60 THE FORMATION OF ENZYMES IN BACTERIA 



results in an alteration of the gene at tlie site of the " hit." 

 It has been found experimentally that whenever a gene is so 

 altered, there is a loss of one enzyme from the enzymic con- 

 stitution. From this work, and from other investigations 

 carried out with yeast cells, it can be concluded that the 

 formation of an enzyme by a cell is controlled in the first 

 place by the presence of the correct gene and that one gene 

 controls one enzyme. When a gene is altered and the enzymic 

 constitution of the resulting cell changes, the new cell is said 

 to be a " mutant." Mutants arise spontaneously, and, as a 

 result of many studies which have been carried out on micro- 

 organisms, it appears that any given gene may alter and give 

 rise to a mutant about once in every 10^ to 10^ generations. 

 Consequently a spontaneous mutant is a very rare thing but 

 when we are dealing with large populations, and in bacterial 

 cultures we normally deal with populations of 10^-10® cells/ml., 

 it is probable that mutants will be present and any change in 

 the environmental conditions which favour the growth of the 

 mutant rather than that of the unaltered mother culture, 

 will give rise to a progressive selective growth of mutant 

 cells. Likewise if the environment is not suitable for growth 

 of the mutant, then it will not multiply and will not exert 

 any significant effect upon the properties of the culture as 

 a whole. 



Biologists working with moulds and higher organisms are 

 accustomed to thinking in terms of genes and mutations, but 

 it is only during recent years that the application of these 

 terms to bacteriology has been investigated in detail. The 

 presence of a heritable factor involved in enzyme control has 

 been shown by some masterly studies by Avery and his 

 colleagues on the conditions governing the formation of the 

 polysaccharide capsule of the Pneumococcus (see jDhap. VI). 

 Pneumococci are divided by serological methods into a number 

 of types, and type specificity is conferred by the chemical 

 structure of the polysaccharide capsule of the organism. Thus 

 the capsule of a Type III Pneumococcus is composed of a 



