COMPARISON OF ANTIBODY DIRECT EPIFLUORESCENT FILTER TECHNIQUE WITH THE MOST PROBABLE NUMBER PROCEDURE FOR RAPID ENUMERATION OF LISTERIA IN FRESH VEGETABLES

The antibody-direct epifluorescent filter (Ab-DEFT) technique was eval uated as a rapid alternative to the most probable number (MPN) method for enumeration of artificially inoculated Listeria monocytogenes in r eady-to-eat packaged salads and other fresh vegetables. Ab-DEFT was pe rformed by homogenization of food in mesh-lined Stomacher bags, follow ed by prefiltration of homogenate through a 5 mu m pore nylon filter, and passage of filtrate through a 0.4 mu m pore black polycarbonate fi lter to collect and concentrate Listeria cells. After cells were stain ed with a fluorochrome-labeled polyclonal antibody to Listeria, the fi lter surface was examined by epifluorescence microscopy, and fluoresce nt cells were counted. A 3-tube MPN procedure was performed by success ive enrichments of homogenized foods in Listeria enrichment and Fraser broths, followed by selective plating. Ab-DEFT provided quantitative determinations of Listeria cells that correlated with plate counts and MPN estimates in a linear response over a range of cell concentration s from 10 to 10(7) colony forming units (CFU)/mL. Microbial background s as high as 10(8) CFU/mL did not affect performance of Ab-DEFT. In co ntrast to the MPN method, which required 5 days to perform, quantitati on by Ab-DEFT could be completed in less than 1 h. Despite cross-react ivities demonstrated by the polyclonal fluorescent antibody, the poten tial of Ab-DEFT as a rapid alternative to MPN for microbial cell enume ration was evident.