Efficacy of Repeated Applications of Bacteriophages on Salmonella enterica-Infected Alfalfa Sprouts during Germination

Infectivity responses of Salmonella enterica to bacteriophages on maize seeds and maize sprouts

Salmonella enterica (S. enterica) is a major foodborne pathogen leading to a large number of outbreaks and bringing food safety concerns to sprouts. The control of S. enterica on maize sprouts is important because raw maize sprouts have been gaining attention as a novel superfood. Compared to conventional chemical methods, the applications of bacteriophages are regarded as natural and organic. This study investigated the effects of a 2 h phage cocktail (SF1 and SI1, MOI 1000) soaking on reducing the populations of three Salmonella enterica strains: S. Enteritidis S5-483, S. Typhimurium S5-536, and S. Agona PARC5 on maize seeds and during the storage of maize sprouts. The results showed that the phage cocktail treatment effectively reduced populations of S. enterica strains by 1-3 log CFU/g on maize seeds and decreased population of S. Agona PACR5 by 1.16 log CFU/g on maize sprouts from 7.55 log CFU/g at day 0 of the storage period. On the other hand, the upregulations of flagella gene pefA by 1.5-folds and membrane gene lpxA by 23-folds in S. Typhimurium S5-536 indicated a differential response to the phage cocktail treatment. Conversely, stress response genes ompR, rpoS, and recA, as well as the DNA repair gene yafD, were downregulated in S. Agona PARC5. This work shows the use of bacteriophages could contribute as a part of hurdle effect to reduce S. enterica populations and is beneficial to develop strategies for controlling foodborne pathogens in the production and storage of maize sprouts.

Tailoring the Host Range of Ackermannviridae Bacteriophages through Chimeric Tailspike Proteins

Host range is a major determinant in the industrial utility of a bacteriophage. A model host range permits broad recognition across serovars of a target bacterium while avoiding cross-reactivity with commensal microbiota. Searching for a naturally occurring bacteriophage with ideal host ranges is challenging, time-consuming, and restrictive. To address this, SPTD1.NL, a previously published luciferase reporter bacteriophage for Salmonella, was used to investigate manipulation of host range through receptor-binding protein engineering. Similar to related members of the Ackermannviridae bacteriophage family, SPTD1.NL possessed a receptor-binding protein gene cluster encoding four tailspike proteins, TSP1-4. Investigation of the native gene cluster through chimeric proteins identified TSP3 as the tailspike protein responsible for Salmonella detection. Further analysis of chimeric phages revealed that TSP2 contributed off-target Citrobacter recognition, whereas TSP1 and TSP4 were not essential for activity against any known host. To improve the host range of SPTD1.NL, TSP1 and TSP2 were sequentially replaced with chimeric receptor-binding proteins targeting Salmonella. This engineered construct, called RBP-SPTD1-3, was a superior diagnostic reporter, sensitively detecting additional Salmonella serovars while also demonstrating improved specificity. For industrial applications, bacteriophages of the Ackermannviridae family are thus uniquely versatile and may be engineered with multiple chimeric receptor-binding proteins to achieve a custom-tailored host range.