Development of an improved selective medium for the detection of Shigella spp.

Abstract

We developed an improved selective medium (HEX medium, modified Hektoen Enteric agar medium) for the detection of Shigella spp. from foods. Conventional culture media for Shigella spp. are neither specific nor sensitive. HEX medium contains lactose, sucrose, d-xylose, and salicin as a differentiation marker. The concentration of bile salts No.3 was reduced to a level of 0.3% which completely inhibited tested gram positive bacteria. All Shigella spp. tested (Shigella flexneri, Shigella dysenteriae, Shigella boydii, and Shigella sonnei) produced green colonies on HEX medium, while Hafnia alvei, found to be false positive for Shigella on HE agar, appeared as differentiable orange colonies on HEX medium. A total of 300 uninoculated and inoculated food samples were used to evaluate the specificity and sensitivity of HEX medium. The specificity was 13.50, 38.50, and 83.50 (%) on MacConkey agar, HE agar, and HEX medium, respectively. The sensitivity was 76.00, and 84.00 (%) for S. flexneri and 80.00, and 92.00 (%) for S. sonnei on HE agar and HEX medium, respectively. HEX medium had superior specificity and sensitivity compared to HE agar. Therefore HEX medium can be an appropriate selective and differential medium for detection of Shigella spp. from foods.