Phenotypic characterization of beta-lactamases and postantibiotic effect of tigecycline on gram-negative bacilli isolated from Lebanese patients

Abstract

The objective of this study was to assess the profiles of resistance and to phenotypically detect and chatacterize the beta-lactamases produced by Gram-negative bacilli isolated from Lebanese patients. In addition, the postantibiotic effects (PAE) of tigecycline on selected strains of the above isolates were quantified. For this purpose, 124 clinical samples (Gram-negative bacilli) were collected from patients at Saint George Hospital-University Medical Center, Beirut, Lebanon from May 1 st to July 1st, 2010. These isolates were screened by disk diffusion method in order to analyze their patterns of susceptibility to different antimicrobials agents. Consequently the production of the major beta-lactamases in these isolates was investigated using phenotypic tests. Macrobroth dilution method was done to determine the MIC of tigecycline and accordingly quantify the in-vitro post-antibiotic effect (PAE) of this antibiotic on 10 different strains representing the different resistance profiles, using the method described by Pankuch and Appelbaum. High levels of antimicrobial resistance were noted in E.coli and K. pneumoniae, respectively, against ampicillin (74.68% and 81.25%), cephalexin (74.68% and 75%), piperacillin (60.76% and 43.75%), and nalidixic acid (73.41% and 50%). No resistance to carbapenems and tigecycline was detected in these isolates. Of the 79 isolates of E.coli and 16 isolates of K. pneumoniae, 31.65% of E. coli and 12.5% of K. pneumoniae were ESBL producers. Production of AmpC was detected in 8.86% of E. coli and 31.25% of K. pneumoniae. Regarding the non-fermenters, of the 19 strains of P. aeruginosa, 84.21% were non significant beta-lactamase producers. Only 1 strain of A. baumanii was identified as carbapenemase producer. No metallo-beta-lactamase was documented in any of the tested strains. The isolates showed high sensitivity to tigecycline (MIC50=0.04 μg/mL) and the PAEs of E. coli, K. pneumoniae, and A. baumannii were 0.95 to 3.93, 1.78, and 1.28 h, respectively, showing a prolonged PAE supporting therefore a relaxed dosage regimen. This is the first study in Lebanon and one of very few in the Middle East to explore PK/PDs of tigecycline against Gram-negative bacilli with different profiles of resistance. It provides new data, essential for the epidemiology of infection and improvement of the treatment of bacterial infection in Lebanon, in an attempt to circumvent the ever increasing problem of antibacterial resistance.