Phenotypic detection and polymerase chain reaction screening of extended-spectrum β-lactamases produced by Pseudomonas aeruginosa isolates.

A growing number of β-lactamases have been reported in Pseudomonas aeruginosa isolates. The aims of this study were to survey the types of extended-spectrum β-lactamases (ESBLs) by polymerase chain reaction (PCR), to evaluate the reliability of phenotypic tests for ESBLs, and to identify the clonal distribution by pulsed-field gel electrophoresis (PFGE) among P. aeruginosa isolates resistant to expanded-spectrum cephalosporins (ceftazidime, aztreonam, or cefepime). The antimicrobial susceptibility of 57 P. aeruginosa isolates from blood specimens were examined according to the recommendations of the Clinical Laboratory Standards Institute. ESBL phenotypes were determined by using cloxacillin-containing double disc synergy test (DDST). The existence of 11 β-lactamase genes was detected by PCR. Of the 57 P. aeruginosa isolates, 35 (61.4%) isolates were PCR-positive for β-lactamase genes. Twelve of 35 isolates were PCR-positive for combination of ampC and ESBL genes, including TEM, GES, SHV, VEB and OXA-I genes. The sensitivity and specificity of cloxacillin-containing DDST (using the criteria of ceftazidime zone diameter increased ≧5 mm) were 84.1% and 54.5%, respectively. Nine clusters were classified among 35 PCR-positive isolates by PFGE. Isolates of clusters B and C were distributed in different wards of this hospital during a period of 3-4 years. ESBL genes are not uncommon in P. aeruginosa isolates. Cloxacillin-containing DDST can enhance the sensitivity and has a potential role for phenotypic detection of ESBL-producing P. aeruginosa, and PCR is also helpful for the identification of specific β-lactamase genes. These P. aeruginosa isolates were classified into several diverse clones which could continue to spread in the hospital over a long period of time.