Method for the detection of Shigella in foods

A method for the detection of Shigella in foods was evaluated. The method consisted of selective enrichment at 37 degrees C in GN-broth supplemented with novobiocin (10 micrograms/ml) followed by subculture after 6 and 24 h incubation of 0.1 ml volumes to SS-agar with and without streptomycin (7.5 micrograms/ml). Test samples were contaminated by adding shigellae to 1:10 suspensions of foods (minced meat, fresh cut vegetables and cooked peeled shrimp) in the GN-broth with novobiocin. Shigella could be detected at a contamination level of 1 per 25 g in the presence of shrimp (in all 20 samples) and at 25 per 25 g in the presence of nearly all samples of fresh cut vegetables (in 13 out of 18 samples). Using this procedure Shigella could not be detected when the organism was present at a relatively high contamination level of 10(2) per 25 g in the presence of minced meat (with all 10 samples). However, the procedure described is an improvement on methods currently available for the detection of Shigella in foods.