In vitro susceptibility of bovine digital dermatitis associated spirochaetes to antimicrobial agents.

Bovine digital dermatitis (BDD) is an infectious lameness in cattle, which has a large global impact in terms of animal welfare and cost. The majority of evidence suggests that spirochaetes are the aetiological agent of this disease. The aim of this study was to identify the susceptibility of BDD associated spirochaetes to a range of antimicrobial agents with a view to potential usage in vivo to treat this widespread cattle disease. A microdilution method was adapted to determine the in vitro susceptibilities of 19 UK digital dermatitis spirochaetes (6 Treponema medium/Treponema vincentii-like, 8 Treponema phagedenis-like and 5 Treponema denticola/Treponema putidum-like) to eight relevant antimicrobials. The BDD spirochaetes exhibited the highest susceptibility to penicillin and erythromycin and this information may now be used to aid development of efficacious treatments. This study has also identified that BDD spirochaete T167 is spectinomycin resistant and that the likely biological basis is a point mutation in the 16S rRNA gene. Interestingly, nearly all Brachyspira isolate 16S rRNA gene sequences in Genbank have this substitution, suggesting it may be responsible for the characteristic spectinomycin resistance reported for the Brachyspira genus.