Nosocomial outbreaks caused by Acinetobacter baumannii and Pseudomonas aeruginosa: Results of a systematic review

•Outbreaks caused by A. baumannii take mainly place on intensive care units (ICUs).•Outbreaks caused by P. aeruginosa affect ICUs and peripheral wards equally.•Mortality in Acinetobacter outbreaks is twice as high as in Pseudomonas outbreaks.•Resistance profiles of either pathogen did not influence the mortality rates.•Infection control measures were more often carried out in Acinetobacter outbreaks. Large outbreaks of infection by Acinetobacter baumannii and Pseudomonas aeruginosa have been reported. This research compares characteristics of such outbreaks. Determination of risk factors for the occurrence and appropriate infection control measures. The Outbreak Database, PubMed, and reference lists of identified articles were used. Key words included nosocomial and (outbreak or epidemic) and (aeruginosa or baumannii). Articles were included if they describe distinct outbreak(s) caused by A baumannii or P aeruginosa and were published between 2000 and 2015. There were no further restrictions with respect to language or type of article. One hundred fifty outbreaks by A baumannii and 131 outbreaks by P aeruginosa were included, including multidrug-resistant strains in 113 Acinetobacter and 49 Pseudomonas outbreaks. Acinetobacter outbreaks were mainly reported from intensive care units, after use of antibiotics, during mechanical ventilation, and presented with a mortality rate of 47% compared with 23% by Pseudomonas. Resistance did not alter mortality by either species. Most infection control measures were implemented or enforced more often in Acinetobacter outbreaks. These findings should support staff in infection control departments and on wards if an outbreak is suspected. Better adherence to the Outbreak Reports and Intervention Studies of Nosocomial Infection guidelines in outbreak reporting is necessary. A precise definition of multidrug resistance for Acinetobacter and Pseudomonas is lacking.