Survival of Shigella flexneri on Vegetables and Detection by Polymerase Chain Reaction

Commercially prepared and packaged fresh vegetables were tested to determine the types and levels of indigenous microflora. Sixteen species of bacteria from 11 genera were identified and titers of up to 1 × 10¹⁰ cells per gram of vegetable were observed. To evaluate the survival of Shigella spp. on packaged vegetables, an avirulent insertion mutant of Shigella flexneri 5 (pHS 1059) was added to vegetables. This strain survived in phosphate-buffered saline at pH 7.3 at 5 to 10°C for more than 3 months. It also survived for several days at both ambient and refrigerator temperatures when inoculated onto various commercially prepared vegetables. A rapid method for detecting Shigella spp. on vegetables was developed by using the polymerase chain reaction (PCR) to amplify a 118-base-pair DNA fragment from the S. flexneri virulence-associated spa region. The PCR also generated the corresponding fragments from S. sonnei , S. boydii , and Shigella sp. This fragment was also observed when S. flexneri cells were used to artificially contaminate sterile and nonsterile vegetables, but no amplified fragment was observed when the normal microflora of the vegetables were eluted and tested by PCR.