EXTENDED-SPECTRUM BETA-LACTAMASES AND OTHER ENZYMES PROVIDING RESISTANCE TO OXYIMINO-BETA-LACTAMS

Since the oxyimino-beta-lactams cefotaxime, ceftazidime. ceftriaxone, and aztreonam were introduced, strains of Klebsiella pneumoniae, Esche richia cell, and other gram-negative pathogens have appeared that are resistant to these antibiotics. Some resistant strains produced extend ed-spectrum beta-lactamases that are derived from common plasmid-media ted TEM-, SHV-, or OXA-type enzymes by one or more amino acid substitu tions that enhance the affinity and hydrolytic capacity of the parenta l beta-lactamases. Other resistant strains have acquired plasmids enco ding AmpC-type beta-lactamases that are normally determined by chromos omal genes in other genera. Current criteria for detecting this type o f resistance underestimate its prevalence and need to be modified. Oxy imino-beta-lactam resistant isolates are usually susceptible to carbap enems, such as imipenem and meropenem, but plasmid-mediated carbapenem resistance has already appeared.