High rate of colistin dependence in Acinetobacter baumannii

Sir, Colistin is currently considered the last treatment option for MDR Acinetobacter infections. Although the current resistance rates to colistin are relatively low, they are increasing and threaten to become a serious problem worldwide. In this study, we report the in vitro development of colistin dependence in many colistin-susceptible Acinetobacter baumannii isolates after exposure to colistin. Initially, we performed population analysis for some colistinsusceptible isolates collected from two different hospitals, Samsung Medical Center (Seoul, Korea) and Samsung Changwon Hospital (Changwon, Korea). We plated 50 mL of bacterial cell suspension ( 10 cfu/mL) and its serial dilutions on Mueller–Hinton agar containing 0, 0.5, 1, 2, 3, 4, 5, 6, 8 and 10 mg/L colistin sulphate. Surviving colonies at 10 mg/L were plated on Mueller– Hinton agar with discs of 10 mg colistin. We found that the bacteria grew only near the disc (Figure 1a), which indicated colistin dependence. Next, we performed colistin susceptibility testing for 170 A. baumannii isolates collected from ICUs of Korean hospitals in 2015 and examined colistin-susceptible isolates for colistin dependence by using population analysis and the colistin disc method. Among 170 A. baumannii isolates, 21 (12.4%) were resistant to colistin (MIC90, 4 mg/L; MIC50, 1 mg/L). Among 149 colistinsusceptible isolates, surviving colonies on agar with 10 mg/L colistin were observed in 96 isolates (64.4%). In addition, we found that 49 colistin-susceptible A. baumannii isolates (32.9%) developed colistin dependence. These isolates were obtained from sputum (23 isolates, 46.9%), blood (17 isolates, 34.7%), wounds (5 isolates, 10.2%) and other locations. Nearly all of the original isolates that developed colistin dependence after exposure to colistin were resistant to imipenem, meropenem, ciprofloxacin, ceftazidime, cefepime, gentamicin, trimethoprim/sulbactam and piperacillin/tazobactam (87.8%–98.0%). Twenty-eight isolates (57.1%) were resistant to ampicillin/sulbactam. Tigecycline non-susceptibility was identified in nine isolates (18.4%). However, colistin-dependent mutants were resistant to polymyxins, tetracycline, ciprofloxacin and piperacillin/tazobactam, but were susceptible to the others, including carbapenems. The 49 isolates developing colistin dependence showed six STs in MLST using the Oxford scheme: ST208 (28 isolates, 57.1%), ST191 (12 isolates, 24.5%), ST368 (6 isolates, 12.2%), ST435 (1 isolate), ST451 (1 isolate) and ST1141 (1 isolate). These STs belong to the global clone 2 and have been identified frequently in Korea. ST208 was identified most frequently among the isolates that developed colistin dependence, although ST191 has recently been identified as the most frequently found carbapenemresistant A. baumannii clone in Korea. The results of PFGE showed that the colistin-dependent mutants originated from their corresponding parental isolates (Figure 1b). Colistin was administered to 45 patients with A. baumannii infection. Among the A. baumannii isolates obtained from these patients, 18 developed colistin dependence, while the remainder did not. We compared 3 and 7 day treatment failure (defined as a death or persistence of bacteria) in patients with and patients without colistin-dependent strains: 66.7% versus 37.0% (3 day; P1⁄40.051) and 44.4% versus 25.9% (7 day; P1⁄40.197). Colistin dependence was previously reported in an isolate of A. baumannii–Acinetobacter calcoaceticus complex. In that study, only 1 of 19 isolates developed colistin dependence. Partial colistin dependence was recently identified in some LPS-deficient strains with mutations in lpxA, lpxC and lpxD. In addition to those studies, our study showed that many colistin-susceptible A. baumannii strains develop colistin dependence after exposure to colistin. Although the clinical implication of colistin dependence remains unclear, the high rate of treatment failure in patients with A. baumannii isolates that develop colistin dependence is notable. Research letters