GENETIC-DIFFERENCES BETWEEN BCG SUBSTRAINS

Setting: University-affiliated Mycobacteriology Reference Laboratory. Objective: To determine the genetic differences of 25 BCG isolates representing 16 referenced substrains. Design: Non-randomized, observational study based on the visual comparison of the large restriction fragment (LRF) patterns created by digesting each BCG isolate's DNA with an infrequent cutting restriction endonuclease (DraI, AsnI, XbaI or SpeI) and separating the resultant DNA fragments with pulsed field gel electrophoresis. Results: The 25 BCG isolates gave 13 different DraI LRF patterns, 11 different XbaI LRF patterns, 11 different AsnI LRF patterns, and 15 different SpeI LRF patterns. Examples of the same BCG substrains from different sources produced the same LRF patterns for only 2 of 6 substrains studied. These findings suggest a significant degree of genetic diversity in this group of isolates despite a common origin. Four clinical BCG isolates gave LRF patterns identical to BCG Tice, BCG Connaught or BCG Glare. The BCG LRF patterns more closely resembled patterns of Mycobacterium bovis than M. tuberculosis. Conclusions: LRF patterns can accurately identify specific BCG substrains and will be useful in epidemiologic studies, monitoring vaccine production and studies of BCG vaccine efficacy.