Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Veterinary Medical Sciences - Pathobiological Sciences

First Advisor

Philip H. Elzer


The genus Brucella is composed of Gram-negative, facultative intracellular pathogens that impact man and animals. Brucella species express a 25 kDa major outer membrane protein termed Omp25. To determine the role of Omp25 in virulence, mutants were created in Brucella abortus (BA25), Brucella melitensis (BM25), and Brucella ovis (BO25) that contain disruptions in the omp25 gene (Deltaomp25 mutants). In the murine brucellosis model, each Deltaomp25 mutant was attenuated when compared to the virulent parental strain. To assess the ability of the B. abortus and B. melitensis mutants to colonize and induce abortions in the ruminant host, cattle and goats in late-gestation were inoculated with BA25 or BM25, respectively. Significant decreases in the abortion and colonization profiles of BA25-infected cattle and BM25-infected goats were observed when compared to animals inoculated with the parental strains (P < 0.05, n = 10). While virulent B. abortus colonized 10/10 calves and induced abortions in 5/10 dams, BA25 colonized only 3/10 calves and caused one abortion. BM25 did not cause abortions and failed to colonize 5/9 caprine dams compared to 6/6 colonized dams and abortions in goats inoculated with the parental strain. The capacity of brucellae to replicate inside macrophages and chorionic trophoblasts plays an important role in fetal pathogenesis. The in vitro replication rate of the B. abortus Deltaomp25 mutant in these cells is significantly less than the replication rate of the parental strain (P < 0.05, n = 5). Such differences may explain the attenuation of both BA25 and BM25 in vivo. Because they appeared attenuated, the effectiveness of the mutants as vaccine candidates was subsequently determined. Vaccination of mice with either BM25 or BO25 provided significant protection against challenge with the homologous virulent parental strain (P < 0.01, n = 5). Vaccination of goats with BM25 provided protection against abortion and colonization at levels equal to the current caprine vaccine, B. melitensis strain Rev. 1. Unlike strain Rev. 1 which causes abortions when given to pregnant goats, BM25 is safe for pregnant goats and may be a possible vaccine alternative to strain Rev. 1. In summary, the described Brucella Delta omp25 mutants are attenuated and efficacious against virulent challenge.