Abstract
The in vitro activities of 10 antipseudomonal antibiotics were tested against recent clinical isolates, of Ps. aeruginosa isolated from various clinical specimens received at the King Khaled University Hospital (KKUH), in Riyadh, Saudi Arabia, Ciprofloxacin, amikacin or tobramycin, and imipenem exhibited the highest activity with resistance rates of 4, 6, and 9% respectively. Piperacillin, ceftazidime, gentamicin, and aztreonam showed respective resistance rates of 17, 19, 20 and 21%, whereas 66% of the isolates were resistant to cefotaxime which exhibited complete cross-resistance with ceftriaxone (90%). Ps. aeruginosa isolates were further serotyped by a slide agglutination test with commercial antisera. A total of 13 serotypes were defined, though 54% of the isolates were nontypable. The most frequent serotypes were 0:6 0:10, and 0:11. There was no apparent correlation between serotypes and antibiotic resistance markers, nor to the origin of the clinical specimens. The majority of the non-typable isolates resistant to ciprofloxacin, extended spectrum Beta lectams or aminoglycoside antibiotics, however, were frequently isolated from urine and sputa and to a lesser extent from general swab specimens. Moreover, analysis of spontaneously selected Ps. aeruginosa variants resistant to ceftazidime (I mg mL(-1)), revealed that their MICs against all Beta lactam agents, except imipenem, were about 4 to 64-fold higher than those of corresponding wild-serotypes; presumably due to selection of stably derepressed chromosomally mediated (class I) beta lactamase. Resistance due to impermeability seems unlikely, since in each of these variants the MICs against aminoglycoside antibiotics or ciprofloxacin remained almost unchanged. The clinical significance of these findings is important in the selection of appropriate empiric treatment of serious Ps. aeruginosa infections in hospitalised patients and emphasises the enforcement of a conservative approach to antibiotic therapy.