184 ROLLIN D. HOTCHKISS AND AuDREY H. EvANS 



of times (e.g. Hotchkiss and Marmur, 1954) they have not 

 been described in any detail. 



Of the fifteen drug resistance traits that have by now been 

 transformed with DNA into Pneumococcus (and one or two 

 into Hemophilus influenzae) all have so far been transferred 

 in essentially the normal form as first encountered. In the 

 case of penicillin-resistant pneumococci, the DNA from 

 multiple-step highly resistant donor strains gave trans- 

 formation to unit resistance steps (Hotchkiss, 1951), but none 

 of the transformants detected bore mutant properties other 

 than those which had been encountered in the history of the 

 donor strain. Essentially the same result was obtained with 

 the first sulphonamide resistance transformations and a 

 number of others. 



In 1955, however, a highly resistant pneumococcal mutant, 

 designated Fn, was isolated after selection in a single ex- 

 posure to sulphanilamide. The indications were that a rare 

 single-step mutation had occurred, resulting in a resistance 

 to more than 600 [ig. sulphanilamide per ml. of standard 

 medium. Like the somewhat analogous single-step mutant 

 obtainable from many bacterial species and highly resistant 

 to streptomycin, this new pneumococcal strain is stable and 

 can be propagated indefinitely without change of resistance 

 level. 



Disseminative Transformations 



By contrast, when the strain Fn was used as donor of DNA 

 to transform the sensitive parent strain, only a very few 

 transformants displayed the high resistance of the donor. 

 A far greater number of transformants were obtained which 

 were resistant only to lower concentrations of sulphanil- 

 amide. Furthermore, when examined at a series of drug con- 

 centrations, several fairly distinct classes of transformants 

 could be identified, having quantitatively different resistance 

 toward sulphonamide, and all but one had a lower resistance 

 than the donor. By this direct observation and isolation, 

 four classes could at once be recognized in a first trans- 



