Abstract Background Identifying the source of an outbreak is the most crucial aspect of any outbreak investigation. In this review, we address the frequently discussed question of whether (rectal) screening of health care workers (HCWs) should be carried out when dealing with outbreaks caused by gram negative bacteria (GNB). A systematic search of the medical literature was performed, including the Worldwide Outbreak Database and PubMed. Outbreaks got included if a HCW was the source of the outbreak and the causative pathogen was an Escherichia coli, Klebsiella spp., Enterobacter spp., Serratia spp., Pseudomonas aeruginosa, or Acinetobacter baumannii. This was true for 25 articles in which there were 1196 (2.1%) outbreaks due to GNB, thereof 14 HCWs who were permanently colonized by the outbreak strain. Rectal screening of HCWs was helpful in only 2 of the 1196 (0.2%) outbreaks. Instead, the hands of HCWs served as a reservoir for the outbreak strain in at least 7 articles â especially when they suffered from onychomycosis or used artificial fingernails or rings. Conclusion Due to very weak evidence, we do not recommend rectal screening of HCWs in an outbreak situation with GNB. However, besides a critical review of hand hygiene habits, it might be useful to examine the hands of staff carefully. This measure is cheap, quick to perform, and seems to be quite effective.