62 THE FORMATION OF ENZYMES IN BACTERIA 



the sugar is slowly acquired in the course of subcultivation, and 

 after several such passages, the culture behaves as though it 

 were a normal lactose-fermenting Esch. coli. Detailed 

 investigation of the individual cells in the cultures (by plating 

 out a high dilution on lactose- and -indicator-plates) shows that 

 all cultures contain a number of fermenting cells and a number 

 of non-fermenting cells. The initial non-fermenting culture is 

 found to contain, on the average, one fermenting " mutant " 

 for every 10^ non-fermenting cells. In the course of cultivation 

 in the presence of lactose, the proportion of fermenting cells 

 increases, since the medium will obviously favour the growth 

 of these mutants but the final fermenting culture will still 

 contain a small proportion of non-fermenting cells. 



It is not certain what is the difference between the fermenting 

 and non-fermenting cells. It is probable that the fermenting 

 cells possess the enzyme lactase, whereas the non-fermenting 

 cells are mutants which have lost this enzyme. However, 

 one investigator has claimed that both types of cell possess 

 lactase but the non-fermenting one has a cell-wall impermeable 

 to the disaccharide. Whatever may be the true difference, it 

 is clear that growth in a lactose medium results in selective 

 growth of the mutant able to utilise the sugar. 



In the next chapter we shall be considering organisms which 

 have lost the ability to synthesise certain amino-acids. A 

 simple example is that of Eberthella typhosa which, when 

 freshly isolated, is unable to synthesise tryptophan. The 

 primitive type is able to synthesise the amino-acid; in the 

 course of multiplication, mutants arise and, in approximately 

 10^ generations, a cell arises which has lost one of the enzymes 

 involved in tryptophan synthesis ; if the organism is growing 

 in the tissues of a host it will find tryptophan supplied in the 

 medium and the synthetic disability will therefore not impose 

 any restriction on growth. The synthesis of a substance 

 such as tryptophan involves the expenditure of energy ; con- 

 sequently if the organism can grow by the assimilation of 

 preformed tryptophan its growth process will be energetically 

 more efi&cient and, in the course of many generations, the 



