Biocontrol validation of a strictly lytic phage A511 and a commercial phage PGL show reduction of Listeria monocytogenes in raw Scottish Salmon fillet post-harvest

Introduction : Listeria monocytogenes is a known food-borne risk associated with ready-to-eat (RTE) foods, including fish. An estimated 90% of cases of invasive human listeriosis result from ingestion of foods containing > 2.0 × 10 3 CFU/g, with growth after purchase being a significant factor. This study aimed to evaluate the biological characteristics, lytic spectra, and determine the biocontrol potential of a strictly lytic L. monocytogenes bacteriophage A511 in comparison with a commercial phage PGL (P100) on raw Scottish salmon fillets. Methodology : The lytic spectra of phage A511 and the commercial phage PGL was determined using spot assays on a panel of 33 L. monocytogenes strains and phage growth kinetics such as burst and latent period was evaluated using standard double agar layer methods. For the biocontrol study, ∼ 4 log 10 CFU/ml of L. monocytogenes were inoculated on raw Scottish salmon fillets, and the two individual lytic phages (viz., phage A511 and commercial phage PGL) at three phage application rates (final concentrations of 10 5 , 10 7 and 10 9 PFU/g) were used to control experimental Listeria contamination of fresh Scottish salmon fillets stored at 4°C and 10°C. Results : The phages A511 and PGL exhibited lytic spectra of 97% and 100%, respectively, against the strains tested and produced burst sizes of ∼ 49 PFU/cell for A511 and a previously reported range of 100-200 PFU/cell for PGL. There was a significant effect of phage titre (p<0.0001) on the levels of L. monocytogenes with reductions in bacterial numbers to below the limit of detection (i.e., no detectable counts were observed) after 1-day incubations at both 4°C and 10°C for both phage A511 and PGL applied at 10 9 PFU/g, with enhanced reductions at 4°C. Conclusion This study conducted a lab‑scale evaluation of the archival phage A511 on raw salmon in direct comparison with a commercial phage (PGL) under a unified dose–time–temperature framework. To our knowledge, a direct comparison of A511 and PGL on raw salmon using an identical experimental design has not been previously reported. While the findings demonstrate effective reduction of L. monocytogenes under controlled laboratory conditions, further work including broader strain coverage, naturally contaminated lots, sensory and shelf‑life assessments, and multi‑batch validation, is required before commercial implementation can be fully assessed.