INDUCED MUTATIONS IN E. COLI 



265 



It appears, however, that the actual mutants isolated 

 after desoxycholate, pyronin or acriflavine treat- 

 ment were not tested for resistance to these chemi- 

 cals. It should be pointed out that only a modest 

 degree of chemical-resistance in only a portion of the 

 isolated mutant clones would allow the observed 

 increased proportion of mutants among the sur- 

 vivors. 



To bring out this point more clearly I may be 

 permitted perhaps to recalculate one of the tables 

 from the lantern slide dealing with acriflavine- 

 treated cultures. The other tables would give very 

 similar results. From the figures given it is pos- 

 sible to recalculate the actual yield of mutants ob- 

 served from each 10 8 bacteria of the original cul- 

 ture. While this column has to be calculated from 

 another in this instance the point must be stressed 

 that it was originally an observed part of the ex- 

 perimental data and tabulating it involves no par- 

 ticular assumptions. 



the individual cells affected within a population or 

 to the characters, genetic or somatic, affected within 

 a given cell. Once a particular cell, by virtue of its 

 permeability, physiological state, internal pH, or 

 other inherent or transient property, has accumu- 

 lated in its interior sufficient chemical mutagen to 

 suffer a mutation at one locus, we would indeed 

 expect that many other changes might become 

 manifest. This picture would furnish one plausible 

 basis for the conservative viewpoint that only a 

 moderate amount of chemically-induced mutation 

 has taken place, and that this has been to some 

 degree associated with temporary or inheritable re- 

 sistance to the chemicals, a finding not to be ex- 

 pected to any great extent in spontaneous or irradia- 

 tion-induced mutants. 



Witkin: Dr. Hotchkiss has emphasized the fact 

 that the absolute recovery of mutants falls off, ex- 

 cept in the range of high survival, as the exposure 

 to the chemical is extended. This is true not only 



It will be noted that, considered from this point of 

 view, the most striking production of mutants was 

 in the low-killing range where an increase of some 

 30% was observed. Detected as it presumably was 

 in counts of some one or two hundred colonies, this 

 increase represents at present the only positive in- 

 dication, free of assumptions, that new mutants have 

 been produced. The particular figure is of course 

 rather dependent upon the value taken for the 

 background content of mutants; what was under- 

 stood to be an average figure was used here. In any 

 case, where the killing has been greater, the "yield" 

 of mutants has also decreased well below the back- 

 ground value. 



Although several theories or explanations come 

 to mind to account for the findings, it should be 

 pointed out that these comments arise as it were 

 from experimental considerations only and are sug- 

 gested mainly as a more conservative way of ex- 

 pressing the data. When it has been demonstrated to 

 what degree, if any, representative mutants recovered 

 from the chemical mutagen solutions are resistant to 

 that mutagen, then it will be more safe to calculate 

 the actual yield of mutants, dead and alive, produced 

 from 10 8 original cells. 



Finally, it may be pointed out that there is no 

 reason to suppose that mutations induced by chemi- 

 cals need show randomness with repect either to 



for the chemicals studied in these experiments, but 

 for radiations as well. An absolute increase of mu- 

 tants is to be expected only in the case of nontoxic 

 mutagens, or in the case of agents whose mutagenic 

 activity is relatively much greater than its toxicity. 

 No such case is known as yet, and it seems unlikely, 

 at least for mutagens acting in an aspecific manner, 

 that such cases are theoretically possible, since in- 

 duced lethal mutations may be expected to exceed, 

 as a class, any single phenotype under consideration. 

 Nevertheless, the fact that absolute increases are 

 not observed, except where survival is high, does 

 create the necessity for caution in interpreting the 

 results. 



First of all, it can be pointed out that, for each 

 of the three chemicals considered to be mutagenic, 

 absolute increases in the recovery of mutants were 

 observed under special circumstances. In the case 

 of acriflavin and pyronin, the actual number of 

 mutants recovered from a given number of original 

 bacteria increases significantly when the killing does 

 not exceed 50% to 60%. In the case of sodium 

 desoxycholate, it is possible to use low concentra- 

 tions where the maximal killing is reached by 24 

 hours, and there is no further killing between 24 and 

 48 hours of exposure. The number of mutants, how- 

 ever, does increase during this period, resulting in 

 actually higher mutant counts on the experimental 



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