Role of subtyping in detecting Salmonella cross contamination in the laboratory.
BACKGROUND
With the exception of M. tuberculosis, little has been published on the problems of cross-contamination in bacteriology laboratories. We performed a retrospective analysis of subtyping data from the National Salmonella Reference Laboratory (Ireland) from 2000-2007 to identify likely incidents of laboratory cross contamination.


METHODS
Serotyping and antimicrobial susceptibility testing was performed on all Salmonella isolates received in the NSRL. Phage typing was performed on all S. Typhimurium and S. Enteritidis isolates while multi-locus variance analysis (MLVA) was performed on selected S. Typhimurium isolates. Pulsed field gel electrophoresis (PFGE) using the PulseNet standard protocol was performed on selected isolates of various serovars.


RESULTS
Twenty-three incidents involving fifty-six isolates were identified as likely to represent cross contamination. The probable sources of contamination identified were the laboratory positive control isolate (n = 13), other test isolates (n = 9) or proficiency test samples (n = 1).


CONCLUSIONS
The scale of laboratory cross-contamination in bacteriology is most likely under recognized. Testing laboratories should be aware of the potential for cross-contamination, regularly review protocols to minimize its occurrence and consider it as a possibility when unexpected results are obtained.
De Lappe
Niall
N
National Salmonella Reference Laboratory, Department of Medical Microbiology, Galway University Hospitals, Galway, Ireland. niall.delappe@hse.ie
Connor
Jean O
JO
Doran
Geraldine
G
Devane
Genevieve
G
Cormican
Martin
M
D001434:Bacteriophage Typing; D016521:Electrophoresis, Gel, Pulsed-Field; D004866:Equipment Contamination; D005506:Food Contamination / Q000032:analysis; D005516:Food Microbiology; D007753:Laboratories; D008826:Microbial Sensitivity Tests; D012475:Salmonella / Q000145:classification / Q000302:isolation & purification*; D012703:Serotyping