Effect of Cobalt-60 radiation on dormant Mycobacterium smegmatis in vitro

The burgeoning tuberculosis epidemic worldwide is mainly due to the reactivation of latent tuberculosis (TB) infection. South Africa is rated as one of the countries with the worst TB epidemic and the population that is mostly affected is the mineworkers. It is suggested that reactivation of latent TB infection is responsible for these high tuberculosis rates in this population. Various risk factors for reactivation of latent TB have been identified and include silicosis, diabete mellitus, immunosuppressive drug therapy, Human Immunodeficiency Virus infection and malnutrition. The aim of this study was to determine if there is any relationship between exposure to low levels of ionizin radiation and reactivation TB by evaluating the effects of radiation on dormant mycobacteria in vitro. The Wayne in Vitro dormancy model was used to induce dormancy in Mycobacterium smegmatis. Dormant cultures were exposed to 0.1, 1 and 5 Gray cobalt-60 radiation. Radiation effects were evaluated using viable counts after 24 and 72 hours of radiation exposure. Exponential phase cultures treated in exactly the same way as the dormant cultures together with the cultures that were not exposed to any radiation served as controls. Subsequent to Cobalt-60 irradiation, the dormant cultures were sensitive to metronidazole and resistant to isoniazid and the inverse was observed in irradiated exponential phase cultures. The results suggested that both dormant and exponential phase Mycobacterium smegmatis cultures retained their antibiotic susceptibility pattern and thus were not affected by Cobalt-60 radiation.