Detection of Multi-drug resistant Acinetobacter baumannii (MDR-AB) in Peri-Rectal Surveillance Cultures of Critically Ill Patients for Infection Control

Background: MDR-AB has emerged as an important cause of nosocomial infections in healthcare facilities globally. MDR-AB is often difficult and time consuming to identify using standard techniques; therefore the choice of detection method may be important for their prevention and spread. The objective of this study was to compare two selective media for active surveillance of MDR-AB. Methods: Peri-rectal surveillance cultures obtained from a cohort of patients admitted to the medical and surgical intensive care units (ICUs) at the University of Maryland Medical Center between January 1, 2007 and February 15, 2007 were used for this study. Peri-rectal surveillance cultures were collected from patients on ICU admission, weekly, and upon discharge from the unit. Each peri-rectal culture was plated onto CHROMagar ™ Acinetobacter (CHROM) (Chromagar; Paris, France) and MacConkey agar with 1 µg/ml of ceftazidime (MAC/CAZ). Plates were incubated at 37°C for 24 and 48 hours. Colonies were identified as A.baumannii using API 20E identification strips or the Vitek II (bioMerieux; Durham, NC). Susceptibility testing was performed by the disk diffusion method. MDR-AB was defined as susceptible to two or fewer antibiotics excluding polymixin B and tigecycline. Results: There were 788 peri-rectal cultures obtained during the study period from 376 unique patients. There were 48 (6%) A.baumannii isolates identified. CHROM detected 44 of 48 (92%) A.baumannii isolates, 26 (59%) of which were MDR-AB. MAC/CAZ detected 39 of 48 (81%) A.baumannii isolates, 21 (54%) of which were MDR-AB. CHROM missed 1 MDR-AB isolate while MAC/CAZ missed 6 MDR-AB isolates. Conclusion: CHROM was more sensitive than MAC/CAZ for the detection of both A.baumannii and MDR-AB from peri-rectal surveillance cultures. This may have implications for identifying patients colonized with MDR-AB for infection control.