Abstract
The burgeoning tuberculosis epidemic worldwide is mainly due to the reactivation of old latent tuberculosis infection. South Africa is rated as one of the countries with the worst tuberculosis epidemic and the population that is mostly affected is the mineworkers. Reports suggest that reactivation of latent tuberculosis 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, diabetes mellitus, immunosuppressive drug therapy, Human Immunodeficiency Virus infection and malnutrition.
The aim of this study was to determine whether there is any relationship between exposure to low levels of ionizing radiation and reactivation tuberculosis 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 and Mycobacterium bovis BCG. Dormant pGFM-11- transformed and non-transformed cultures were then exposed to 0.1, 1 and 5Gray Cobalt-60 radiation. The radiation effects were evaluated using viable counts, the bacillary adenosine triphosphate assay and quantification of the green fluorescent protein expression using flow cytometry 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.
Dormancy was successfully induced as determined by the sensitivity of the dormant cultures to metronidazole, resistance to isoniazid and assumption of synchronous replication on dilution into oxygen-rich medium. Subsequent to exposure to Cobalt-60 radiation, 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 cultures of the tested mycobacteria retained their antibiotic susceptibility pattern and thus were not affected by Cobalt-60 radiation. It was concluded that the doses of Cobalt-60 radiation used in this study did not cause the reactivation of in vitro dormant mycobacterial strains tested.
Dr. H. Abrahamse
Mrs. J.V. Hind