Antimicrobial susceptibility of Acinetobacter baumannii isolated from hospital patients

Abstract

The incidence of multidrug-resistant Acinetobacter baumannii (MDR A. baumannii) is increasing worldwide and is leading to therapeutic problems. We investigated the in vitro activities of cefoperazone/sulbactam, colistin, imipenem, and rifampicin alone and in double combinations against 100 A. baumannii isolates from patients at Songklanagarind Hospital in Songkhla Province, Thailand. The E-test method was used to determine antimicrobial susceptibility, the minimal inhibitory concentration (MIC) and for antimicrobial combination testing. A. baumannii isolates were susceptible to colistin (97%), cefoperazone/sulbactam (69%), imipenem (45%), and rifampicin (13%). Fifty-nine percent of them were MDR A. baumannii. Colistin was superior to cefoperazone/sulbactam, rifampicin and imipenem against MDR A. baumannii and the MIC50, MIC90 of colistin were 0.75 and 1 μg/ml, respectively. Non-MDR A. baumannii isolates were susceptible to cefoperazone/sulbactam (100%), colistin (95%), imipenem (93%) and rifampicin (2%). Combinations of cefoperazone/sulbactam plus colistin or rifampicin, imipenem plus colistin or rifampicin and colistin plus rifampicin showed indifferent effects against most MDR isolates. Of all the antimicrobial combinations tested, cefoperazone/sulbactam plus rifampicin produced the highest percentages (42%) of synergy, partial synergy, and additive results. The activity rate of cefoperazone/sulbactam against MDR A. baumannii was higher when combined with rifampicin than colistin. Thus colistin had the greatest activity against most MDR and non-MDR A. baumannii isolates among all of the antibiotics tested. Cefoperazone/sulbactam and imipenem showed good activity against non-MDR isolates, and cefoperazone/sulbactam combined with rifampicin may be useful in treating infections caused by MDR isolates.