Persistent Listeria monocytogenes subtypes isolated from a smoked fish processing facility included both phage susceptible and resistant isolates

Contamination of Ready-To-Eat foods with Listeria monocytogenes can typically be traced back to post-processing contamination from environmental sources; contamination is often linked to subtypes that persist in food associated environments. Although phage-based biocontrol strategies have been proposed for controlling this pathogen, information on the efficacy of phage treatment against diverse L. monocytogenes subtypes from food associated environments is still limited. We identified subtypes that were repeatedly found (" persistent") in a smoked fish processing facility by using EcoRI ribotyping data for isolates obtained in 1998-2009. PFGE analysis of 141 isolates (9 ribotypes) supported persistence for up to 11 years. Characterization of selected isolates, representing persistent subtypes, against a panel of 28 listeriaphages showed a wide range of likelihood of phage susceptibility, ranging from 4.6% (for 7 ribotype DUP-1043A isolates) to 95.4% (for 7 ribotype DUP-1044A isolates). In challenge studies with 10⁵ and 10⁶ CFU/ml L. monocytogenes, using phage cocktails and a commercial phage product at different phage-host ratios, one isolate (ribotype DUP-1043A) was not affected by any treatment. A reduction in L. monocytogenes counts of up to 4 log units was observed, after 8 h of treatment, in isolates of two ribotypes, but subsequent re-growth occurred. Survivor isolates obtained after 24 h of treatment showed decreased susceptibility to individual phages included in the phage cocktail, suggesting rapid emergence of resistant subtypes.