AN ALLELIC VARIANT OF THE CHROMOSOMAL GENE FOR CLASS-A BETA-LACTAMASEK2, SPECIFIC FOR KLEBSIELLA-PNEUMONIAE, IS THE ANCESTOR OF SHV-1

Fecal Klebsiella isolates from neonates in 22 Swedish special care uni ts 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. pneumoni ae reference strains (including the K. pneumoniae subsp. pneumoniae, o zaenae, and rhinoscleromatis type strains) tested were positive, where as all 67 K. oxytoca isolates and the K. oxytoca, K. planticola, and K . terrigena type strains tested were negative. Resistance to beta-lact ams in K. pneumoniae was not transferable by conjugation, and the beta -lactamase gene was never found on a plasmid. Southern blot analysis s howed that the gene had a defined chromosomal location. Isoelectric fo cusing and sequencing of 231-bp PCR amplicons from different isolates revealed many variants of the enzyme, with the two main groups being S HV-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 rathe r than cephalosporin resistance), pIs, and sequence data showed that t he chromosomal beta-lactamase of K. pneumoniae is a class A, group 2 e nzyme distinct from the chromosomal AmpC enzymes found in several othe r gram-negative bacteria and from the chromosomal beta-lactamase K1 of K. oxytoca. We propose that the chromosomal beta-lactamase of K. pneu moniae 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.