Observations on the Numbers of Virus-Resistant Mutants Developing in the Host-Virus System of the Lactic Streptococci

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Abstract
Virus-resistant mutants present in 6 single-strain cultures of lactic streptococci were enumerated by 2 plate culture methods and a multiple subculture method. The number of resistant mutants by plating methods varied from 0 to 66.2 per 1 x 106 colony count of the test cultures. The mutation rate of two Streptococcus lactis cultures was higher than that of four cultures of S. cremoris. The multiple subculture method revealed a mutation rate for one culture of S. cremoris, of about 1 per 3 x 109 host colonies. The age of culture and temperature of incubation appeared to have little effect on the mutation rate.