PHASE OF DECLINE 97 



oxygen starvation. It seems not improbable that, once the simpler food-stuffs 

 have been broken down, the attack on the more complex compounds can be suc- 

 cessful only in the presence of abundant oxygen. 



It must not be thought that during the stationary phase all growth has ceased. 

 In most cases multiplication continues for some time after the cessation of the 

 logarithmic phase, but the rate of division progressively diminishes. The time taken 

 to reach the maximum number varies. The temperature of incubation is important ; 

 thus, Graham-Smith (1921), working with Staphylococcus aureus, found that the 

 maximum number of viable organisms was reached on the 2nd day at 37° C, on 

 the 5th day at 2T^ C, and on the 8th day at 17° C. Much depends, too, on the 

 nature of the medium. 



Not only are fresh organisms being formed during the stationary phase, but 

 large numbers are dying. Once the maximum number of organisms has been 

 reached, and before the phase of decline, the multiplication rate just suffices to 

 balance the death rate, so that the number of living bacteria remains constant. 



Phase of Decline 



After lasting for a variable time — from about an hour to several days — ^the 

 stationary phase passes gradually into the phase of decline. The numbers decrease 

 slowly over a period of days, weeks, or months, till all the organisms are dead. 

 With some organisms, such as the pneumococcus, this phase is short, and the culture 

 may be sterile in 2 to 3 days ; with other organisms, such as Bad. coli, it 

 may last for months. It is possible that growth is not entirely in abeyance during 

 this stage, for if counts are performed at daily intervals, spasmodic rises are some- 



71-0 



3 15 6 7 8 9 10 V 12 13 14 15 16 U 18 13 20 Zl ^ 23 24 25 26 27 28 29 30 31 3233 34 35 36 37 38 39 40 4142 4J 44 



Time, in D-'^ys 



Fio. 19." — Growth of Sahn. typhi-murium in Broth, showing the prolongeu Decline 



Phase. 

 Continuous line = Total number of bacteria alive and dead. 

 Interrupted line = Number of living, or viable bacteria. 



P.B. B 



