Effect of subinhibitory antibiotics concentrations on biofilm formation by clinical Acinetobacter baumannii isolated from endotracheal tubes in ventilator-associated pneumonia

Abstract

The treatment of ventilator-associated pneumonia caused by Acinetobacter baumanii requires a probabilistic antibiotic therapy followed by a specific antibiotic therapy. However, this treatment is faced with two problems; first, the great ability of A. bauamnii to form a biofilm on the surface of endotra-cheal tube, and second the exposure of bacteria to a sub-minimal inhibitory concentration (Sub-MIC) of antibiotics due to their weak diffusion through the tissues. The purpose of the present study was to determine the effects of Sub-MICs of ceftazidime (CAZ), piperacillin/tazobactam (PIP/TAZ), imipenem (IMP) and colistin (CST) on biofilm formation of six A. baumanii clinical iso-lates and as well as on its representative strain ATCC 19606. Sub-MIC of ceftazidime was found to increase biofilm formation in all six isolates. Alt-hough, in the presence of Sub-MIC of PIP/TAZ, biofilm formation of three isolates was reduced. Also, Sub-MIC of CST had a stimulating effect on bio-film formation in three isolates and a reducing effect on one single isolate. While, no impact was noted on all isolates in the presence of Sub-MIC of IMP. The ATCC 19606 biofilm formation exhibited no significant change in the presence of Sub-MIC of IMP. However, it decreased in the presence of Sub-MIC of PIP / TAZ and CST and increased in the presence of Sub-MIC of CAZ Finally, an inappropriate choice of antibiotic therapy, combined with a lower concentration can in some cases stimulate the potential of clinical A. baumanii strains to form a biofilm in ventilator-associated pneumonia (VAP).