Collaborative Evaluation of Two Methods for the Detection of Shigella sonnei in Oysters and Young Corn

A new two-step enrichment method for the detection of Shigella sonnei in oysters and young corn was compared to the Bacteriological Analytical Manual (BAM) /Association of Analytical Communities (AOAC) method in a collaborative study.Twenty-one laboratories participated in the evaluation. Each laboratory used both methods for the detection of S. sonnei which had been inoculated into chilled or frozen oysters and young corn. The inoculum sizes for S. sonnet were 0, 14 and 140 cfu/25 g in oysters and 0, 18 and 180 cfu/25 g in young corn. Detection rates by PCR (invE and ipaH) and rates of S. sonnel colony appearance on three selective isolation agar media were compared. Statistical analyses were performed by the Welch test. The two methods gave detection rates of S. sonnei in 15/20 tests on oysters that were significantly different (p< 0.05). The new method showed superior detection rates for S. sonnei that had been inoculated into chilled and frozen oysters compared to the BAM / AOAC method with regard to the five detection items mentioned above. Both methods showed similar high detection rates for S. sonnel in chilled and frozen young corn. Based on these results, the new two-step enrichment method is recommended for the detection ofS. sonnei in food.