Evolution of plasmid-coded resistance to broad-spectrum cephalosporins.

A clinical isolate of Klebsiella ozaenae with transferable resistance to broad-spectrum cephalosporins produces a beta-lactamase determined by plasmid pBP60. The beta-lactamase had the same isoelectric point as SHV-1 (7.6). From heteroduplex analysis, an extensive homology between the two bla genes could be deduced; therefore, the new beta-lactamase was designated SHV-2. Enzymatic studies revealed that SHV-2 was able to hydrolyze broad-spectrum cephalosporins due to an increased affinity of these compounds for the enzyme. The assumption that SHV-2 is a natural mutant of SHV-1 was strongly supported by the isolation of a laboratory mutant of SHV-1 that showed activities similar to those of SHV-2.