Unexpected finding of Fusobacterium varium abundance in cattle rumen: implications for liver abscess 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. Importance Judicious antibiotic use is essential to mitigate the spread of antimicrobial resistance. Dogmatic prophylactic use of in-feed tylosin to control cattle liver abscesses hinges on the assumption that F. necrophorum in the rumen is the main etiologic agent. However, our unexpected finding of abundance of F. varium in the rumen and its resistance to antibiotics, in hand with the potential pathogenicity of this species, calls for increased attention to F. varium . Further investigation into F. varium is necessary to better understand bovine liver abscess development and devise higher-precision alternatives to antibiotic treatment. ### Competing Interest Statement The authors have declared no competing interest.