Naturally Attenuated, Orally Administered Mycobacterium microti as a Tuberculosis Vaccine Is Better than Subcutaneous Mycobacterium bovis BCG

Mycobacterium microti is phylogenetically closely related to Mycobacterium tuberculosis and is a member of that complex of organisms. It is a curved, acid-fast bacillus that is naturally attenuated with a narrow host range for Microtus species only. In this study, we confirm the unique susceptibility of voles to infection with M. microti and the relative resistance of mice with a significantly lower organism burden after 8 weeks of infection. In addition, histopathologic examination of lungs reveals a lack of cellular, granulomatous aggregates characteristically seen in murine M. tuberculosis infection. In the past, M. microti has been used successfully in humans as a vaccine against tuberculosis but was associated with cutaneous reactions. In an attempt to circumvent this adverse effect, we report the efficacy of aerosol and oral vaccination with M. microti. High-dose orogastric vaccination with M. microti resulted in a statistically significant improvement in protection against aerosol challenge with virulent M. tuberculosis in the murine model compared with subcutaneous M. bovis BCG Pasteur vaccination.