High-Level Expression of Chromosomally Encoded SHV-1 β-Lactamase and an Outer Membrane Protein Change Confer Resistance to Ceftazidime and Piperacillin- Tazobactam in a Clinical Isolate of Klebsiella pneumoniae

Abstract

ABSTRACT We describe Klebsiella pneumoniae 15571, a clinical isolate resistant to ceftazidime MIC = 32 μg/ml) and piperacillin-tazobactam (MICs = 1,024 and 128 μg/ml). K. pneumoniae 15571 expresses a single β-lactamase with a pI of 7.6. However, when cloned in a high-copy-number vector in Escherichia coli , this bla SHV-1 gene did not confer resistance to ceftazidime, a spectrum consistent with the nucleotide sequence, which was nearly identical to those of previously described bla SHV-1 genes. Outer membrane protein (OMP) analysis of K. pneumoniae 15571 revealed a decrease in the quantity of a minor 45-kDa OMP in comparison to that in K. pneumoniae 44NR, a low-level ampicillin-resistant strain that also expresses a chromosomally determined bla SHV-1 . Crude β-lactamase enzyme extracts from K. pneumoniae 15571 produced roughly 200-fold more β-lactamase activity than K. pneumoniae 44NR. Northern hybridization analysis revealed that this difference was explainable by quantifiable differences in transcription of the bla SHV-1 gene in the two strains. Primer extension analysis of bla SHV-1 mRNA from K. pneumoniae 15571 and 44NR indicated that the transcriptional start sites were identical in the two strains. DNA sequencing of the promoter regions upstream of the of bla SHV-1 open reading frames in the two K. pneumoniae strains revealed an A→C change in the second position of the −10 region in K. pneumoniae 44NR compared to that in 15571. Site-directed mutagenesis of the cloned K. pneumoniae 15571 bla SHV-1 , in which the A in the second position of the 15571 −10 region was changed to a C, resulted in a substantial lowering of the MIC of ampicillin. When the levels of β-lactamase enzyme expression in E. coli were compared, the bla SHV-1 downstream of the altered −10 region produced 17-fold less β-lactamase enzyme. These results indicate that elevated levels of ceftazidime resistance can result from a combination of increased enzyme production and minor OMP changes and that levels of chromosomally encoded SHV-1 β-lactamase production can vary substantially with a single-base-pair change in promoter sequence.