Implementation of a Streamlined Testing Algorithm to Decrease Turnaround Time for Listeria monocytogenes Foodborne Outbreak Investigations

Introduction: During a multi-state investigation of a Listeria monocytogenes illness cluster in 2014, the Virginia Division of Consolidated Laboratory Services (DCLS) isolated L. monocytogenes from the implicated food which possessed an indistinguishable Pulsed Field Gel Electrophoresis (PFGE) pattern to five previously identified clinical cases. Utilization of a rapid food sample testing algorithm enabled DCLS to more promptly identify contaminated food products to include in the public recall notification, enabling regulatory authorities to remove the product from the market.

Purpose: The algorithm for isolation of L. monocytogenes, biochemical confirmation, and molecular subtyping by PFGE prior to the outbreak required approximately ten days from sample receipt. Given the critical need for expedited results, algorithm modifications were proposed and evaluated

Methods: The existing DCLS Food Microbiology and PFGE methods were evaluated using a risk-based approach to maximize efficiency while maintaining quality laboratory practices. The testing algorithms were compared, redundancies identified, and a harmonized L. monocytogenes algorithm developed.

Results: Four process redundancies in the Listeria testing algorithms were identified: (i) transfer of suspicious colonies to blood agar plates (BAP) on Day 5, (ii) confirming L. monocytogenes isolates prior to submission to PFGE on Day 7, (iii) subculture of isolates to blood heart infusion agar (BHI) from BAP in PFGE, and (iv) overnight incubation of PFGE plugs for cell lysis. The response time for L. monocytogenes was reduced from 10 days to 6 days by (i) streaking for isolation on BHI and BAP on Day 4, (ii) submitting BHI plates to PFGE on Day 5, (iii) performing biochemical confirmation concurrently with PFGE analysis, and (iv) PFGE processing using a "1 day" method.

Significance: Use of an improved laboratory testing algorithm allowed for rapid notification to regulatory agencies, and consumers, thereby, reducing or eliminating potential illnesses associated with contaminated product.