Subtle Imipenem Resistance In an ICU Outbreak of Acinetobacter-baumanii calcoaceticus (ACBC) Sandy J. Close, Pharm.D., BCPS, Steven J. Martin, Pharm.D., BCPS, FCCM, Eric Sahloff, Pharm.D., Diane Cappelletty, Pharm.D The Infectious Disease Research Laboratory; College of Pharmacy, The University of Toledo 2801 W. Bancroft St., Toledo, Ohio Introduction: Multi-drug resistant isolates of ACBC were collected from an outbreak in our 20 bed medical/surgical ICU. Imipenem monotherapy has been the treatment of choice for these ACBC infections. In vitro modeling uncovered the presence of subtle, underlying imipenem-resistant subpopulations in 4 antibiotic-naive ACBC isolates from this outbreak. The purpose of the present study was to determine the extent of imipenem-resistant subpopulations in ACBC isolates from our ICU to more accurately predict the likelihood of successful monotherapy with imipenem. Hypothesis/Methods: 25 consecutive isolates from nosocomial infections were collected, and baseline broth-dilution minimum inhibitory concentrations (MIC’s) for imipenem were determined. Isolates (inoculum 10 8 cfu/ml) were then plated on Mueller-Hinton agar (MHA) containing 2X and 4X the baseline MIC. Growth on antibiotic plates was evaluated at 24 and 48 hours for the presence of resistant subpopulations in the inoculum. Results: Baseline imipenem MICs ranged from 0.25 to 1  g/ml. At 24 hours, 9 (36%) isolates had growth on 2X plates (mean 9.17 x 10 4 cfu/ml, and 3 isolates on 4X plates (mean 1.63 x 10 2 cfu/ml). All growth at 24 hours had MICs 2-4X baseline. At 48 hours, 18 (72%) isolates had growth on 4X plates (mean 4.0 x10 5 cfu/ml). For 48 hour growth, 8 isolates had MICs >2X baseline, and 10 had no change in MIC. Isolates with rapid reversion to baseline MICs likely represent either unstable resistance, or drug degradation in agar at 37  C. ABSTRACT In vitro modeling confirmed the presence of underlying imipenem-resistant subpopulations in 4 antibiotic-naive ACBC isolates from this outbreak. An example of resistance selection is shown For more detail refer to Poster # 617. METHODS 25 consecutive isolates from nosocomial infections were collected, and baseline broth-dilution minimum inhibitory concentrations (MICs) for imipenem were determined. The isolates (inoculum 10 8 cfu/ml) were then grown on Mueller-Hinton agar (MHA) containing 2X and 4X the baseline MIC. Growth on antibiotic plates was evaluated at 24 and 48 hours for the presence of resistant subpopulations in the inoculum. For isolates with subpopulation growth on the antibiotic plates at 48 hours, MIC’s were re-determined, for comparison with baseline. In an additional project, four isolates from this sample were selected for additional testing in an in-vitro pharmacodynamic model. Pulse field gel electrophoresis studies are in currently in progress. M = Meropenem at 1 gram q 8 hours I = Imipenem at 500 mg q 6 hours

Over one half of the ACBC outbreak isolates demonstrated subtle but significant imipenem resistance at baseline, which is not evident in routine MIC testing. Resistance was evident after brief exposure to low drug concentrations (similar to clinical conditions). An unstable resistance pattern was observed with higher drug concentration exposure. The subtle presence of significant imipenem- resistant subpopulations during an ICU ACBC outbreak raises the concern for imipenem monotherapy in the management of these infections. Further study including combination therapies is warranted. Pulse-field gel analysis is currently being undertaken to further examine these isolates. CONCLUSIONS REFERENCES 1 BergogneBérézin E. The increasing significance of outbreaks of Acinetobacter spp.: the need for control and new agents. J Hosp Infect 1996;30(suppl): Jellison TK, McKinnon PS, Rybak MJ. Epidemiology, Resistance, and Outcomes of Acinetobacter baumanii Bacteremia Treated with Imipenem-Cilastatin or Ampicillin-Sulbactam. Pharmacotherapy 2001;21: Koeleman JGM, Parlevliet GA, Dijkshoorn L, Saveldoul PHM, Vandenbroucke-Grauls CMJE. Nosocomial outbreak of multi-resistant Acinetobacter baumanii on a surgical ward: epidemiology and risk factors for acquisition. J Hosp Infect 1997;37: Acinetobacter strains have become significant nosocomial pathogens in recent years. Intensive cares units, burn units and extended care facilities are the most common locations of these outbreaks. 1,2,3 S Susceptibilities to antimicrobial agents vary greatly from institution to institution and can rapidly change during an outbreak. The incidence of ACBC infections at our institution has been on the rise during the last two years (Figure 1). Simultaneously susceptibilities to commonly prescribed antimicrobial agents have been decreasing. Traditionally, the drug or drug regimen of choice for the treatment of these infections has been determined by evaluating the MIC data provided by the microbiology laboratory. Imipenem monotherapy has been the treatment of choice for these ACBC infections. The purpose of the present study was to determine the extent of imipenem-resistant subpopulations in ACBC isolates from our ICU to more accurately predict the likelihood of successful monotherapy with imipenem. BACKGROUND Conclusion: Nearly 75% of ACBC outbreak isolates demonstrated subtle but significant elevations in imipenem MICs at baseline, which are not evident in routine MIC testing. Resistance was evident after brief exposure to low drug concentrations. An unstable resistance pattern was observed with higher drug concentration exposure. The presence of significant imipenem- resistant subpopulations in ACBC raises the concern for imipenem monotherapy in the management of these infections. 48 hour growth: 8 isolates had MICs  2X baseline 10 isolates had no change in MIC from baseline Baseline24 hours48 hours MIC (mcg/ml) range MIC 90 (mcg/ml) Growth: 2X MIC--9/25 (36%)24/25 (96%) Growth 4X MIC--3/25 (12%)18/25 (72%) RESULTS Preliminary Pulse-Field Gel Lanes 1-3 = Different ACBC Strains Lane 4 = Insufficient DNA to Assess Lane 5 = Control