Unexpected finding of Fusobacterium varium as the dominant Fusobacterium species in cattle rumen: potential implications for liver abscess etiology and interventions

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 ribosomal ribonucleic acid 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 encourage further investigation into the ecological role of F. varium within the bovine rumen and potential role in liver abscess development, and proactive interventions.

Characterization of Fusobacterium isolates from the respiratory tract of white-tailed deer (Odocoileus virginianus)

A total of 23 clinical isolates of Fusobacterium spp. were recovered at necropsy over a 2-year period from the respiratory tract of white-tailed deer ( Odocoileus virginianus). Isolates were identified as Fusobacterium varium (18/23), Fusobacterium necrophorum subsp. funduliforme (3/23), and Fusobacterium necrophorum subsp. necrophorum (2/23). Using polymerase chain reaction–based detection of virulence genes, all F. necrophorum isolates were positive for the promoter region of the leukotoxin operon and the hemagglutinin-related protein gene, while all F. varium isolates were negative. The presence of the leukotoxin gene in F. necrophorum isolates and the absence of this gene in F. varium isolates were confirmed by Southern hybridization using 2 separate probes. Toxicity to bovine polymorphonuclear leukocytes was observed with all F. necrophorum isolates, but was not observed in any F. varium isolates. Susceptibility to antimicrobials was markedly different for F. varium as compared to F. necrophorum. In summary, no evidence of leukotoxin production was detected in any of the 23 F. varium isolates used in the current study. The data suggests that F. varium, the most common species isolated, may be a significant pathogen in deer with a different virulence mechanism than F. necrophorum.

Identification of a 43-kDa outer membrane protein of Fusobacterium necrophorum that exhibits similarity with pore-forming proteins of other Fusobacterium species

A pair of primers was designed in an attempt to amplify outer membrane protein (OMP) gene of Fusobacterium necrophorum based on nucleotide sequence of the OMP of Fusobacterium nucleatum. Further analysis was performed to characterize its molecular properties and phylogeny in the genus Fusobacterium. We identified a predicated 43kDa outer membrane protein (43K OMP) in F. necrophorum, which showed the same properties as other pore-forming proteins of Gram-negative anaerobic bacteria according to analysis of signal peptide, AT-rich, membrane-spanning region and conserved motifs. The predicated 43K OMP exhibited 70.22%, 62.04%, 56.75%, 58.72%, 51.59%, 31.49% and 50.26% amino acid identity with the OMPs of F. nucleatum, Fusobacterium varium, Fusobacterium ucerans, Fusobacterium periodonticum, Fusobacterium mortiferum, Fusobacterium gonidiaformans and F. necrophorum (hypothetical protein), respectively. 11 common conserved domains and 10 common variable domains were found among the 45 aligned OMPs of Fusobacterium species. Distributions of the conserved and variable domains were highly associated with predicted membrane-spanning regions, cell surface exposed regions and B-cell epitope regions. Phylogenetic analysis revealed the predicated 43K OMP of F. necrophorum was closely related with the OMPs from F. nucleatum and F. periodonticum. These data will increase understanding of pathogenesis and genetic evolution of F. necrophorum.