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 involving 18 laboratories across Canada. Shrimp, coleslaw, ice cream and wieners were inoculated with low levels (5 × 10² and 10³/g) of L. monocytogenes and shipped to participants. Foods were diluted and then plated onto either lithium chloride phenylethyl and moxa-lactam agar (LPM), Oxford agar (OXA), modified Oxford agar (MOX) or Palcam agar (PAL). Recovery was good for all foods, except coleslaw. Of the four plating media tested, all were more or less equivalent in their ability to recover colonies for enumeration, except that more colonies were enumerated on LPM than on PAL agar. Recovery of L. monocytogenes ranged from <50 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 indicate that the direct plating method can be used for the recovery of low levels of Listeria monocytogenes in Category 3 foods, as presently suggested for use in the Canadian Listeria compliance guide.