244 R. Knox 



situation which is not easy to analyse, and that until more 

 precise methods are available for labelling tubercle bacilli the 

 problem of drug resistance in this group of organisms is not a 

 profitable field. But while it is true that precise genetic 

 analyses in tubercle bacilli may be more rapidly advanced 

 by discoveries going on in other groups of organisms, it is after 

 all possible that some of the problems are unique to mycobac- 

 teria, and therefore it is worth while trying to collect more 

 information about the different patterns of resistance which 

 they show with different drugs. Szybalski and Bryson (1952), 

 Middlebrook (1954), Mitchison (1952, 1953) and others have 

 already contributed much in this field. 



The experiments here described show how careful we must 

 be before we talk about mutation rates in tubercle bacilli. 

 For example, Middlebrook (1956) has stated that by a simple 

 plating technique it is possible to show that the frequency of 

 mutants resistant to high levels of isoniazid is 1 in 10^, and to 

 streptomycin 1 in 10^, and that the frequency of double 

 mutants seems to be something like the product of these two 

 rates. We have repeatedly tried to demonstrate this but have 

 come up against this phenomenon of pseudomutants. We are 

 trying to find a medium in which isoniazid does not decay, or 

 some means of maintaining the level constant by regular 

 small additions of drug. In the meantime we can say that the 

 pseudomutant phenomenon is especially marked in Kirchner 

 medium in which the drug decays rapidly, that it is less marked 

 in Fisher (1952) medium in which it decays more slowly, and 

 that it does not occur with streptomycin or PAS, both of which 

 are more stable than isoniazid in the media we have used. 



One of the most interesting features of isoniazid resistance 

 is the relation between resistance, catalase activity and 

 virulence. It seems to be generally agreed that there are at 

 least two types of isoniazid-resistant tubercle bacilli: (1) stable, 

 highly resistant catalase-negative strains (Middlebrook, 1954) 

 and (2) unstable, catalase-positive strains of lower resistance. 

 A possible explanation for these two kinds of resistant 

 culture is suggested here. 



