Efflux pump, the masked side of beta-lactam resistance in Klebsiella pneumoniae clinical isolates.
BACKGROUND
Beta-lactamase production and porin decrease are the well-recognized mechanisms of acquired beta-lactam resistance in Klebsiella pneumoniae isolates. However, such mechanisms proved to be absent in K. pneumoniae isolates that are non susceptible to cefoxitin (FOX) and susceptible to amoxicillin+clavulanic acid in our hospital. Assessing the role of efflux pumps in this beta-lactam phenotype was the aim of this study.


RESULTS
MICs of 9 beta-lactams, including cloxacillin (CLX), and other antibiotic families were tested alone and with an efflux pump inhibitor (EPI), then with both CLX (subinhibitory concentrations) and EPI against 11 unique bacteremia K. pneumoniae isolates displaying the unusual phenotype, and 2 ATCC strains. CLX and EPI-dose dependent effects were studied on 4 representatives strains. CLX MICs significantly decreased when tested with EPI. A similar phenomenon was observed with piperacillin+tazobactam whereas MICs of the other beta-lactams significantly decreased only in the presence of both EPI and CLX. Thus, FOX MICs decreased 128 fold in the K. pneumoniae isolates but also 16 fold in ATCC strain. Restoration of FOX activity was CLX dose-dependent suggesting a competitive relationship between CLX and the other beta-lactams with regard to their efflux. For chloramphenicol, erythromycin and nalidixic acid whose resistance was also due to efflux, adding CLX to EPI did not increase their activity suggesting differences between the efflux process of these molecules and that of beta-lactams.


CONCLUSIONS
This is the first study demonstrating that efflux mechanism plays a key role in the beta-lactam susceptibility of clinical isolates of K. pneumoniae. Such data clearly evidence that the involvement of efflux pumps in beta-lactam resistance is specially underestimated in clinical isolates.
Pages
Jean-Marie
JM
UMR-MD-1, Facultés de Médecine et de Pharmacie, Université de la Méditerranée, IFR88, Marseille, France. Jean-Marie.PAGES@univmed.fr
Lavigne
Jean-Philippe
JP
Leflon-Guibout
Véronique
V
Marcon
Estelle
E
Bert
Frédéric
F
Noussair
Latifa
L
Nicolas-Chanoine
Marie-Hélène
MH
D002352:Carrier Proteins / Q000502:physiology*; D004352:Drug Resistance, Microbial; D006801:Humans; D007711:Klebsiella pneumoniae / Q000187:drug effects* / Q000302:isolation & purification; D008826:Microbial Sensitivity Tests; D047090:beta-Lactams / Q000494:pharmacology*
D002352:Carrier Proteins; D047090:beta-Lactams