Short Communication 



DEVELOPMENT OF RESISTANCE TO STREPTOMYCIN 

 IN SERRATIA MARCESCENS 



B. Gyorffy and I. Kallay 

 Institute of Genetics, Hungarian Academy of Sciences^ Budapest 



Our work in the field of bacterial genetics was begun three years 

 ago, and the results of our studies concerning the problem of the 

 development of streptomycin resistance in Serratia marcescens are 

 summarized here. 



The bactericidal action of streptomycin was determined by the 

 proportion of cells surviving on exposure to streptomycin for a 

 limited time. Our results agreed with those already obtained for 

 other bacteria (Demerec, 1951; Linz and Lecocq, 1955). 



The bacteriostatic action of streptomycin was measured by 

 counting colonies formed on streptomycin-agar. The overall picture 

 of the distribution of the surviving fraction was similar to that 

 reported by other workers (Demerec, 1948; Welsch, 1952). The facts 

 that repeated experiments gave the same fraction of survivors and 

 that not all of the cells survived even at low concentrations sug- 

 gested the pre-existence of resistant variants in the populations 

 (Sneath, 1956). 



Colonies picked at random from streptomycin plates were retested. 

 Many of these clonal populations, and in particular those from the 

 faintly pigmented colonies, when grown at 4, 6 and 8 [ig. strepto- 

 mycin/ml. consisted almost wholly of cells having the same sensitivity 

 as that of the original population. Some colonies, however, with 

 normal pigment production, consisted of cells capable of forming 

 the same number of colonies on plates containing 8 and 16 \xg. 

 streptomycin/ml., respectively, as they did on control plates; and 

 giving about 10 per cent survival on plates containing 24 and 36 ]ig. 

 streptomycin/ml., respectively. This low degree of resistance of 

 substrains was maintained after several subcultures in drug-free 

 medium. 



Above the threshold concentration, when strong selection had 

 already taken place, the possibility of resistant colonies occurring 

 was increased. Although some of the colonies formed on plates con- 

 taining 16-36 fxg. streptomycin/ml. consisted of " persistors ", many 



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