Fusobacterium varium has been generally overlooked in cattle rumen microbiome studies relative to the presumably more abundant liver abscess-causing Fusobacterium necrophorum. However, F. varium was found to be more abundant in the rumen fluid of cattle and under culture conditions tailored to enrich F. necrophorum. Using near-full length 16S rRNA sequencing, we demonstrate that F. varium grows under restrictive conditions commonly used to enumerate F. necrophorum, suggesting that previous F. necrophorum abundance assessment may have been inaccurate and that F. varium may be an underestimated member of the ruminal bacterial community. Fusobacterium varium were not as susceptible as F. necrophorum to in-feed antibiotics conventionally used in feedlots. Exposure to tylosin, the current gold standard for liver abscess reduction strategies in cattle, consistently hindered growth of the F. necrophorum strains tested by over 67% (P < 0.05) relative to the unexposed control. In contrast, F. varium strains were totally or highly resistant (0 - 13% reduction in maximum yield, P < 0.05). Monensin, an ionophore antibiotic, had greater inhibitory activity against F. necrophorum than F. varium. Finally, preliminary genomic analysis of two F. varium isolates from the rumen revealed the presence of virulence genes related to those of pathogenic human F. varium isolates associated with active invasion of mammalian cells. The data presented here encourages further investigation into the ecological role of F. varium within the bovine rumen and potential role in liver abscess development, and proactive interventions.

Fusobacterium varium has been generally overlooked in cattle rumen microbiome studies relative to the presumably more abundant, liver abscess-causing Fusobacterium necrophorum. Here, we revisit that conventional wisdom and report greater relative abundance of F. varium than F. necrophorum in both raw rumen samples and in lactate-supplemented enrichments tailored for F. necrophorum growth, despite its consistent inadvertence in past ruminal surveys and putative inability to metabolize lactate. Our observation that F. varium grows under restrictive conditions used to enumerate F. necrophorum suggests that previous estimations were inaccurate and F. varium is an underestimated player within the ruminal community. Exposure to tylosin, the current gold standard among prophylactic liver abscess prevention strategies in cattle, consistently reduced growth of all F. necrophorum strains screened by greater than 67% relative to unexposed controls. In contrast, F. varium strains were completely or highly resistant (0 - 11% reduction in maximum yield). Monensin, an ionophore fed to cattle to improve feed efficiency also had stronger inhibitory activity against F. necrophorum than against F. varium. Finally, preliminary genomic analysis of two F. varium bovine isolates revealed the presence of virulence genes related to those of pathogenic F. varium human isolates associated with active invasion of mammalian cells.