In vitro activities of colistin, imipenem and ceftazidime against drug-resistant Pseudomonas aeruginosa and Acinetobacter baumannii isolates in the south of Iran.

This in vitro study performed on a collection of non-fermenting Gram-negative bacilli (NFB) consist of 18 A. baumannii and 21 P. aeruginosa isolates. Non-duplicated isolates (one per patient) were isolated from blood, endotracheal tube and sputum samples of hospitalized patients in the south of Iran. The minimum inhibitory concentrations (MICs) of each isolate was determined using Epsilometer (E)-test strips containing colistin, imipenem, and ceftazidime. In overall, all A. baumannii isolates were non-susceptible to imipenem and ceftazidime. In contrast, all isolates were susceptible to colistin with MIC50 and MIC90 of 0.75/1.5 µg/mL, respectively. Antibiotic susceptibility results showed that 81% and 23.8% of P. aeruginosa isolates were susceptible to ceftazidime and imipenem, respectively. While, all of the P. aeruginosa isolates were susceptible to colistin with MIC50 and MIC90 of 0.5/1 µg/mL, respectively. In summary, colistin showed the promising in vitro activity against drug-resistant strains of two clinically important NFB in our region. However, investigation on a larger collection of drug-resistant strains demands to support these observations in the near future.