TWO-STEP RESISTANCE BY ESCHERICHIA COLI B TO BACTERIOPHAGE T2

Richard E. Lenski ¹

¹ Department of Zoology, University of Massachusetts, Amherst, Massachusetts 01003

Numerous authors have noted the difficulty in obtaining mutants of E. coli B that are resistant to bacteriophage T2 using standard procedures of plating large numbers of cells in the presence of excess phage. Yet, T2-resistant mutants appear in continuous culture at rates inconsistent with this difficulty. This paradoxical result derives from the fact that resistance to T2 usually arises as a consequence of two nonindependent mutations. Mutant bacteria resistant to phage T4 are very common and increase rapidly in continuous culture with phage T2 owing to an approximate halving of the rate at which T2 adsorbs to and kills these partially resistant mutants. The rate at which these partially resistant mutants then give rise to fully resistant mutants is approximately two orders of magnitude higher than the rate obtained by direct selection. These results are consistent with biochemical evidence that T2 adsorption to E. coli B involves both the bacterial lipopolysaccharide (to which phage T4 adsorbs) and a bacterial surface protein. However, this genetic evidence suggests that T2 can adsorb to either receptor type alone, whereas the biochemical evidence suggests that T2 requires a complex of the two receptors for adsorption to E. coli B. These results also indicate that the effects of genetic background can influence not only the selective advantage associated with particular mutations but also the rate at which certain selectively defined characteristics arise via mutation.

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