Genetic Aspects of Drug Resistance 49 



(1) induction, either in non-genie or in genie components of 

 the bacterial cell, by the drug used in treatment; (2) gene 

 mutation, independent of the treatment, and subsequent 

 selection of the mutants by the drug. The primary object 

 of these studies has been to determine whether or not resistant 

 variants appear spontaneously in a bacterial population and 

 give rise to clones of resistant individuals, as would be ex- 

 pected if they originate by mutation but not if their origin 

 were dependent upon non-genetic change of phenotype. 

 Several methods have been used. 



The first evidence of genie origin of resistance was obtained 

 with Staphylococcus aureus and penicillin (Demerec, 1945), by 

 means of the so-called fluctuation test developed by Luria 

 and Delbriick (1943) in their studies of the origin of resistance 

 to bacteriophages. The fluctuation test was later performed 

 with several other bacteria and several other drugs (Oakberg 

 and Luria, 1947; Demerec, 1948; Newcombe and Hawirko, 

 1949), with results that confirmed the genetic origin of resist- 

 ance. Exceptions that have been reported (Eriksen, 1949; 

 Welsch, 1952; Banic, 1954) are readily explainable (Cavalli- 

 Sforza and Lederberg, 1953) in terms of the nature of the 

 materials used or the techniques employed, and do not 

 constitute evidence against the genetic origin of resistant 

 variants. The fluctuation test allows quantitative predictions 

 of the magnitude of any variance dependent upon mutation. 

 By means of statistical models (Lea and Coulson, 1949; 

 Armitage, 1952) it has been possible to predict the exact 

 distribution and size of clones on the basis of the mutational 

 theory. 



A second method of testing the spontaneous origin of 

 resistant variants is based on a technique devised by New- 

 combe (1949) for studies of phage resistance. Newcombe's 

 technique compared the number of resistant colonies observed 

 on plates to which phage had been added after the growth of 

 microcolonies with the number observed on control plates on 

 which the microcolonies had been dispersed by spreading just 

 before the application of the phage. By the introduction of 



