engleski

Effect of Subinhibitory Concentrations of Antibiotics on the Morphology and Serum Sensitivity of Wild-Type Pseudomonas aeruginosa Strains

Antibiotics are often present at the site of infection at subinhibitory concentrations, because in human treated with intermittent dosage schedules of antibiotics, suprainhibitory concentrations will always be followed by subinhibitory levels. The aim of this study was to determine the effect of subminimal inhibitory concentrations (subMICs) of ceftazidime, meropenem, ciprofloxacin and gentamicin on the serum sensitivity and morphology of Pseudomonas aeruginosa strains isolated from a variety of isolation sites. A total of 20 serum-resistant P. aeruginosa strains isolated from different specimens (urine, bronchial secretions, endotracheal and wound aspirates) were selected. The MICs of antibiotics were determined for each investigated strain by a standard microdilution technique using Mueller-Hinton broth medium according to the methods of the National Committee for Clinical Laboratory Standards. Bacteria were exposed to 1/2, 1/4, 1/8, 1/16 and 1/32 of the MIC of each antibiotic tested as described previously (Sandberg et al., Rev Infect Dis 1979 ; 1:838-44). Sensitivity of P. aeruginosa strains to the bactericidal action of normal human serum before and after bacterial exposure to subMICs of antibiotics was determined. Bacterial susceptibility to serum killing was measured by assessing regrowth after incubation of bacteria in normal human serum according to Schiller and Hatch method (Diagn Microbiol Infect Dis 1983 ; 1:145- 7). Morphological changes of bacteria after exposure to subMICs of antibiotics were detected by using a phase-contrast microscopy. Significant difference in serum sensitivity of the strains was observed after the bacteria were exposed to subMICs of ceftazidime, meropenem and ciprofloxacin, while the exposure to subMICs of gentamicin did not affect the resistance of investigated strains to the bactericidal action of the serum. Upon comparison of the number of serum- resistant strains before and after exposure to subMICs of antibiotics, statistically significant differences were determined after exposure of the strains to 1/2, 1/4, 1/8 and 1/16 MIC of meropenem, after exposure of the strains to 1/2, 1/4 and 1/8 MIC of ceftazidime, and after exposure of the strains to 1/2 and 1/4 MIC of ciprofloxacin (p<0.01). SubMICs of antibiotics alter not only bacterial susceptibility to serum killing but also their morphology. Morphological changes were most prominent after bacterial exposure to 1/2, 1/4 and 1/8 MIC of ceftazidime and meropenem. Higher concentrations of those antibiotics caused formation of very long filaments. Moreover, higher concentrations of meropenem beside filamentous forms of bacterial cells formed also irregular swelling of such filaments. No filamentous forms or any morphological changes were observed when the investigated strains were exposed to subMICs of gentamicin. The alteration in bacterial morphology caused by subMICs of ceftazidime, meropenem and ciprofloxacin was related to subsequent bacterial sensitivity to serum bactericidal activity.

antibiotics; subinhibitory concentrations; Pseudomonas aeruginosa; serum sensitivity

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