2 SEX IN MICROORGANISMS 



both phages, but each burst coming from one bacterium will consist 

 entirely either of Tl or of T2. This effect was called mutual exclusion. 

 At that time Delbriick and Luria made another pecuhar observation. 

 If a single bacterium was infected with many identical phage parti- 

 cles instead of one, the latent period and the yield did not change. 

 This made it appear that actually only one particle took part in the 

 growth. The principle of mutual exclusion could thus be extended 

 to particles of the same strain. As soon as one particle begins to grow 

 in a bacterium, a reaction is started that results in the exclusion of all 

 other particles, even if they are already adsorbed. Quoting from Del- 

 briick: "The mutual exclusion effect is so novel that its explanation 

 calls for a bold hypothesis. We assume that the first virus which 

 penetrates the cell wall will make the cell wall impermeable to other 

 virus particles just as the fertilization of an egg by one spermatozoon 

 makes the tgg membrane impermeable to other spermatozoa." 



In 1944 Luria found the first mutation in phage T2 (Luria, 

 1945). If a high concentration of bacteria is seeded on an agar plate, 

 the bacteria will grow in a thin continuous film on the surface. If a 

 bacteriophage is present on this surface, it will multiply by lysing the 

 bacteria, producing a colony of phage called a plaque, which appears 

 like a clear visible hole in the film of turbid bacteria covering the sur- 

 face of the agar. Strain B of the colon bacillus, sensitive to T2, can 

 mutate to B/2, resistant to T2. However, if a very large number of 

 T2 phage particles are plated on B/2, some plaques are obtained. 

 These plaques are due to T2/; mutants which lyse B/2. Therefore we 

 have two types of phage: T2 and T^2h, and two types of bacteria: 

 B and B/2. T2^ lyses both bacterial strains, T2 only B and not B/2. 

 Suppose we plate on B: T2 cannot be distinguished from Tlh. If 

 we plate on B/2, only T2/:) will give plaques. As a crucial test for 

 the theory of mutual exclusion, Luria infected the same bacteria first 

 with T2 and then with T2h; he found that T2h was excluded. He 

 concluded that exclusion exists also among closely related phages 

 like T2 and its mutant b. But the actual result of Luria's experiment 

 was due to a different phenomenon discovered by Dulbecco several 

 years later (1952) and called mutual exclusion between related 

 phages. This exclusion is due to the interval of time allowed between 

 the first and second infection, and not to exclusion in the previous 

 sense. The longer this interval of time, the more complete is the 

 exclusion. 



