Antigenic Differences Between Mycobacterium bovis Strain BCG and an Isoniazid-Resistant Mutant.

The antigenicities of Mycobacterium bovis strain BCG and an isoniazid (INH) - resistant mutant were compared. Sonic extracts of heat-killed, washed bacteria were injected into rabbits, and antisera were produced (anti-BCG and anti-BCG-R). The sonic extracts were also subjected to ultracentrifugation, and test antigens were prepared from the resulting supernatants (BCG and BCG-R). The antigens were labeled with (131)I, and the antisera were tested for their capacity to bind (131)I-BCG and (131)I-BCG-R by precipitating (131)I-antigen-antibody complexes with anti-rabbit gamma globulin. Significant differences were detected between the two test antigens. Immunodiffusion studies with unabsorbed antisera and antisera absorbed with BCG and BCG-R suggested that an antigenic component in BCG is lacking or diminished in BCG-R and that anti-BCG has antibodies not found in anti-BCG-R. These quantitative and qualitative differences were confirmed by inhibition studies. Complete inhibition was achieved in all systems by both test antigens when either was added in sufficient concentration. As expected, BCG-R inhibited the binding of (131)I-BCG-R by anti-BCG-R or anti-BCG (or both) to a greater extent than BCG, and BCG was a better inhibitor in an (131)I-BCG-anti-BCG-R system. However, in an (131)I-BCG-anti-BCG test system, BCG was the better inhibitor only at high concentrations. It was concluded that the test antigens possess different proportions of identical or cross-reacting components and that INH-sensitive organisms possess a component(s) that is diminished or absent in the INH-resistant mutant. Whether this component(s) is responsible for INH-resistance is not known.