Development of a bacteriophage-based Method for Detection of Escherichia Coli O157:H7 in Fresh Vegetables

In this study, a method using a recombinant phage for detection of E. coli O157:H7 in fresh vegetables was investigated. Four kinds of fresh vegetables, i.e. lettuce (Lactuca sativa), mustard greens (Brassica juncea), coriander (Coriandrum sativum), and soybean sprouts were selected since they are commonly used in meals in Vietnam. Firstly, a phage-based method was investigated for detection of E. coli O157:H7 in the four types of vegetables. To support the detection by suppressing growth of background bacteria in vegetables, selective antibiotics, i.e. novobiocin (N) and vancomycin (V) in combination with BHI medium were examined. Secondly, quality of the method was evaluated in terms of sensitivity, specificity, and rapidity. The method enabled the detection of E. coli O157:H7 inoculated at 10³, 10², or 10¹ CFU/ 10 mL of sterile 0.8% NaCl containing 5 g of vegetable and in the presence of several Gram-positive and Gram-negative bacteria inoculated at 10⁷ CFU/10 mL. The time for detection was approximately 16.5 hours for E. coli O157:H7 inoculated at 10 CFU/10 mL of sterile 0.8% NaCl containing 5 g of vegetable. The limit of detection was considered to be 2 CFU g^-1 vegetable.