IN VITRO ACTIVITY OF IMIPENEM AND CEFTAZIDIME AGAINST MUCOID AND NON-MUCOID STRAINS OF PSEUDOMONAS AERUGINOSA ISOLATED FROM PATIENTS IN IRAN

Background . Mucoid strains of Pseudomonas aeruginosa are surrounded by a capsule composed of β-1, 4 linked D-mannuronic acid and L-guluronic acid. The capsule provides a protective barrier against antimicrobial agents and the immune system, and is an important pathogen in immunocompromised hosts. In this study, we compared the activities of imipenem and ceftazidime against mucoid and non-mucoid clinical strains of P. aeruginosa. Methods . From April to September 2000, one-hundred and thirty-three P. aeruginosa cultures were isolated from hospitalized patients in Tehran. In addition to determination of the minimum inhibitory concentrations (MICs) of two antipseudomonal antibiotics, ceftazidime and imipenem, against these isolates, mucoid strains were selected by capsule staining. Mucoid phenotype was confirmed by precipitation of alginate in cold ethanol. Data were analyzed by the Chi-square method. Results . More than 70.6% of the strains were sensitive to imipenem, which inhibited most strains at an MIC of 4 µg/mL. However, more than 72.9% of strains were resistant to ceftazidime, and only concentrations of 128 µg/mL or more inhibited their growth. There was a statistically significant relationship between the presence of the alginate capsule and resistance against ceftazidime. Such a relation did not exist for imipenem. Conclusion . Considering the extensive use of ceftazidime in Iran, performing susceptibility tests, as well as determining the mucoid phenotype in strains of P. aeruginosa is strongly recommended prior to antimicrobial therapy in immunocompromised patients.