Detection, isolation and enumeration of Yersinia enterocolitica from raw pork

The methods available for the isolation of Yersinia enterocolitica from foods are generally considered to be less than optimal, and methods for estimation of numbers are lacking. Such methods are needed to understand better the significance of foodborne yersiniosis and to provide data for exposure assessment. We describe a method for the detection and enumeration of Y. enterocolitica containing the pYV virulence plasmid (YeP+) in samples from pork surfaces. The method uses a multiplex PCR targeting the ail and virF genes to detect Y. enterocolitica after incubation of surface swabs in Yersinia enrichment broth according to Ossmer. Enumeration was achieved by adapting the enrichment to a most probable number (MPN) method format. A presumptive result was available within 24 h of sample receipt, and YeP+ isolates were confirmed within four days. The presence/absence and MPN methods were evaluated in a pilot survey of 34 packs of raw pork meat purchased from retail outlets in Christchurch, New Zealand. YeP+ was detected by PCR on meat from 32% of the packs, and YeP+ isolates were obtained from 18% of the samples. YeP+ were present at numbers ranging from 0.30 to 5.42 MPN/cm2. This improved method for the detection and enumeration of YeP+ from meat samples can be used for microbiological surveys to obtain data for assessments of consumer exposure to virulent Y. enterocolitica, and in outbreak investigations.