Deep Cleans Reduce Persistence of Listeria monocytogenes in Retail Delis

Introduction: Listeria monocytogenes is commonly found in the retail deli environment, where it can persist over long periods of time and potentially cross-contaminate Ready-to-Eat foods. Previous studies by our group determined daily interventions failed to significantly reduce L. monocytogenes persistence in retail delis.

Purpose: The goal of this study was to determine the effect aggressive cleaning strategies have on L. monocytogenes persistence.

Methods: We recently performed an aggressive deep clean SSOP in nine delis. Samples were collected from ten food contact surfaces, fifteen non-food contact surfaces, and three transfer points immediately before and after each deep clean, then once monthly for three months. Pulsed-field Gel Electrophoresis (PFGE) was used to type 107 L. monocytogenes isolates collected. Isolates from the same store and sharing the same AscI and ApaI fingerprints were considered the same strain. A strain was defined as persistent if detected from the same store in at least three month’s sampling.

Results: Three of eight strains previously identified as long-term persistent (i.e., in two separate six-month sampling studies from July 2010-October 2011) became transient after execution of the deep clean SSOP. Four additional strains previously identified as short-term persistent (i.e., persistent in only one six-month sampling study) were absent post deep clean, and one short-term persistent strain became transient.

Significance: Deep cleans were moderately effective at reducing or eliminating long-term persistent strains of L. monocytogenes. Additional development is needed to remove all persistent L. monocytogenes from the deli environments.