Restriction enzymes: The (billion dollar) consequences of studying why certain isolates of phage γ infect only certain strains of E. coli

In 1953, Bertani and Weigle1 reported that samples of bacteriophage γ that had been propagated on certain strains of Escherichia coli retained the ability to infect that same strain of E. coli but were unable to infect other strains. A contemporary version of their study is shown in Figure 1; phage γ, obtained by infecting E. coli strain K (γK), is applied to two different E. coli strains, K and B. The γK particles infect E. coli K with an efficiency of 1: i.e., every phage produces a plaque in a lawn of E. coli K cells. But the same preparation of γK shows a pathetic efficiency of infection on E. coli B, about 10−4: i.e., 10,000 phage particles yield just one plaque on the lawn of E. coli B cells. The phage obtained from the few productive infections of E. coli B (γB) are then tested against the same two strains, K and B. This time, the phage have largely lost the ability to infect E. coli K, as they now show an efficiency of 10−4 instead of 1, but they infect E. coli B much more readily than before, with an efficiency raised from 10−4 to 1.