Detection of highly active β-lactamases in Pseudomonas aeruginosa and its impact on antimicrobial susceptibility

Objective: To investigate the prevalence of derepressed hyperproduction of AmpC β-lactamase and extended-spectrum β-lactamases (ESBLs) in Pseudomonas aeruginosa and its impact on antimicrobial susceptibility. Methods: A modified three-dimensional extract method was used to detect the phenotypes of P. aeruginosa strains harboring AmpC β-lactamase and /or ESBLs. Antimicrobial susceptibility was tested by Kirby-Bauer method. Results: Highly active β-lactamases were detected in 83 (34. 4%) of 241 P. aeruginosa strains isolated in our hospital from March 2001 to October 2002. Of the 83 isolates, 7 (2.9%) produced AmpC β-lactamase, 48(19.9%) produced ESBLs, 21 (8.7%) produced both, 7 (2.9%) produced non-AmpC non-ESBLs β-lactamases. The majority of highly active (3-lactamases producing strains were isolated from sputum (56 strains, 67. 5%) and wound exudates of burn patients (23 strains, 27. 7%). Most patients were ill with severe pneumonia (34 cases, 41. 0%) , burn infections (23 cases, 27. 7%) and malignant tumors (10 cases, 12. 0%). These 83 strains were multiple-drug resistant with resistance rate of 10. 8% to imipenem and 49. 4% - 86. 7% to the other 11 antimicrobials tested. The antimicrobial resistance rates were higher than those in β-lactamase negative strains except that to imipenem (X2≥16. 1, P 0. 005). Conclusions: The most frequently detected β-lactamases in P. aeruginosa isolated from our hospital are ESBLs (28. 6%) , followed by derepressed hyperproducing AmpC β-lactamase (11.6%). β-lactamases producing strains are mainly i-solated from patients with severe pneumonia, burn infections, and maligant tumors. Antimicrobial agents should be chosen according to the results of antimicrobial susceptibility.