encoding plasmids. Mutants were selected by serial passages with the 0.5-fold MIC of ciprofloxacin, and were randomly selected to determine mutations. Bacterial fitness was evaluated by competition assays in vitro and in vivo .Results: The oqxAB -carrying plasmids contributed to a 4-8-fold increase in the ciprofloxacin MIC and increased the ciprofloxacin mutation prevention concentration by 8-16-fold. The MIC of ciprofloxacin for the two transformants increased faster than that of E. coli TOP10 by serial passaging. Novel mutations in gyrB (A468P or F458V) were first observed. Mutations in gyrA were distributed at codons 87 and 83 in the two transformants, whereas mutation A119E in gyrA dominated in the TOP10 mutants. Although the two oqxAB -bearing plasmids caused a decrease in fitness in vitro , their fitness increased when combined with more than one chromosomal mutation, and clear biological benefits were observed in vivo . The mutations in gyrB were associated with a fitness cost, which could be compensated for by additional mutations. The novel mutation gyrA ΔS83 significantly reduced biological fitness both in vitro and in vivo , and was thus quickly replaced by more beneficial mutations in the population.Conclusions: The possession of plasmid-borne oqxAB may facilitate the evolution of fluoroquinolone resistance, and the fitness cost of OqxAB-encoding plasmids could be compensated by additional chromosomal mutations. [ABSTRACT FROM AUTHOR]

The article presents a report on emergence of Escherichia coli co-producing New Delhi metallo-b-lactamase-1(NDM-1) and Klebsiella pneumoniae carbapenemase 2 (KPC-2) from a retail vegetable, China. Topics discussed include use of agar dilution method for determining of minimum inhibitory concentration of antimicrobial agents; its resistance to imipenem, ceftazidime, cefotaxime; and a table depicting characteristics of E. coli TS79e and its transconjugants.

Objectives To investigate the occurrence of 16S rRNA methylases conferring high-level resistance to aminoglycosides in Enterobacteriaceae isolated from two pig farms in China. Methods Enterobacteriaceae isolated from 151 pig rectal swab samples and 9 environmental samples were screened for the presence of the rmtA, rmtB, armA and rmtC genes by PCR and sequencing. Conjugation experiments were carried out to study the transferability of the 16S rRNA methylase genes. All isolates and their transconjugants were tested for susceptibility to antimicrobial agents. The clonal relatedness of RmtB-producing Escherichia coli was assessed by PFGE with XbaI. Results Of 152 Enterobacteriaceae isolates recovered from pigs, 49 (32%) were positive for the rmtB gene, including 48 E. coli and a single isolate of Enterobacter cloacae. Of the nine Enterobacteriaceae isolates from environmental samples, no 16S rRNA methylase gene was identified. The 49 rmtB-positive isolates showed resistance to ampicillin, tetracycline and trimethoprim and also carried a blaTEM gene. Transfer of the rmtB and blaTEM genes by conjugation experiments of all 49 isolates was successful, suggesting that the rmtB-containing plasmids in the E. coli and E. cloacae isolates were self-transmissible. Conjugative transfer frequencies varied from 2.2 × 10−10 to 1.3 × 10−6 transconjugants per recipient. The transfer of non-aminoglycoside antimicrobial resistance traits was also observed in most cases. Forty-four rmtB-positive E. coli showed 30 different PFGE types. Conclusions The rmtB gene was detected on conjugative plasmids of porcine E. coli and E. cloacae isolates. Both horizontal gene transfer and clonal spread were responsible for the dissemination of the rmtB gene. The emergence of 16S rRNA methylases in Enterobacteriaceae isolates is described for the first time in China. This is also the first report of rmtB-positive Enterobacteriaceae among healthy food-producing animals. [ABSTRACT FROM AUTHOR]