Evaluation of the Cephalosporins, Cefepime, Cefpirome and Ceftazidime, against Clinical Isolates of Imipenem-Resistant Pseudomonas aeruginosa.

The level of resistance to imipenem in our institution has increased significantly in recent years, particularly in isolates of Pseudomonas aeruginosa. At present, 30% to 40% of P. aeruginosa isolated in clinical samples are imipenem resistant. The objective of this study was to test therapeutic alternatives within the class of beta-lactam antibiotics in the treatment of infections caused by imipenem-resistant strains of P. aeruginosa (IRPA). We tested 160 isolates of IRPA collected consecutively from in patients at Hospital São Paulo-UNIFESP (Federal University of São Paulo). Using the E-test method, these isolates were tested for cefepime, cefpirome, and ceftadizime susceptibility or resistance. The E-test was also used to confirm the resistance to imipenem. One group of samples was studied epidemiologically using the pulsed-field gel electrophoresis (PFGE) method to determine how this type of resistance spread in our institution. Cefepime was the most active antibiotic (MIC50, 24 µg/mL; MIC(90), 64 µg/mL) and only 57 (35.6%) of the samples showed cross-resistance between imipenem and this fourth-generation cephalosporin. The number of samples that were highly resistant (MIC(90), >256 µg/mL) to cefepime, ceftazidime or cefpirome was 8 (5.0%), 22 (13.8%) and 38 (23.8%), respectively. Molecular typing revealed the presence of 9 molecular profiles among 26 IRPA samples tested, suggesting that selection of resistant mutants occurred in each patient. However, identical molecular profiles were also found in more than one patient. This study showed that cefepime is the most effective cephalosporin against the IRPA samples isolated in our institution. In addition, we conclude that P.areuginosa resistant strains are selected in each patient, but patient to patient transmission also occurs.