beta-Lactamase of Pseudomonas pseudomallei and its contribution to antibiotic resistance.

beta-Lactamase production was examined in nine strains of Pseudomonas pseudomallei isolated from human, animal and environmental sources in Thailand and Hong Kong. All produced the same weakly inducible, membrane associated chromosomal cephalosporinase, which had a molecular weight of 29,500 and an isoelectric point of 7.4-7.7. The enzyme resembled the cefuroximases of Ps. cepacia and Proteus vulgaris, but differed from the Class I cephalosporinases typical of Ps. aeruginosa and most enterobacteria, in being strongly active against carbenicillin, cefotaxime and cefuroxime and in being inactivated readily by clavulanic acid. Synergy experiments with clavulanic acid investigated the enzyme's contribution to antibiotic resistance, and these results broadly correlated with those of in-vitro hydrolysis assays. Thus, ampicillin, carbenicillin, cefoperazone, cefotaxime, cefuroxime and cephalothin, which were hydrolysed in vitro, were potentiated four to 64-fold by 2 mg/l clavulanic acid; but cefoxitin, ceftazidime, cloxacillin and imipenem, which appeared stable in vitro, were potentiated four-fold or less.