Evaluation of novel Brucella melitensis unmarked deletion mutants for safety and efficacy in the goat model of brucellosis

Pregnant goats were employed to assess unmarked deletion mutant vaccine candidates BMΔasp24, BMΔcydBA, and BMΔvirB2, as the target host species naturally infected with Brucella melitensis. Goats were assessed for the degree of pathology associated with the vaccine strains as well as the protective immunity afforded by each strain against abortion and infection after challenge with wild-type Brucella melitensis 16M. Both BMΔasp24 and BMΔvirB2 were considered safe vaccine candidates in the pregnant goat model because they did not cause abortion or colonize fetal tissues. BMΔasp24 was isolated from the maternal tissues only, indicating a slower rate of clearance of the vaccine strain than for BMΔvirB2, which was not isolated from any maternal or fetal tissues. Both strains were protective against abortion and against infection in the majority of pregnant goats, although BMΔasp24 was more efficacious than BMΔvirB2 against challenge infection.