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

Abstract

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.

Detection of Tylosin Resistance in Fusobacterium necrophorum subspecies necrophorum

“…Here, we report the two strains containing the erm(B) gene have dose dependent, growth phase, and preconditioning dependent responses, showing a lack of inhibition at environmentally relevant concentrations and up to 100 μg/mL for FN38. We also ran the same experiments on a strain previously observed to be susceptible to tylosin: 40 ATCC 25286, whose genome was previously sequenced and annotated. 42 FNC, a strain highly susceptible to tylosin, and whose genome we sequenced, was also included.…”

“…Previously, the tylosin susceptibility of F. necrophorum has been discussed at length, using a range of dosages in both laboratory experiments and in vivo (4 μg/mL, 9.4–16 μg/mL, 10.5–11.5 μg/mL, 12 μg/mL, 45 μg/mL, 90 mg/steer/day). Here, we report the two strains containing the erm (B) gene have dose dependent, growth phase, and preconditioning dependent responses, showing a lack of inhibition at environmentally relevant concentrations and up to 100 μg/mL for FN38.…”

Investigating the impact of feed-induced, subacute ruminal acidosis on rumen epimural transcriptome and metatranscriptome in young calves at 8- and 17-week of age

“…In agreement with our results, Petri et al ( 66 ) found a high abundance of the genera Olsenella and Comamonas in animals with induced ruminal acidosis. The Fusobacterium genus is a normal inhabitant of the rumen of cattle, however, some species from this genus are opportunistic pathogens and primary causative of rumen abscesses ( 67 , 68 ). The presence of this microbial genera in the treated animals highlights the possibility of using microbial taxa as biomarkers for subacute acidosis.…”