In the US, beef cattle feedlots commonly use antibiotic feed additives, such as tylosin and virginiamycin, to prevent liver abscesses, which can cause significant economic losses and animal welfare concerns. Ionophores, such as monensin, are also routinely provided to enhance feed efficiency and improve animal performance. However, growing public and regulatory pressure to curb the proliferation of antibiotic resistant bacteria has prompted a need to reduce or eliminate their use. As an alternative, our group is working on bacteriophage-based feed additives that target Fusobacterium necrophorum, the primary cause of bovine liver abscesses, and Streptococcus bovis complex bacteria with the aim of reducing or eliminating the dependence on antibiotic feed additives.

Using a combination of phage isolation techniques, we identified and characterized fifteen new F. necrophorum phages belonging to six genetically distinct groups. These phages exhibited a broad intraspecific host range and could infect various contemporary F. necrophorum strains isolated from different regions of the United States. All phages were of temperate origin, with dsDNA genomes ranging from around 36 kbp to 114 kbp. In vitro evolution resulted in the recovery of a potential virulent mutant, φBB37, which inhibited the growth of the high leukotoxin-producing F. necrophorum 8L1 strain for over 40 hours in vitro. Phage cocktails proved effective for up to 60 hours in vitro. After minimal optimization, most phages demonstrated high productivity (> 10⁹ PFU/mL) and little yield variation during scale-up, highlighting their commercial potential. To assess the safety and in vivo dynamics of F. necrophorum phage cocktails, a small-scale animal trial was conducted with nine cattle split into three groups (control, low dose, and high dose). Relative to the control group, F. necrophorum subsp. necrophorum was suppressed by 86% over three days (AUC) using high dose (10¹¹ PFU), rotated cocktails. F. necrophorum concentrations returned to baseline two days after phage feeding was ceased.

Fifteen phages active against S. bovis complex bacteria were isolated from rumen fluid and bovine feces. Four were selected for genome sequencing based on their host ranges and killing efficiencies. All four genomes had high similarity, ranged from 33 – 36 kbp, and appear to be obligately lytic. Phages displayed relatively high strain specificity, but were able to maintain host inhibition up to 24 hours in vitro. Appleman’s protocol was effective for expanding phage host range from three to seven strains. Since phage-based feed additives are likely to be subject to elevated temperatures during manufacturing or milling, thermal stability was assessed. All phages were initially inactivated by incubation at 50°C for 15 mins or 65°C for 5 mins. Adaptive laboratory evolution significantly increased phage thermal stability, with little inactivation observed after 1 hour at 60°C. Overall, these results support further research into the use of S. bovis phages to mitigate ruminal acidosis and improve cattle performance.