In vitro Activities of Colistin and Ampicillin/Sulbactam against Acinetobacter baumannii
Warunee Punpanich MD, PhD*, Anurak Munsrichoom BSc, MSc**, Suchada Srisarang BSc (Statistics)***, Vipa Treeratweeraphong BSc, MSc**
Affiliation : * Department of Pediatrics, Queen Sirikit National Institute of Child Health, College of Medicine, Rangsit University, Bangkok, Thailand ** Division of Microbiology, Queen Sirikit National Institute of Child Health, Bangkok, Thailand *** Pediatric Infectious Disease Research Unit, Queen Sirikit National Institute of Child Health, Bangkok, Thailand
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 MIC E-test, respectively.
The colistin MIC50 and MIC90 for A. baumannii were 0.38 and 1 μg/mL, respectively. The ampicillin/sulbactam MIC50 and
MIC90 were 16 and 89.6 μg/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.
Keywords : In vitro susceptibility, Acinetobacter baumannii, Multi-drug resistance, Colistin, Ampicillin/Sulbactam
