Detection of Listeria in Probiotic Cultures

Introduction: Consumption of probiotics is increasingly popular. The World Health Organization defines probiotics as “live microorganisms which when administered in adequate amounts confer a health benefit on the host”. There is a substantial body of literature describing the reduction of high Listeria counts in the presence of competing organisms. However, no literature describes effective methods for detecting low levels of Listeria in probiotic cultures, suggesting that food-safety monitoring of probiotics may be difficult.

Purpose: This study set out to establish the detectability of Listeria species in probiotic cultures and estimate the sensitivity of detection.

Methods: A dehydrated probiotic culture was spiked with known concentrations of Listeria monocytogenes, enriched in different broth media, and then tested using the Roka Atlas^® Listeria species test protocol.

Results: No recovery was detected from probiotic samples spiked with approximately 30 CFU per 25 g and enriched at 1/10 and 1/100 in Demi-Fraser broth for 24 h at 35°C. Supplementation of the enrichment medium with antibiotics (Colistin Sulfate 10.0 mg/L with Moxalactam 20.0 mg/L; Clindamycin plus Erythromycin both at 8 mg/L: and all four together) did not lead to Listeria recovery. However, a 30 CFU/25 g spike was detectable after thermal pasteurization of the probiotic, showing that it was purely competitive pressure inhibiting Listeria recovery. A spike level as low as 100 CFU/25 g sample was detected following 24 h incubation at 1/100 in Demi-Fraser broth at 35°C, establishing that the limit of detection of Listeria in this type of matrix is between 30 and 100 CFU/25 g sample.

Significance: This study demonstrated the challenging nature of Listeria detection from concentrated probiotic cultures. However, the observed detection limit is likely still protective of public health given the extensive literature on death of Listeria in these environments.