title: Screening agars for MRSA: evaluation of a stepwise diagnostic approach with two different selective agars for the screening for methicillin-resistant Staphylococcus aureus (MRSA).
abstract: BACKGROUND
  Colonization with methicillin-resistant Staphylococcus aureus (MRSA) poses a hygiene risk that does not spare field hospitals or military medical field camps during military deployments. Diagnostic options for unambiguously identifying MRSA isolates are usually scarce in military environments. In this study, we assessed the stepwise application of two different selective agars for the specific identification of MRSA in screening analyses.
  
  
  METHODS
  Nasal swabs from 1541 volunteers were subjected to thioglycollate broth enrichment and subsequently screened on CHROMagar MRSA selective agar for the identification of MRSA. The MRSA identity of suspicious-looking colonies was confirmed afterwards or excluded by another selective agar, chromID MRSA. All isolates from the selective agars with MRSA-specific colony morphology were identified by biochemical methods and mass spectrometry.
  
  
  RESULTS
  The initial CHROMagar MRSA screening identified suspicious colonies in 36 out of 1541 samples. A total of 25 of these 36 isolates showed MRSA-like growth on chromID agar. Out of these 25 isolates, 24 were confirmed as MRSA, while one isolate was identified as Staphylococcus kloosii. From the 11 strains that did not show suspicious growth on chromID agar, 3 were methicillin-sensitive Staphylococcus aureus (MSSA, with one instance of co-colonization with Corynebacterium spp.), 2 were confirmed as MRSA (with 1 instance of co-colonization with MSSA), 2 were lost during passaging and could not be re-cultured, one could not be identified by the applied approaches, and the remaining 3 strains were identified as Staphylococcus saprophyticus, Staphylococcus hominis (co-colonized with Macrococcus caseolyticus) and Staphylococcus cohnii, respectively.
  
  
  CONCLUSIONS
  The application of the selective agar CHROMagar MRSA alone proved to be too non-specific to allow for a reliable diagnosis of the presence of MRSA. The combined use of two selective agars in a stepwise approach reduced this non-specificity with an acceptably low loss of sensitivity. Accordingly, such a stepwise screening approach might be an option for resource-restricted military medical field camps.
authors.affiliation: Department of Tropical Medicine at the Bernhard Nocht Institute, German Armed Forces Hospital of Hamburg, Hamburg, Germany.
authors.forename: Volker
authors.initials: V
authors.lastname: Micheel
authors.affiliation: Department of Infectious Disease Epidemiology, Bernhard Nocht Institute for Tropical Medicine Hamburg, Hamburg, Germany.
authors.forename: Benedikt
authors.initials: B
authors.lastname: Hogan
authors.affiliation: Institute for Microbiology, Virology and Hygiene, University Medicine Rostock, Rostock, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Köller
authors.affiliation: Institute for Microbiology, Virology and Hygiene, University Medicine Rostock, Rostock, Germany.
authors.forename: Philipp
authors.initials: P
authors.lastname: Warnke
authors.affiliation: Institute for Microbiology, Virology and Hygiene, University Medicine Rostock, Rostock, Germany.
authors.forename: Sabine
authors.initials: S
authors.lastname: Crusius
authors.affiliation: Department of Tropical Medicine at the Bernhard Nocht Institute, German Armed Forces Hospital of Hamburg, Hamburg, Germany.
authors.forename: Rebecca
authors.initials: R
authors.lastname: Hinz
authors.affiliation: Department of Tropical Medicine at the Bernhard Nocht Institute, German Armed Forces Hospital of Hamburg, Hamburg, Germany.
authors.forename: Ralf Matthias
authors.initials: RM
authors.lastname: Hagen
authors.affiliation: Department of Infectious Disease Epidemiology, Bernhard Nocht Institute for Tropical Medicine Hamburg, Hamburg, Germany.
authors.forename: Norbert Georg
authors.initials: NG
authors.lastname: Schwarz
authors.affiliation: Department of Tropical Medicine at the Bernhard Nocht Institute, German Armed Forces Hospital of Hamburg, Hamburg, Germany.|Institute for Microbiology, Virology and Hygiene, University Medicine Rostock, Rostock, Germany.
authors.forename: Hagen
authors.initials: H
authors.lastname: Frickmann
keywords: CHROMagar; ChromID agar; Chromogenic agar; Hygiene; Methicillin-resistant Staphylococcus aureus; Screening; Selective agar; Stepwise diagnostics