98 THE GROWTH AND DEATH OF BACTERIA 



times noticed, indicative of the production of fresh organisms. Moreover, there 

 is a gradual rise in the total count, pointing to the same conclusion (Fig. 19). 



This is the curve obtained for a culture of a coliform or similar organism in a 

 nutrient broth medium. In a medium containing a fermentable sugar, such as 

 glucose broth, the curve is very different. Instead of descending slowly and 

 irregularly, it passes rapidly down in an oblique straight line, till it reaches the 

 abscissa (Fig. 18). 



This difierence is almost certainly due to the disinfectant action of the acid 

 produced in the culture. The death of the organisms in Curve B is similar to the 

 velocity of a imimolecular reaction, and can be represented by the formula 



K = - log - 



t ^2 



where K is the velocity constant, t the interval of time between successive observ- 

 ations, Til the number of living bacteria present at the beginning, and W2 the 

 number present at the end of time t. This point will be more fully discussed in 

 Chapter 5. For further information on the growth phases of bacteria, the reader 

 is referred to a detailed consideration of this subject by Buchanan and Fulmer 

 (1928). 



Dormancy of Bacteria. 



G. S. Burke (1923) inoculated a series of tubes of glucose peptic digest agar 

 and of glucose peptic digest broth with CI. botulinum, seeding one spore into each 

 tube. She sealed the tubes to prevent desiccation, incubated them at 37° C, and 

 noted each day in how many tubes growth had occurred. In the agar medium the 

 majority developed in 10 days, but occasional spores continued to germinate up 

 till the 92nd day. In the broth the majority developed in 14 days, but one 

 or two germinated daily until the 33rd day, and thereafter at intervals till the 

 144th day. 



V. Burke, Sprague and Barnes (1925) obtained similar results with B. suhtilis, 

 B. megatherium, and Bad. coli, thus showing that the phenomenon is not con- 

 fined to anaerobic bacteria or the germination of spores. It is clear from these 

 experiments that bacteria may lie dormant for long periods without multiplying. 

 Superficially, this resembles the phenomenon of lag, but it is possible that dormancy 

 and lag are dependent on different factors. G. S. Burke (1923) draws attention to 

 the similarity in behaviour of dormant spores and the seeds of certain of the higher 

 plants. She suggests that in each instance the cause of the dormancy lies in the 

 cell itself, and is connected with the degree of permeability of the wall. 



It is this dormancy which appears to be responsible for the occasional failure 

 of the intermittent process of sterilization. Probably owing to this, too, is the 

 fact that a culture may be contaminated with an organism without signs of the 

 contamination becoming evident till after three or four subcultures have been 

 made. 



Mitogenetic Rays. — The extensive work of Gurwitsch and his followers (for 

 references see Bateman 1935) has suggested that certain plant and animal tissues 

 under suitable conditions may emit so-called mitogenetic rays, which are able 

 to stimulate growth in tissue cells placed in a position favourable for their absorp- 

 tion. According to some workers, the radiation is of the short ultra-violet type, 

 but attempts to confirm this by physical methods have not so far been successful. 

 The rays, if they exist at all, have therefore to be detected by biological means. 



