The genus Brucella consists of bacterial pathogens that cause brucellosis, a major zoonotic disease characterized by undulant fever and neurological disorders in humans. Among the different Brucella species, Brucella melitensis is considered the most virulent. Despite successful use in animals, the vaccine strains remain infectious for humans. To understand the mechanism of virulence in B. melitensis, the proteome of vaccine strain Rev 1 was analyzed by two-dimensional gel electrophoresis and compared to that of virulent strain 16M. The two strains were grown under identical laboratory conditions. Computer-assisted analysis of the two B. melitensis proteomes revealed proteins expressed in either 16M or Rev 1, as well as up-or down-regulation of proteins specific for each of these strains. These proteins were identified by peptide mass fingerprinting. It was found that certain metabolic pathways may be deregulated in Rev 1. Expression of an immunogenic 31-kDa outer membrane protein, proteins utilized for iron acquisition, and those that play a role in sugar binding, lipid degradation, and amino acid binding was altered in Rev 1.Brucellosis is a major infectious disease afflicting humans and a wide range of domesticated animals worldwide. The disease is caused by several Brucella species, which are aerobic, nonmotile, gram-negative, facultative intracellular coccobacilli. The genus Brucella belongs to the âŁ-2 subgroup of the class Proteobacteria. It is subdivided, on the basis of its pathogenicity and host preference, into six nomen species: Brucella abortus, B. canis, B. melitensis, B. neotomae, B. ovis, and B. suis (12). In addition, a new strain affecting marine mammals was recently isolated and tentatively named B. maris (32). On the basis of DNA hybridization data, it was suggested that all of these organisms be placed into a single species, B. melitensis (39). Among the various nomen species, B. abortus, B. canis, B. suis, B. maris, and B. melitensis have been reported to infect humans (21,35,13). B. melitensis is a pathogen of goats and sheep and is considered the most virulent species for humans. Human infection can result from either occupational contact or ingestion of contaminated food.Vaccination and eradication of infected hosts have been key factors in the control of brucellosis. Rev 1, an attenuated strain of virulent B. melitensis, was developed in 1957 (19). It is considered the most effective vaccine for the control of brucellosis in small ruminants and was used in comprehensive vaccination programs in many countries, including Saudi Arabia, Kuwait, Mongolia, Spain, and Turkey (8).Our laboratory has been involved in a comprehensive analysis of the B. melitensis 16M proteome, and initial results have been published recently (40). Previous proteomics studies using B. melitensis cells grown under different conditions have been reported (36, 37), and initial work on the B. abortus proteome has been described (26, 29). A comparative study was conducted with B. abortus vaccine strains S19 and RB51 and...