Activity of tigecycline on planktonic and sessile cells of Acinetobacter baumannii

Acinetobacter baumannii is an important pathogen, capable of survival for very long periods on various surfaces in the hospital environment. Tigecycline is a commonly used antimicrobial agent especially for the treatment of resistant infections. The aim of this study was to investigate the activity of tigecycline on both planktonic and sessile biofilm cells of A. baumannii strains isolated from blood cultures and to compare the efficiency in terms of biofilm synthesis. Tigecycline activity on 59 A. baumannii strains was examined by agar dilution technique. The ability of strains to form biofilm was evaluated by adherence on polystyrene surfaces in brain heart infusion broth supplemented with 0.25% glucose. Time-kill technique was used for determination of the time and concentration dependent activity of tigecycline on biofilm positive and negative strains. The planktonic cells in logarithmic growth phase were exposed to tigecycline at 0.5, 1, 2, 4, ve 8 x minimum inhibitory concentration (MIC) concentrations and colony counts were evaluated after 0, 2, 4, 6, 24 and 48 hours. The effect of tigecycline on sessile cells was studied on biofilm matrix composed around plastic beads. Tigecycline susceptibility rate of planktonic cells was 89.8% and MIC50 and MIC90 values were 1 and 2 microg/ml, respectively. Biofilm formation was detected in 52.5% of isolates and no significant correlation was found between MIC values and biofilm production of the strains (p > 0.05). Tigecycline showed a potent antibacterial activity against planktonic cells regardless of biofilm forming capability of strains. Biofilm inhibitory concentrations of sessile cells were elevated significantly. As a result, tigecycline showed a potent activity on planktonic A. baumannii cells however, the effect was decreased significantly on sessile cells in biofilm environment. The results suggest that, the possibility of decreased sensitivity of cells in biofilm environment should be considered as well as antibiotic sensitivity test results during the treatment of infections caused by A. baumannii.