944Outbreak of Colistin-Resistant Acinetobacter baumannii in Intensive Care Units in Detroit, Michigan

Background: A. baumannii is commonly recognized as an emerging multi-drug resistant (MDR) organism frequently impervious to majority of the commonly prescribed antibiotics. Colistin is one of the few therapeutic agents which possess activity against this pathogen, and its use has dramatically increased. As a result, colistin resistance is increasingly reported among A. baumannii and presents a unique challenge. Methods: A cluster of colistin-resistant A. baumannii cases at Detroit Receiving Hospital were identified from May 1st, 2013 to October 31st, 2013. Colistin resistance was defined as an MIC of >2 μg/ml (E-test). Epidemiologic data for these cases were collected and isolates assayed for clonality with Diversilab rep-PCR and multi-locus sequence typing (ST). Results: 11 cases were identified. The mean age of the patients was 48.8 years (range 17-76) and 10 (91%) resided in one of two intensive care units. All patients were treated with broad spectrum antimicrobials (but not colistin) prior to isolation of the colistin-resistant isolate. 9 (82%) patients were mechanically ventilated and the pathogen was detected from sputum specimen in 8 (73%) of patients. Other features frequently identified in these cases were the use of glucometer (73%) and tube feeds (82%). Colistin MIC ranged from 3-32. Environmental surveillance cultures were performed, but only one specimen was positive for the same organism. Genotyping was performed on 5 patient isolates which revealed 95% similarity between strains (Figure 1) and all isolates were ST281. In order to contain the outbreak, optimal infection prevention practices were reinforced, active surveillance screening of high risk patients implemented, with presumptive contact isolation. Conclusion: To our knowledge this is the first reported outbreak of colistin-resistant A. baumannii in United States. The hands of healthcare workers and environmental reservoirs are hypothesized to be the source of the outbreak. We observed a wide range of colistin MICs among outbreak strains, possibly due to 1) the emergence of heteroresistance or 2) differences in the population structure. A case-control study will help to further help delineate the cause of the outbreak. OXA23