Purpose: This project was focused on the evaluation of several elements in the environmental sampling for Listeria spp.: sampling device, storage time after sampling, volume of enrichment broth.
Methods: Listeria monocytogenes was cultured overnight in Brain Heart Infusion (BHI) broth, diluted to proper concentrations, and then dried at room temperature overnight on stainless steel surface. The evaluation followed AOAC microbial method validation guidelines, comparing fractional positive results generated by each variable. The method described in FDA’s Bacteriological Analytical Manual (BAM) was followed in the whole process. Different materials of swabs (cotton, polyester, and polyurethane) and sponges (cellulose and polyurethane) used for collecting L. monocytogenes were compared. After sampling, three storage conditions of the samples were compared: 2 hours at room temperature, 24 h at 4°C, 48 h at 4°C. Two different enrichment volumes were evaluated: 90 ml and 225 ml.
Results: All experiments generated fractional positive results required by the AOAC guidelines. The 3 storage conditions after sampling (2 hours at room temperature, 24 h and 48 hours at 4°C) generated statistically equivalent results. The two enrichment volumes (90ml and 225ml), also, generated statistically equivalent results. Different materials of swabs and sponges generated statistically equivalent results as well. Statistical analyses were performed by probability of detection (POD) analysis according to AOAC guidelines, as well as Fisher’s Exact method.
Significance: This study contributed information that can be used to develop and optimize a complete procedure for environmental sampling of Listeria spp.