Purpose: The purpose of this study was to measure the spectrum of a listeriophage cocktail against a library of L. monocytogenes strains with varied phenotypic and genotypic characteristics.
Methods: Lytic capacity of the listeriophage was tested against 381 L. monocytogenes isolates representing 61 strains by standard spot assay in LB agar (supplemented with MOPS and glucose) at 21.5°C and 30°C. Lytic capacity of each isolate was evaluated on a scale of 0 to 3 (0: no lysis; 1: partial lysis; 2: ring of confluent lysis around lawn in center; 3: confluent lysis). Multinomial logistic regression was used to determine proportional odds estimates for phage susceptibility based on isolates’ persistence vs. transience, PFGE type, and attachment capacity to abiotic surfaces at different temperatures and concentrations.
Results: If a L. monocytogenes isolate was historically persistent in an environment, the odds of its lysis score being zero, one, and two increased by 11.4, 8.0, and 3.7 times, respectively, relative to lysis score three. Similarly, for incubation temperature 21.5°C, the odds of lysis score being zero, one and two increased by 4.0, 3.5, and 2.5 times, respectively, relative to score of three. The effect of attachment capacity and concentration of phage was not significant (P>0.05) when persistence and temperature were included in the model.
Significance: This study showed that the lytic capacity of listeriophage may be reduced if L. monocytogenes strains have persistent phenotypes. Whether and how these characteristics impact the efficacy of phage in controlling L. monocytogenes, in commercial food processing environments, is the subject of further study.