Development of Listeria monocytogenes Enumeration Method Using FSIS Guidelines in Comparison with FDA BAM L. monocytogenes Detection and Enumeration

Introduction: Listeria monocytogenes (L. mono) is a foodborne pathogenic bacteria commonly associated with listeriosis.  Listeriosis is associated with poorly prepared and/or handled foods, affecting individuals with weakened immune systems.  Enumeration testing can help determine the extent of L. mono contamination by providing a direct measure of viable cell counts.

Purpose: The purpose of this study was to develop a simple, cost effective enumeration method for L. mono using methodology from the United States Department of Agriculture, Food Safety and Inspection Service Microbiology Laboratory Guidebook (FSIS MLG).  The goal was to show that L. mono enumeration using FSIS MLG methodology was equivalent to results obtained using the Bacteriological Analytical Manual (FDA BAM) method.

Methods: L. mono was enumerated in naturally and artificially contaminated ice cream, soft cheeses, and deli sandwiches. A side by side enumeration was performed using most probable number (MPN) analysis.

Results: Twenty samples were analyzed on both methods (n=20). Eight samples had the same MPN result on both methods, seven of these eight were <0.3 MPN/g.  Four of the samples had a higher MPN/g on the FSIS method.  Eight of the samples had a higher MPN/g on the BAM method. The difference in the number of tubes positive was only greater than one tube per sample on three samples.

Significance: Enumerating L. mono will provide valuable information to the food industry about the extent of L. mono contamination in manufacturing and distributing facilities. The FSIS MLG enumeration has the advantage of being a quicker, easier method while using fewer materials throughout confirmation.