A study of the relationship between the resistance of Mycobacterium tuberculosis to isonicotinic acid hydrazide (isoniazid) and to thiophen-2-carboxylic acid hydrazide.

Thiophen-2-carboxylic acid hydrazide (TCH) is used to distinguish TCH-sensitive bovine, 'Asian human' and 'African' strains from TCH-resistant 'classical' human strains of Mycobacterium tuberculosis. It has been claimed that this test cannot be applied to isoniazid-resistant strains as these also become resistant to TCH. Although such cross-resistant mutants were readily isolated in vitro, a study of the incidence of TCH resistance in a large series of INH-sensitive and -resistant strains isolated from patients indicated that the emergence of this type of mutant is the exception rather than the rule in vivo. Thus, the use of TCH for subdividing the species M. tuberculosis remains valid for epidemiological purposes, irrespective of the occurrence of isoniazid resistance.