Unexpected finding ofFusobacterium variumabundance in cattle rumen: implications for liver abscess interventions

Fusobacterium variumhas been generally overlooked in cattle rumen microbiome studies relative to the presumably more abundant, liver abscess-causingFusobacterium necrophorum. Here, we revisit that conventional wisdom and report greater relative abundance ofF. variumthanF. necrophorumin both raw rumen samples and in lactate-supplemented enrichments tailored forF. necrophorumgrowth, despite its consistent inadvertence in past ruminal surveys and putative inability to metabolize lactate. Our observation thatF. variumgrows under restrictive conditions used to enumerateF. necrophorumsuggests that previous estimations were inaccurate andF. variumis 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 allF. necrophorumstrains screened by greater than 67% relative to unexposed controls. In contrast,F. variumstrains 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 againstF. necrophorumthan againstF. varium. Finally, preliminary genomic analysis of twoF. variumbovine isolates revealed the presence of virulence genes related to those of pathogenicF. variumhuman isolates associated with active invasion of mammalian cells.ImportanceJudicious 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 thatF. necrophorumin the rumen is the main etiologic agent. However, our unexpected finding of abundance ofF. variumin the rumen and its resistance to antibiotics, in hand with the potential pathogenicity of this species, calls for increased attention toF. varium. Further investigation intoF. variumis necessary to better understand bovine liver abscess development and devise higher-precision alternatives to antibiotic treatment.