Genotypic characterization and in vitro activities of tigecycline and polymyxin B for members of the Enterobacteriaceae with decreased susceptibility to carbapenems

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Carbapenem resistance in members of the Enterobacteriaceae is increasing. To evaluate the effects of tigecycline and polymyxin B against carbapenem-non-susceptible pathogens, 89 representative clinical carbapenem-non-susceptible Enterobacteriaceae isolates were recovered from seven hospitals from four cities in China during 2006–2009: 30 Serratia marcescens, 35 Klebsiella pneumoniae, seven Enterobacter cloacae, six Enterobacter aerogenes, five Escherichia coli, four Citrobacter freundii and two Klebsiella oxytoca isolates. Twenty-eight S. marcescens isolates were indistinguishable. The 35 K. pneumoniae isolates belonged to 12 clonal strains. Among the 89 Enterobacteriaceae isolates, 82 produced KPC-2, seven produced IMP (three produced KPC-2 simultaneously), three did not produce any carbapenemases and nine were deficient in porins. Polymyxin B was much more active than tigecycline against carbapenem-non-susceptible Enterobacteriaceae. The MIC50 and MIC90 of imipenem, meropenem, ertapenem, polymyxin B and tigecycline were 8 and 32 μg ml−1, 8 and 32 μg ml−1, 16 and 128 μg ml−1, 0.5 and 16 μg ml−1, and 4 and 16 μg ml−1, respectively. Rates of susceptibility to imipenem, meropenem, ertapenem and polymyxin B were 30.0 %, 27.5 %, 2.5 % and 89.2 % by CLSI criteria. The rate of susceptibility to tigecycline was 40 % and 17.5 % by Food and Drug Administration (MIC ⋜2 μg ml−1) and European Committee on Antimicrobial Susceptibility Testing (MIC ⋜1 μg ml−1) criteria, respectively. KPC-2- or IMP-producing E. coli transconjugants exhibited reduced susceptibility to carbapenems but were susceptible to polymyxin B and tigecycline with an MIC range of 0.5–2 μg ml−1, 0.25–2 μg ml−1, 0.5–4 μg ml−1, 0.5 μg ml−1 and 0.5–1 μg ml−1. In conclusion, carbapenem resistance in Enterobacteriaceae is mainly due to production of KPC-2, and polymyxin B is active for the carbapenem-resistant Enterobacteriaceae.

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