Identification of plasmid-mediated extended-spectrum and AmpC beta-lactamases in Enterobacter spp. isolated from dogs.

The genetic determinants involved in reduced susceptibility to third-generation cephalosporins and aztreonam were identified in ten canine Enterobacter isolates associated with opportunistic infections in three veterinary hospitals in Brisbane, Australia. All isolates were evaluated by a combination of phenotypic (broth microdilution and disc susceptibility, modified disc diffusion and IEF) and genotypic (PFGE, plasmid analysis, Southern blot hybridization, bacterial conjugation, PCR and sequencing) methods to investigate genetic relatedness and to identify plasmid-mediated resistance genes, in particular beta-lactamase genes responsible for extended-spectrum cephalosporin resistance. The ten canine isolates were genotypically diverse based on PFGE and belonged to either Enterobacter cloacae or Enterobacter hormaechei on the basis of 16S rRNA gene sequence analysis. Plasmid profiles were also diverse. Nine isolates contained a transmissible blaSHV-12-carrying plasmid (approximately 140 kb) that also conferred resistance to chloramphenicol, gentamicin, spectinomycin, tetracycline, trimethoprim and sulfonamides. In all plasmid-mediated extended-spectrum beta-lactamase (ESBL)-producing isolates including transconjugants, blaSHV-12 was shown to reside in a approximately 6.5 kb plasmid fragment. The remaining isolate that was not an ESBL producer possessed an AmpC beta-lactamase gene (blaCMY-2) on a approximately 93 kb transmissible plasmid. This plasmid did not contain any other antimicrobial resistance genes. Additional plasmid-mediated beta-lactamases identified in some isolates included bla(TEM) and blaOXA-10. This is the first report of canine Enterobacter isolates containing transmissible plasmid-mediated blaSHV-12 and blaCMY-2 resistance genes. Therefore, Enterobacter isolated from opportunistic infections in dogs may be an important reservoir of plasmid-mediated resistance genes, which could potentially be spread to other members of the Enterobacteriaceae.

In vitro development of resistance to DX-619 and other quinolones in enterococci.

OBJECTIVES: To investigate the molecular events involved in the development of quinolone resistance in enterococci. METHODS: Clinical isolates of Enterococcus faecium and Enterococcus faecalis were exposed to inhibitory and subinhibitory concentrations of DX-619, ciprofloxacin, levofloxacin, gatifloxacin and moxifloxacin. Mutational frequencies were calculated and susceptibility changes were determined. The quinolone resistance determining regions (QRDRs) of gyrA and parC in less-susceptible mutants were amplified by PCR and sequenced. RESULTS: Single-step mutants of E. faecalis and E. faecium were selected with all drugs. There were no differences in the frequencies of mutant selection among drugs, with frequencies ranging from 10(-5) to 10(-8). All single-step mutants were inhibited by 0.03-1 mg/L DX-619, 0.25-8 mg/L moxifloxacin, 0.5-8 mg/L gatifloxacin, 1-16 mg/L levofloxacin and 1-32 mg/L ciprofloxacin. No QRDR changes were observed in single-step mutants. Less-susceptible mutants selected after five passages on agar containing subinhibitory quinolone concentrations were inhibited by 0.12-8 mg/L DX-619, 1-64 mg/L moxifloxacin, 2-64 mg/L gatifloxacin and 2-128 mg/L levofloxacin and ciprofloxacin. QRDR changes were detected in only 9 of the 20 fifth-passage mutants, suggesting that mutations outside the purported QRDRs and/or other resistance mechanisms were also involved. CONCLUSION: The relatively high frequencies at which single-step mutants were selected with all drugs indicate that caution is necessary if quinolones are to be considered for monotherapy of serious enterococcal infections. DX-619, the most potent quinolone, may have potential as an anti-enterococcal agent if sufficient concentrations can be safely attained in vivo.

Multidrug resistance associated with mexXY expression in clinical isolates of Pseudomonas aeruginosa from a Texas hospital.

Fluoroquinolones and carbapenems are important drug classes used for the treatment of Pseudomonas aeruginosa infections. However, overexpression of the P. aeruginosa efflux pump, MexEF-OprN, can provide dual resistance to both fluoroquinolones and carbapenems. Recently, a hospital in Texas encountered several P. aeruginosa isolates resistant to both of these drug classes. The purpose of this study was to determine whether the mechanism responsible for this multidrug resistance involved the overexpression of MexEF-OprN. To test this hypothesis, 7 clinical isolates from the Texas hospital were analyzed for clonality, antimicrobial susceptibility, expression of the porin, oprD, and four multidrug-resistant efflux pumps (mexAB-oprM, mexCD-oprJ, mexEF-oprN, and mexXY), quinolone resistance-determining region mutations, and beta-lactamase production. Two groups of isolates possessed similar pulse field gel electrophoresis patterns indicating their genetic relatedness. In addition to fluoroquinolone and carbapenem resistance, each isolate also displayed varying degrees of susceptibility to additional beta-lactams tested and resistance to gentamicin. Interestingly, none of the 7 clinical isolates overexpressed mexEF-oprN as determined by semiquantitative reverse transcriptase polymerase chain reaction, but overexpression of mexXY was observed in 6 of the 7 isolates. All 7 isolates showed a decrease of OprD in the outer membrane and a reduction in transcriptional expression of oprD compared to wild-type strain, PAO1. These results demonstrate that multidrug resistance to the fluoroquinolones and carbapenems in these clinical isolates was not a result of the overexpression of the mexEF-oprN pump. Instead, resistance to these two agents seemed to arise through independent mutational events.

Plasmid-mediated, carbapenem-hydrolysing beta-lactamase, KPC-2, in Klebsiella pneumoniae isolates.

Four isolates of Klebsiella pneumoniae obtained from patients at a Maryland medical centre exhibited reduced susceptibility to carbapenems and were found to produce the novel, class A, plasmid-mediated, carbapenem-hydrolysing enzyme, KPC-2. This enzyme has 99% identity with the plasmid-mediated, carbapenem-hydrolysing enzyme KPC-1, reported previously in a North Carolina K. pneumoniae isolate. The KPC-2-producing isolates were either susceptible or intermediate to imipenem and meropenem, unlike the KPC-1-producing isolate, which was resistant to these agents. Detection of KPC-2 may be a problem for clinical laboratories because in this study it was associated with positive extended-spectrum beta-lactamase (ESBL) confirmation tests (clavulanate-potentiated activities of ceftriaxone, ceftazidime, cefepime and aztreonam). Therefore, a failure to recognize the significance of reduced carbapenem susceptibility in the isolates that remained susceptible to imipenem or meropenem could have resulted in the isolates being incorrectly identified as ESBL producers.