In vitro activities of colistin and ampicillin/sulbactam against Acinetobacter baumannii.

OBJECTIVE: The aim of the present study was to examine the in vitro antimicrobial activity of colistin, and ampicillin/sulbactam against A. baumannii isolated from pediatric patients and to compare the susceptibility testing using disc diffusion with minimal inhibitory concentration (MIC) E-test method.

MATERIAL AND METHOD: One hundred strains of A. baumannii from various clinical isolates were included in the present study. Antimicrobial susceptibilities of A. baumannii to colistin, ampicillin/sulbactam were determined by disc diffusion and minimal inhibitory concentration (MIC) using E-test method. The analysis was stratified by carbapenem resistance status. Sensitivity and specificity of the disc diffusion test compared to the MIC E-test were estimated.

RESULTS: Ninety-seven strains of all isolates (97%) were sensitive to colistin using both disc diffusion and E-test methods. In contrast, 41% and 34% of the isolates were sensitive to ampicillin/sulbactam by disc diffusion and MICE-test, respectively. The colistin MIC50 and MIC90 for A. baumannii were 0.38 and 1 microg/mL, respectively. The ampicillin/sulbactam MIC50 and MIC90 were 16 and 89.6 microg/mL, respectively. Based on the results of MIC E-test, ninety-eight (n = 49) and six (n = 3) percent of carbapenem-resistant A. baumannii (n = 50) were susceptible to colistin and ampicillin/sulbactam, respectively. Sensitivity and specificity of disc diffusion test compared to MIC E-test were 99% and 66.7% for colistin and 80.5% and 98.3% for ampicillin/sulbactam, respectively.

CONCLUSION: The antimicrobial activities of colistin against A. baumannii isolates remained high for both cabapenem-susceptible and -resistant strains. However, the in vitro activity of ampicillin/sulbactam against A. baumannii was low. Thus, a combination, rather than monotherapy, of ampicillin/sulbactam with other antibiotics is strongly recommended when dealing with A. baumannii infection. In addition, disc diffusion test appeared to be a useful screening method for susceptibility testing for colistin and ampicillin/sulbactam against A. baumanii.