EFFECTIVENESS OF THE BACTERIOLOGICAL-ANALYTICAL MANUAL CULTURE METHODFOR THE RECOVERY OF SHIGELLA-SONNEI FROM SELECTED FOODS

The relative retention of the indigenous morphological, biochemical, a nd serological characteristics by Shigella sonnei was tested under var ious 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 (bra in heart infusion) agar gave a lower conversion rate of smooth to roug h colonies, and the percentage of rough colonies derived from cultures stored for prolonged periods increased under all conditions. With res pect to biochemical characteristics, there were no major differences i n the reactions of smooth vs rough variants. For serological character istics, smooth variants agglutinated more readily in homologous antise ra than did rough variants. S. sonnei populations maintained at -70-de grees-C with glycerol remained reasonably stable and were used in reco very studies. Up to six foods (potato salad, chicken salad, cooked sal ad shrimp, lettuce, raw ground beef, and raw oysters) were inoculated with unstressed, chill-stressed, or freeze-stressed S. sonnei cells. T est portions (25 g) were inoculated with serial 10-fold dilutions of c ulture and subsequently analyzed by the culture method described in th e U.S. Food and Drug Administration's Bacteriological Analytical Manua l. It was found that the method was relatively ineffective for the rec overy of S. sonnei from raw ground beef and raw oysters.