BACTERIOPHAGE-RESISTANT MUTANTS IN ESCHERICHIA COLI

Abstract

ACTERIA are very promising as materials for studies of mutability, be-B cause of their great speed of reproduction and because of the opportunity they present for working with populations that comprise enormous numbers of individuals.Changes in bacterial cultures which make the bacteria resistant toward certain specific bacteriophage strains have been known for a long time and have been assumed by several workers to be due to mutations (GRATIA 1921;BURNET 1929; and others).This assumption has recently been confirmed in an important study by LURIA and DELBRUCK (1943).The experimental method used by these investigators consists of determining how many resistant bacteria are present in small cultures of sensitive bacteria, especially when the size of cultures is kept so small that only a few resistants are found, on the average, in each culture.Under these conditions some cultures are still found to contain a comparatively large number of resistants.This indicates that such bacteria probably belong to a single clone stemming from an ancestor which mutated to bacteriophage resistance earlier in the growth of the culture.Changes to bacteriophage resisJance h,ave been known to be fairly specific (BAIL 1923;BURNET 1929), inasmuch as each observed change usually involves resistance to only some of the bacteriophage strains capable of attacking the original strain of bacteria.The problem of resistance in bacteria therefore offers an unusual opportunity for a quantitative study of the occurrence of specific types of mutants, which outweighs the disadvantage of not being able to check the behavior of the mutants during a sexual process.The purpose of the present study is to extend the investigation of LURIA and DELBRUCK to cover resistance to several bacteriophage strains active on the same line of Escherichia coli.In particular, we wished to determine how bacteria attain simultaneous (or "multiple") resistance to various bacteriophage strains-that is, whether or not this resistance is due to separate, independent mutational steps. MATERIALS AND METHODSThe materials used in our experiments consisted of the same bacterial strain -E. coli B-previously used by LURIA and DELBRUCK (1943), of seven phage strains active on B, and of various strains of bacteria, derived by mutation from B, which were resistant to one or more of the phages.The phage strains were indicated as type I to type 7 (TI to T7).TI and T2, with which DR.LURIA supplied us, are the alpha and gamma strains of DELBRUCK and LURIA (1942) and are identical with the P28 and PC strains of DR.J. BRONFENBREN-NER (KALMANSON and BRONFENBRENNER 1939); T3, T4, Tg, and T6 were isolated from a mixture of phages supplied by DR.TONY L. RAKIETEN; T7 was