Development of an Improved Selective Medium for the Detection of Shigella spp.

Abstract

Shigella known as a waterborne pathogen causes outbreaks of Shigellosis. The outbreaks have been traced to contaminated water and food. To detect the Shigella present in contaminated foods is very important for helping to ensure food hygiene. There are several selective media including MacConkey agar, Xylose-Lysine Deoxycholate agar, Hektoen-Enteric agar approved by The U.S. Food and Drug Administration (FDA) and International Organization for Standardization (ISO) for the isolation of Shigella from foods. But conventional culture media for Shigella spp. are neither specific nor sensitive. Therefore, I developed an improved selective medium (HEX, HE-Xylose medium) for the detection of Shigella spp. from foods. HEX 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 (S. flexneri, S. dysenteriae, S. boydii, and S. sonnei) produced green colonies on HEX, while Hafnia alvei, found to be false positive for Shigella on HE agar, appeared as differentiable orange colonies on HEX. HEX supported a higher recovery of heat- or acid-injured Shigella than conventional medium. A total of 300 uninoculated and inoculated food samples were used to evaluate the specificity and sensitivity of HEX. The specificity was 13.50, 38.50, and 83.50 (%) on MacConkey agar, HE agar, and HEX, 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, respectively. HEX had superior specificity and sensitivity compared to HE agar. Therefore HEX can be an appropriate selective and differential medium for detection of Shigella spp. from foods.