SURVIVAL OF SHIGELLA-FLEXNERI ON VEGETABLES AND DETECTION BY POLYMERASE CHAIN-REACTION

Commercially prepared and packaged fresh vegetables were tested to det ermine the types and levels of indigenous microflora. Sixteen species of bacteria from II genera were identified and titers of up to 1 x 10( 10) cells per gram of vegetable were observed. To evaluate the surviva l of Shigella spp. on packaged vegetables, an avirulent insertion muta nt of Shigella flexneri 5 (pHS1059) was added to vegetables. This stra in survived in phosphate-buffered saline at pH 7.3 at 5 to 10 degrees C for more than 3 months. It also survived for several days at both am bient and refrigerator temperatures when inoculated onto various comme rcially 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 8. flexneri viru lence-associated spa region. The PCR also generated the corresponding fragments from 8. sonnei, S. boydii, and Shigella sp. This fragment wa s also observed when S. flexneri cells were used to artificially conta minate sterile and nonsterile vegetables, but no amplified fragment wa s observed when the normal microflora of the vegetables were eluted an d tested by PCR.