236 B. Gyorffy and I. Kallay 



cultures. This was compatible with the spontaneous occurrence of 

 resistant variants (Demerec, 1948; Newcombe and Hawirko, 1949). 

 The actual distribution of numbers of resistant cells in independent 

 cultures, in three fluctuation tests, was compared with the theoretical 

 distribution (Lea and Coulson, 1949; Armitage, 1953). The good 

 correspondence between the observed and theoretical distribution 

 of resistants was satisfactory ; a better correspondence could not be 

 hoped for within the limits of experimental error. This result can 

 be considered as quantitative evidence in support of the spontaneous 

 occurrence of streptomycin-resistant cells. 



The mutation rates were calculated by means of the following 

 four methods: estimating the number of mutations (1) from the 

 proportion of cultures with no mutants (Luria and Delbriick, 1943); 

 (2) from the mean (Luria and Delbriick, 1943), (3) from the median 

 (Lea and Coulson, 1949) and (4) from the maximum value of the 

 number of resistants (Newcombe, 1948). The mutation rates were 

 found to be approximately of the order of 10~^ at a screening level 

 of 100 (Jig. streptomycin/ml., and progressively lower at screening 

 levels of 80, 50 and 30 [xg./ml. (in a single experiment). 



Indirect-selection experiments, by the replica plating technique 

 of Lederberg and Lederberg (1952), have been carried out repeatedly 

 without success ; indirect-selection experiments by the use of enrich- 

 ment cycles (Cavalli-Sforza and Lederberg, 1956) are now in 

 progress. 



On the whole, our observations seem to support the theory of the 

 mutational origin of resistance to streptomycin, in the case of 

 Serratia marcescens . These results will be published in detail. 



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Abraham, E. P. (1953). Symp. Soc. gen. Microbiol., 3, 201. 



Akiba, T. (1955). In Origins of Resistance to Toxic Agents, p. 82, Ed. 



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 Armitage, P. (1953). J. Hyg., 51, 162. 

 Barer, G. R. (1953). J. gen. Microbiol., 5, 1. 



Cavalli-Sforza, L. L., and Lederberg, J. (1956). Genetics, 41, 367. 

 Demerec, M. (1948). J. BacL, 56, 63. 

 Demerec, M. (1951). Genetics, 36, 585. 



Eagle, H., Fleischman, R., and Ltlvy, M. (1952). J. Bad., 63, 623. 

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