246 R. Knox 



have so far had no success. In some such way it might 

 be possible to explain intermediate stages between a stable 

 drug-resistant mutant on the one hand, and a purely pheno- 

 typic and temporary adaptation on the other. 



REFERENCES 



Bigger, J. W. (1944). Lancet, ii, 497. 



Dean, A. C. R., and Hinshelwood, C. N. (1953). Sym. Soc. gen. 



Microbiol., 3, 21. 

 Fisher, M. W. (1952). Amer. Rev. Tuberc, 66, 626. 

 Fisher, M. W. (1954). Amer. Rev. Tuberc, 69, 797. 

 Hinshelwood, C. N. (1946). The Chemical Kinetics of the Bacterial 



Cell. Oxford: Clarendon Press. 

 Knox, R. (1955). Lancet, ii, 110. 

 Knox, R., Swait, E. M., and Woodroffe, R. (1956). J. gen. Microbiol., 



15, 359. 

 Knox, R., and Woodroffe, R. (1957). J. gen. Microbiol., 16, No. 3, 



in press. 

 MiDDLEBROOK, G. (1954). Amcr. Rev. Tuberc, 69, 471. 

 MiDDLEBROOK, G. (1956). Amcr. Rev. Tuberc, 74, No. 1, 42. 

 Mitchison, D. a. (1952). Lancet, ii, 858. 

 MiTCHisoN, D. A. (1953). J. din. Path., 6, 118. 



SzYBALSKi, W., and Bryson, V. (1952). Amer. Rev. Tuberc, 65, 768. 

 YuDKiN, J. (1953). Nature, Lond., Ill, 541. 



DISCUSSION 



Pollock: What is the range of different isoniazid concentrations 

 between the end-point for the majority of cells and the end-point for the 

 " pseudomutants " ? 



Knox: It depends a great deal on the inoculum size, but the arbitrary 

 end-point is usually about 0-01 [ig. in cultures grown for 2-6 days. 

 These pseudomutants eventually grow to a range of about "10 [ig. ". 

 The method which we used in these tubes is simply to put rubber bungs 

 in which prevent evaporation ; and we can then put the tube in without 

 disturbing them and look at them when we like. Where a lot of growth is 

 occurring, a lack of oxygen will develop very quickly, and we are 

 investigating that; but where no growth occurs that does not apply. 



Pollock: A similar effect is seen with penicillin resistance in B. cereus at 

 the end-point ; but there is not such a big difference in concentration as 

 you have found. The likely explanation in the case of B. cereus, and one 

 which might apply in your case, is that there may be an inducible enzyme 

 responsible for destroying the drug. You would have a very delicate 

 balance between two opposite effects of the isoniazid. One would be the 

 inducing effect on the enzyme which destroys it ; the other would be its 



