Rarity of transferable -lactamase production by Klebsiella species

Klebsiella species normally produce a chromosomally encoded group 2b, or molecular class A, -lactamase. In most strains of Klebsiella pneumoniae the enzyme has an isoelectric point (pI) of 7.6, identical to that of the plasmidencoded -lactamases SHV-1 and SHV-2, and is assumed to be SHV-1. This -lactamase is transposon-encoded and it has been postulated that it might have arisen as a klebsiella gene that has been transposed relatively recently on to plasmids. A minority of strains of K. pneumoniae produce chromosomal -lactamases with other pIs, mostly in the range 7.1–8.6. Although it is not certain that the chromosomal pI 7.6 enzymes are identical to SHV-1 since DNA or protein sequences have not been reported, they are probably closely related to it since the DNA sequence of one K. pneumoniae chromosomal -lactamase (LEN-1, pI 7.0–7.1) had 90% homology with SHV-1 and isolates of K. pneumoniae with chromosomally determined pI 7.6 -lactamases hybridized weakly with an SHV-specific DNA probe. Klebsiella oxytoca also produces chromosomal group 2b enzymes, but they are distinct from those of K. pneumoniae, with 40% homology with SHV-1. The chromosomal -lactamases of Klebsiella species are usually produced in small amounts and confer relatively low degrees of resistance to compounds such as ampicillin and carbenicillin. A few strains of K. pneumoniae and rather more (approximately 10%) of K. oxytoca hyperproduce the chromosomal enzymes. Hyperproducing strains of K. oxytoca are said by Livermore to have a characteristic antibiogram, being resistant to all penicillins apart from temocillin, resistant to cefuroxime and aztreonam (MICs 32 mg/L), moderately resistant to cefotaxime and ceftriaxone (MICs 4–32 mg/L), but normally susceptible to ceftazidime. Hyperproduction in both laboratory mutants and clinical isolates of K. oxytoca is attributable to mutations in the promoter region of the -lactamase gene. Klebsiella spp. can produce plasmid-encoded -lactamases in addition to their chromosomal enzymes. Indeed, K. pneumoniae is the species in which plasmid-encoded, extended-spectrum -lactamases are most commonly reported. Although there have been several surveys of lactamases in Klebsiella spp., chromosomally and plasmid-borne -lactamases with isoelectric points of 7.6 have not always been clearly differentiated. We have therefore conducted a survey of -lactamases and their transferability to Escherichia coli in Klebsiella spp. isolated from blood over the past four years.