92 M. R. Pollock 



must be admitted that good evidence for "self-reproducing" 

 enzyme-forming systems is so far non-existent. 



It is true that there have been a number of interesting 

 claims to have demonstrated that heritable changes can be 

 specifically induced in micro-organisms by environmental 

 factors [acquisition of ability to ferment sucrose in yeast 

 (Kossikov, 1950); development of streptomycin resistance in 

 Pseudornonas (Linz, 1950); loss of penicillin resistance in 

 staphylococci after treatment with chloramphenicol (Voureka, 

 1952); lactose-induced development of ability to ferment 

 lactose in Escherichia coli ("mutabile") (Dean and Hinshel- 

 wood, 1954); proflavine-induced proflavine resistance in 

 Aerobacter aerogenes (Dean, 1955) etc.]. But either these 

 have been open to theoretical objections or the phenomena 

 themselves have not yet been fully confirmed by other workers. 

 It would appear that specifically induced heritable change in 

 individual cells has so far been firmly substantiated only for 

 cases of transformation of bacterial strains by deoxyribo- 

 nucleic acid-containing extracts from other types of bacteria. 

 Nevertheless, other instances of "specifically directed muta- 

 tions" may well exist — and indeed should be looked for with 

 increasing persistence. 



It has already been emphasized that the changes in peni- 

 cillin resistance of B, cereus reported have involved simple 

 quantitative changes in amounts of penicillinase produced. 

 Similar, though less marked, quantitative changes (due to 

 single gene mutations) have been reported by Markert and 

 Owen (1954) and Owen and Markert (1955) for Glomerella 

 tyrosinases and by Yanofsky (1952) for Neurospora trypto- 

 phan synthase. Different strains of staphylococci (from the 

 same cultures and therefore probably derived one from 

 another by mutations) were found by Rogers (1953) to fall 

 into distinct groups with widely differing rates of hyaluroni- 

 dase production. Even the penicillin-sensitive parent strain 5 

 of the penicillin-resistant 5/B mutant forms minute amounts 

 of the same enzyme which, when produced in large quantities, 

 is responsible for the resistance of 5/B. There has been no 



