Phenotypic Detection and Polymerase Chain Reaction Screening of Extended-spectrum β-lactamases Produced by Pseudomonas Aeruginosa Isolates

Background/purpose: 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).

Methods: 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.

Results: 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.

Conclusion: 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.