Six strains of of different virulence in guinea-pigs were compared with regard to their resistance to low pH, to hydrogen peroxide (O) at different pH values and to superoxide (·O-). Low virulence was associated with susceptibility to HO in native and isoniazid-resistant strains but not in laboratory-attenuated strain H37Ra. HO resistance was only partly related to catalase content. Low virulence was not associated with susceptibility to an acid environment but the tuberculocidal effect of HO was significantly increased at low pH. The strains were uniformly resistant to ·O- and contained similar amounts of superoxide dismutase. The implications of these observations are discussed in the context of mechanisms of host defence in tuberculosis.


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