Tuberculosis (TB) remains an enormous global health problem, and a new vaccine against TB more potent than the current inadequate vaccine, Mycobacterium bovis BCG, is urgently needed. We describe a recombinant BCG vaccine (rBCG30) expressing and secreting the 30-kDa major secretory protein of Mycobacterium tuberculosis, the primary causative agent of TB, that affords greater survival after challenge than parental BCG in the highly demanding guinea pig model of pulmonary TB. Animals immunized with rBCG30 and then challenged by aerosol with a highly virulent strain of M. tuberculosis survived significantly longer than animals immunized with conventional BCG. The parental and recombinant vaccine strains are comparably avirulent in guinea pigs, as they display a similar pattern of growth and clearance in the lung, spleen, and regional lymph nodes. The pMTB30 plasmid encoding the 30-kDa protein is neither self-transmissible nor mobilizable to other bacteria, including mycobacteria. The pMTB30 plasmid can be stably maintained in Escherichia coli but is expressed only in mycobacteria. The recombinant and parental strains are sensitive to the same antimycobacterial antibiotics. rBCG30, the first vaccine against TB more potent than nearly century-old BCG, is being readied for human clinical trials.Tuberculosis continues as a major global health problem, especially in the developing world where 1 in 6 adults between the ages of 15 and 59 dies from this disease (10, 22). The AIDS epidemic, whose victims are severalfold more susceptible to tuberculosis than the general population, and the worldwide emergence of drug-resistant strains of Mycobacterium tuberculosis, the primary causative agent of tuberculosis, compound the problem (8,14). A better vaccine against tuberculosis is urgently needed (11, 23). The current vaccine, Mycobacterium bovis BCG, a live attenuated vaccine derived from the bovine tuberculosis bacillus in the early 1900s by Calmette and Guérin, while protective against disseminated forms of tuberculosis such as meningitis and miliary tuberculosis, is of inconsistent efficacy against pulmonary tuberculosis, the dominant form (9, 12).In a previous study (20), two recombinant BCG vaccines (rBCG30) overexpressing the major secretory protein of M. tuberculosis, a 30-kDa mycolyl transferase (2, 27), were described. This protein is not only the major secretory protein of M. tuberculosis in broth culture (19) but it is also among the major proteins of all M. tuberculosis proteins expressed in human macrophages (15, 21). Derived from the commercially available Connaught (Conn) and Tice strains of BCG, the recombinant vaccines were tested in the demanding guinea pig model of pulmonary tuberculosis. In contrast to other small animals used for models of tuberculosis, guinea pigs are much more susceptible to tuberculosis than humans, yet they develop disease that closely mimics human disease clinically, immunologically, and pathologically. Guinea pigs immunized with the recombinant vaccines and then challenged by aerosol...