Klebsiella pneumoniae are important human pathogens, particularly as causes of nosocomial
respiratory tract infections. Intrinsically resistant to
ampicillin, in recent years K. pneumoniae strains have acquired resistance to a broad variety of extended-spectrum
cephalosporins. This resistance is most commonly mediated by the extended-spectrum
beta-lactamases (ESBLs), plasmid-mediated
enzymes that have evolved through point mutations in the genes encoding the more susceptible penicillinases TEM-1 and SHV-1. In a small minority of cases, K. pneumoniae have also been found to express extended-spectrum cephalosporin resistance by the elaboration of plasmid-mediated AmpC-type
enzymes. These mutant
enzymes confer resistance to extended-spectrum
cephalosporins,
penicillins, and in some cases cefamycins or
beta-lactam-
beta-lactamase inhibitor combinations. The most reliable and effective antimicrobial treatment of
infections caused by these strains are the
carbapenems imipenem and
meropenem.