ANTIBIOTIC-RESISTANCE AND SEROTYPES OF CLINICAL ISOLATES OF PSEUDOMONAS-AERUGINOSA IN RIYADH

The in vitro activities of 10 antipseudomonal antibiotics were tested against recent clinical isolates, of Ps. aeruginosa isolated from vari ous clinical specimens received at the King Khaled University Hospital (KKUH), in Riyadh, Saudi Arabia, Ciprofloxacin, amikacin or tobramyci n, and imipenem exhibited the highest activity with resistance rates o f 4, 6, and 9% respectively. Piperacillin, ceftazidime, gentamicin, an d aztreonam showed respective resistance rates of 17, 19, 20 and 21%, whereas 66% of the isolates were resistant to cefotaxime which exhibit ed complete cross-resistance with ceftriaxone (90%). Ps. aeruginosa is olates were further serotyped by a slide agglutination test with comme rcial antisera. A total of 13 serotypes were defined, though 54% of th e isolates were nontypable. The most frequent serotypes were 0:6 0:10, and 0:11. There was no apparent correlation between serotypes and ant ibiotic resistance markers, nor to the origin of the clinical specimen s. The majority of the non-typable isolates resistant to ciprofloxacin , extended spectrum Beta lectams or aminoglycoside antibiotics, howeve r, were frequently isolated from urine and sputa and to a lesser exten t from general swab specimens. Moreover, analysis of spontaneously sel ected Ps. aeruginosa variants resistant to ceftazidime (I mg mL(-1)), revealed that their MICs against all Beta lactam agents, except imipen em, were about 4 to 64-fold higher than those of corresponding wild-se rotypes; presumably due to selection of stably derepressed chromosomal ly mediated (class I) beta lactamase. Resistance due to impermeability seems unlikely, since in each of these variants the MICs against amin oglycoside 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 conserva tive approach to antibiotic therapy.