Effectiveness of the Bacteriological Analytical Manual culture method for the recovery of Shigella sonnei from selected foods.

The relative retention of the indigenous morphological, biochemical, and serological characteristics by Shigella sonnei was tested under various storage conditions (room temperature, refrigeration, freezing at -20 degrees C and at -70 degrees C, and lyophilization). The use of a selective (desoxycholate citrate) agar rather than a nonselective (brain heart infusion) agar gave a lower conversion rate of smooth to rough colonies, and the percentage of rough colonies derived from cultures stored for prolonged periods increased under all conditions. With respect to biochemical characteristics, there were no major differences in the reactions of smooth vs rough variants. For serological characteristics, smooth variants agglutinated more readily in homologous antisera than did rough variants. S. sonnei populations maintained at -70 degrees C with glycerol remained reasonably stable and were used in recovery studies. Up to six foods (potato salad, chicken salad, cooked salad shrimp, lettuce, raw ground beef, and raw oysters) were inoculated with unstressed, chill-stressed, or freeze-stressed S. sonnei cells. Test portions (25 g) were inoculated with serial 10-fold dilutions of culture and subsequently analyzed by the culture method described in the U.S. Food and Drug Administration's Bacteriological Analytical Manual. It was found that the method was relatively ineffective for the recovery of S. sonnei from raw ground beef and raw oysters.