An allelic variant of the chromosomal gene for class A beta-lactamase K2, specific for Klebsiella pneumoniae, is the ancestor of SHV-1.

Fecal Klebsiella isolates from neonates in 22 Swedish special care units were examined by a PCR we developed for detection of the SHV-1 beta-lactamase gene. All 105 K. pneumoniae isolates and all 11 K. pneumoniae reference strains (including the K. pneumoniae subsp. pneumoniae, ozaenae, and rhinoscleromatis type strains) tested were positive, whereas all 67 K. oxytoca isolates and the K. oxytoca, K. planticola, and K. terrigena type strains tested were negative. Resistance to beta-lactams in K. pneumoniae was not transferable by conjugation, and the beta-lactamase gene was never found on a plasmid. Southern blot analysis showed that the gene had a defined chromosomal location. Isoelectric focusing and sequencing of 231-bp PCR amplicons from different isolates revealed many variants of the enzyme, with the two main groups being SHV-1 like (pI 7.6; 68 isolates) and LEN-1 like (pI 7.1; 14 isolates). Clavulanic acid markedly reduced the MICs of ampicillin for all the K. pneumoniae isolates tested. This fact, MIC profiles (penicillin rather than cephalosporin resistance), pIs, and sequence data showed that the chromosomal beta-lactamase of K. pneumoniae is a class A, group 2 enzyme distinct from the chromosomal AmpC enzymes found in several other gram-negative bacteria and from the chromosomal beta-lactamase K1 of K. oxytoca. We propose that the chromosomal beta-lactamase of K. pneumoniae be designated K2 and suggest that an allelic pI 7.6 variant of this enzyme is the ancestor of the SHV family of plasmid-mediated beta-lactamases.