Detection of Environmental Listeria spp. after 18-hour Enrichment Using Actero Listeria Enrichment Medium and the Atlas System

Introduction: Current environmental Listeria detection methods prescribe a primary enrichment time ranging from 22 to 48 hours. Environmental monitoring can be particularly problematic as organisms are often in an injured state and slow to grow in primary enrichments. To address this, we comparatively evaluated enrichment times for sub-lethally injured cells using Actero Listeria Enrichment Medium (ALEM), which is designed to select for and resuscitate injured Listeria spp.

Purpose: To use the combination of a specialized media and the Atlas System to achieve a reduced time-to-result for detecting environmental Listeria.

Methods: Sponge-style environmental samples from a variety of food-processing facilities were collected and inoculated with various levels of L. monocytogenes. After acclimating for 24 hours at 4°C, samples were enriched with either Half-Fraser or ALEM broth. Samples were collected, in duplicate, at t = 18, 20, and 24 hours into Roka G2 Transfer Tubes via transfer of 2 ml of enrichment. Transfer tubes were then prepared for analysis by the Atlas Environmental Listeria LE Detection Assay according to the product insert on the automated Atlas instrument. Samples were analyzed via culture at the 24-hour timepoint according to USDA MLG 8.09.

Results: Across spike levels, Atlas detection of Listeria after 18 and 20 hours of enrichment was 98 and 100% concordant, respectively, with detection via cultural methods at 24 hours in samples enriched with ALEM, Half-Fraser enrichment under the same conditions was 98% concordant at 20 and 24 hours.

Significance: Most assays for detecting Listeria spp. in environmental samples, including the Atlas Listeria Environmental Detection Assay, are designed to do so using non-proprietary media with a 24 to 48 hour enrichment time. For laboratories seeking more flexibility in their workflows, or simply a shorter time-to-result, the combination of ALEM and the Atlas system provides a viable alternative.