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, Escherichia cell, and other gram-negative pathogens have appeared that are resistant to these antibiotics. Some resistant strains produced extended-spectrum beta-lactamases that are derived from common plasmid-mediated TEM-, SHV-, or OXA-type enzymes by one or more amino acid substitutions that enhance the affinity and hydrolytic capacity of the parental beta-lactamases. Other resistant strains have acquired plasmids encoding AmpC-type beta-lactamases that are normally determined by chromosomal genes in other genera. Current criteria for detecting this type of resistance underestimate its prevalence and need to be modified. Oxyimino-beta-lactam resistant isolates are usually susceptible to carbapenems, such as imipenem and meropenem, but plasmid-mediated carbapenem resistance has already appeared.