Acid tolerance of Shigella sonnei and Shigella flexneri

Aims: Determination of the behaviour of Shigella sonnei and Sh. flexneri under acid conditions. Methods and results: The growth and survival of Shigella spp. (9 isolates) in acidified Brain Heart Infusion (BHI) (pH 5.0-3.25 with pH intervals of 0.25) was determined after 6, 24 and 30 h incubation at 37degreesC. Subsequently, survival of shigellae was studied in apple juice and tomato juice stored at 7degreesC and 22degreesC for up to 14 days and in strawberries and a fresh fruit salad, kept at 4degreesC for 4 and 48 h. Conclusions: The minimum pH for growth in acidified BHI for Sh. flexneri and Sh. sonnei was, respectively, pH 4.75 and pH 4.50. Survival in fruit juices and fresh fruits depended upon their pH, the type of strain and the incubation temperature. Shigella spp. Survived for up to 14 days in tomato juice and apple juice stored at 7degreesC. The shortest survival time (2-8 d) was observed in apple juice at 22degreesC. Sh. sonnei but not Sh. flexneri was recovered after 48 h from strawberries and fruit salad kept at 4degreesC. Significance and Impact of the Study: Acid foods, especially if kept at refrigeration temperatures, suppport survival of Shigella spp. and may cause Shigella food poisoning.