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

In 1953, Bertani and Weigle reported that samples of bacteriophage λ 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 λ, obtained by infecting E. coli strain K (λK), is applied to two different E. coli strains, K and B. The ëK 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 λK 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 (ëB) 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.