Cross‐protection from Bacteroides nodosus vaccines and the interaction of pili and adjuvants

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Abstract
SUMMARY The effects of vaccination of Merino sheep with the purified pili or the whole cells of Bacteroides nodosus strain 198, either in oil or alum-oil adjuvant, on the severity of footrot induced with the homologous strain (198) and a heterologous strain (217) were determined in a field experiment, on flood irrigated pasture. The efficacy of the whole call vaccines was comparable to that of purified pili vaccines, against homologous challenge, when both had a similar content of pilus antigen although the purified pili vaccines induced significantly greater homologous pilus agglutinating antibody titres than the whole cell vaccines. However, against heterologous challenge, the whole cell vaccines in oil (CO) or alum-oil (CAO) provided significantly greater protection than a purified pili-in-oil (PPO) vaccine, the number of severely affected feet in sheep vaccinated with PPO being similar to that of the unvaccinated group. The group vaccinated with purified pili In alum-oil (PPAO) was intermediate between these two extremes. The superior performance of the PPAO in comparison to the PPO vaccine, against heterologous challenge, was associated with significantly higher mean ELISA titres to the outer membrane complex. Western blot analyses implicated a role in cross-protection for outer membrane proteins, in particular a protein M, 78,000. The PPO vaccine produced fewer, smaller and less persistent vaccination reactions at the inoculation sites than did the other vaccines. Bodyweight gains in the period prior to challenge were much lower for the groups vaccinated with CO and CAO than for the controls and those vaccinated with purified pili, due presumably to the larger vaccination reactions. During the foot-rot transmission period bodyweight gains were greater for all vaccinated groups than for the controls for those sheep challenged with the homologous strain. However, no such differences were observed between the groups challenged with the heterologous strain.