Detection of CTX-M-β Lactamases in Isolated Klebsiella pneumoniae

Organisms producing CTX-M β-lactamases are emerging as a source of resistance to oxyiminocephalosporins such as ceftriaxone and ceftazidime. However, the laboratory detection of these strains is not well defined. In this study, phenotypic assay for screening of extended-spectrum β-lactamases producing strains and molecular assay for the identification of CTX-M β-lactamases genes was developed and used to investigate the prevalence of these enzymes among clinical isolates of Klebsiella pneumoniae in three general hospitals of Tehran, Iran. Phenotypic detection was used for screening of isolates by agar dilution method. A decrease of ≥3 doubling dilution in an MIC for either ceftriaxone or ceftazidime tested in combination with 4 mg/l clavulanic acid (prepared from Glasco Smith company) versus its MIC when tested alone, confirmed an ESBL-producing organism. The PCR assay consisted of four primer sets. In initial screening test, 117 (69%) from 168 clinical isolates were positive and 51 isolates (31%) were negative. From the positive isolates, 96 isolates were positive in phenotypic confirmatory test. Using molecular assay, 117 strains potentially producing extended-spectrum-β-lactamases were examined for the presence of CTX-M enzymes. 88 strains (75.2%) were positive for blactx-m group І genes, 1 strain (0.85%) was positive for blactx-m group ІІІ genes, and 2 strains (1.7%) were positive for blactx-m group ІV. The prevalence of extended-spectrum β-lactamases (ESBLs) are increasing significantly in hospitals of Tehran. In other side, we found that the CTX-M І group had the most prevalence than other CTX-M groups.