A COLLABORATIVE STUDY FOR DIRECT ENUMERATION OF LOW-LEVELS OF LISTERIA-MONOCYTOGENES FROM VARIOUS FOODS

A direct plating method for the enumeration of low levels of foodborne Listeria monocytogenes was evaluated in a collaborative study involvi ng 18 laboratories across Canada. Shrimp, coleslaw, ice cream and wien ers were inoculated with low levels (5 x 10(2) and 10(3)/g) oft. monoc ytogenes and shipped to participants. Foods were diluted and then plat ed onto either lithium chloride phenylethyl and moxalactam agar (LPM), Oxford agar (OXA), modified Oxford agar (MOX) or Palcam agar (PAL). R ecovery was good for all foods, except coleslaw. Of the four plating m edia tested, all were more or less equivalent in their ability to reco ver colonies for enumeration, except that more colonies were enumerate d on LPM than on PAL agar. Recovery of L. monocytogenes ranged from <5 0 to 1250 cfu/g for wieners, <50 to 800 cfu/g for shrimp, <100 to 1440 cfu/g for ice cream and <50 to 700 cfu/g for coleslaw. Results indica te that the direct plating method can be used for the recovery of low levels of Listeria monocytogenes in Category 3 foods, as presently sug gested for use in the Canadian Listeria compliance guide.