In vitro activity of colistin against multidrug-resistant Pseudomonas aeruginosa isolates from patients in Songklanagarind Hospital, Thailand.

Multidrug-resistant Pseudomonas aeruginosa (MDR-PA) infection creates problems for therapy. Previous studies have found MDR-PA is susceptible to colistin. We studied the in vitro susceptibility of MDR-PA to colistin and determined the minimum inhibitory concentration (MIC). One hundred MDR-PA isolates were obtained from patients at Songklanagarind Hospital, in southern Thailand, during January 2008-March 2011. Antimicrobial susceptibilities to amikacin (AK), ceftazidime (CAZ), ciprofloxacin (CIP), imipenem (IMP) and colistin (CO) were tested by standard disk diffusion method. The antimicrobial susceptibility to colistin and the MIC were determined with the E-test. The MDR-PA isolates were susceptible to ceftazidime, ciprofloxacin, amikacin and imipenem in 1, 5, 11 and 32%, respectively. There were 5 antimicrobial resistance patterns of MDR-PA: AK-CAZ-CIP-IMP (50%), AK-CAZ-CIP (32%), CAZ-CIP-IMP (11%), AK-CAZ-IMP (6%) and AK-CIP-IMP (1%). Colistin had good efficacy against MDR-PA (98% susceptibility rate). The MIC50 and MIC90 for colistin were 1.0 and 1.5 jig/ml, respectively. Only 2 MDR-PA isolates were resistant to colistin with the MICs of 3 and 12 microg/ml, respectively. The majority of MDR-PA isolates remained susceptible to colistin; therefore, colistin is a good option for treatment of MDR-PA.