Evaluation of a preharvest bacteriophage therapy for control of Salmonella within bovine peripheral lymph nodes

Authors: Wottlin, Lauren; Edrington, Thomas; Brown, Tyson; ZOOK, CHRISTOPHER - Zoetis; SULAKVELIDZE, ALEXANDER - Intralytix, Inc; Droleskey, Robert - Bob; Genovese, Kenneth - Ken; Nisbet, David

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/28/2021
Publication Date: 2/1/2022
Citation: Wottlin, L.R., Edrington, T.S., Brown, T.R., Zook, C.A., Sulakvelidze, A., Droleskey, R.E., Genovese, K.J., Nisbet, D.J. 2022. Evaluation of a preharvest bacteriophage therapy for control of Salmonella within bovine peripheral lymph nodes. Journal of Food Protection. 85(2):254-260. https://doi.org/10.4315/jfp-21-292.
DOI: https://doi.org/10.4315/jfp-21-292

Interpretive Summary: Salmonella is sometimes an issue in the beef supply, and other research has shown that Salmonella in the peripheral lymph nodes (LN) of cattle may play a part in the contamination of ground beef. Phages are viruses that specifically attack only their target bacteria, making them a potential tool to help in food safety. A series of 4 studies were carried out to test if a phage product specially targeted to kill Salmonella could be injected into calves to kill Salmonella hiding in the LN. Salmonella was first inoculated into the calves using an allergy testing device which put the bacteria in the skin of the calves. The bacteria then migrated to the nearest LN by riding in immune cells called macrophages. Next the phage solution was injected subcutaneously in a few locations near the LN, so it could be tested against the bacteria. After a few days, the calves were euthanized and their LN harvested and cultured for Salmonella. In two of the four studies, the phage product successfully reduced the amount of Salmonella in the LN, but there were no differences in the other two studies. A 1:1 ratio of phage:bacteria was used in this experiment, while other research has used 10 - 1000:1 ratio, so it is likely that a higher phage dose would have yielded more favorable results. More research is needed and should focus on determining the best dose and route of administration for phage products in cattle.

Technical Abstract: Salmonella remains an issue in the food supply, and the few preharvest interventions available for producers have shown limited benefit. Previous research has shown that Salmonella in the peripheral lymph nodes (LN) of cattle contributes to contamination of ground beef. A series of proof-of-concept studies evaluated a commercial bacteriophage (phage) cocktail with demonstrated efficacy in vitro, for mitigation of Salmonella in bovine LN. Salmonella was inoculated intradermally, and the phage cocktail was administered subcutaneously. At the conclusion of each study, calves were euthanized and the popliteal, prescapular, and subiliac LN were cultured for Salmonella and phage. In study I, fewer Salmonella were recovered in the prescapular LN of phage-treated calves than controls. The LN of control and phage-treated calves were found to contain phage; however, transmission electron microscopy of LN culture fluid revealed that control calves had endogenous phage, while phage-treated cattle had both endogenous and exogenous phages originating from the cocktail. Study II resulted in adverse reactions due to endotoxin in the phage cocktail, therefore subsequent studies used a purified product. Phage-treated cattle in study III had reduced Salmonella in prescapular and subiliac LN, yet similar quantities of phages compared to controls. Based on consensus that Salmonella in LN is intramacrophage, it was hypothesized that a 48-h carcass cooling period prior to LN removal would aid in release of macrophage contents and improve phage efficacy. In the final study, cooling carcasses resulted in greater Salmonella recovery; however, there was no effect of phage treatment. To our knowledge, this was the first experiment to administer Salmonella-specific phages to live cattle. As two of four studies showed reduction in LN Salmonella despite a low phage: bacterium ratio, with more refinement phage therapy may be a preharvest intervention for Salmonella in the peripheral LN of cattle.