Long-term survival of tuberculosis complex mycobacteria in soil

Abstract

While there is evidence for the persistence of Mycobacterium bovis in soil, there are no reports for the other Mycobacterium tuberculosis complex (MTC) mycobacteria. Here, soil was inoculated with 108 c.f.u. g−1 M. tuberculosis, M. bovis and M. canettii and subcultured monthly for 12 months. The pathogenicity of mycobacterial colonies, identified by using matrix-assisted laser desorption/ionization time of flight mass spectrometry, was assessed in a mouse model. Moreover, mice were fed with food that contained 16.7 % M. tuberculosis-contaminated soil. The three tested MTC species survived in soil for 12 months with a final inoculum of 2×103 c.f.u. g−1 for M. tuberculosis, 150 c.f.u. g−1 for M. bovis and 2×104 c.f.u. g−1 for M. canettii. In an experiment that included negative controls, all (5/5) mice inoculated with such M. tuberculosis and M. canettii developed 0.03–0.3 granulomas mm−2 in their lungs and spleen and grew mycobacteria; five mice that were inoculated with M. bovis from soil did not develop granulomas but grew mycobacteria. Furthermore, 0.2–0.4 granulomas mm−2 were observed in the lungs and spleen of 3/5 mice fed with M. tuberculosis-contaminated soil in the presence of two negative control mice. M. tuberculosis grew in the stomach, intestine, spleen and lung in 5/5 challenged mice, whereas the negative controls remained M. tuberculosis-free (P = 0.008, Fisher exact test). This study provides clear evidence that MTC mycobacteria survive in soil, and that M. tuberculosis remains virulent while in the soil, outside its hosts, for extended periods of time.