Purpose: Laboratories were instructed to isolate Listeria monocytogenes from samples of queso fresco cheese or alfalfa sprouts. Each food matrix represented a separate PT exercise that consisted of 5 samples each.
Methods: Fifteen laboratories received inoculated queso fresco cheese in which samples 1 , 3, and 5 contained L. monocytogenes (7500 – 9800 CFU/g), 3 and 5 also contained L. grayii (600 CFU/g) and L. innocua (500 CFU/g), respectively, and 2 and 4 had no added inoculum. In a separate exercise, sixty-one laboratories received inoculated alfalfa sprouts in which samples 1, 2, and 5 contained L. monocytogenes ( 200 CFU/g), sample 2 also contained L. grayii (200 CFU/g) and 5 also contained L. innocua (200 CFU/g). Samples 3 and 4 both contained 200 CFU/g each of L. grayii and L. innocua only.
Results: All participants (100%) isolated L. monocytogenes from inoculated queso fresco cheese samples. Although not required 27% identified L.grayii and 67% identified L. innocua. In the alfalfa sprout samples, 92% of participants correctly identified L. monocytogenes. In addition to L. monocytogenes participants detected confounder organisms in the following samples: 2, L. grayii (15%); 3, L. innocua (41%) and L. grayii (20%); 4, L. innocua (44%) and L. grayii (16%); and 5, L. grayii (10%).
Significance: For both exercises a high percentage of laboratories correctly identified L. monocytogenes in food matrices that can be difficult to analyze. Laboratories that participate in PT exercises are able to demonstrate their ability to detect pathogens effectively and demonstrates the need for a PT program that can create samples tailored to specific needs.