Bacterial Multiplication 141 



stant. A balance exists between the number of new cells being 

 created and the number of bacteria dying. 



6. The accelerated death phase, from 6 to 7, is the period dur- 

 ing which cells are beginning to die at an increased rate, with a net 

 loss in viable cells. 



7. The logarithmic death phase, from 7 to 8, indicates that cells 

 are dying at about a geometric rate, a reverse of the logarithmic 

 growth phase. There is some evidence that the death rate is not 

 logarithmic, and many persons prefer to designate this period in 

 the curve as the phase of decline. The segment from 8 on in the 

 above growth curve has no technical designation, but were the 

 phase of decline accepted as the proper terminology, the latter 

 phase in the curve could be included. 



Examination of the organisms during the senescent phase of 

 decline will frequently reveal the presence of weird sizes and 

 shapes of bacteria, and these unusual morphological types are 

 called involution forms. Their formation is probably induced by 

 extremely unfavorable conditions in their environment— the accu- 

 mulation of waste products of metabolism. This is another reason 

 for examining young, actively growing cultures if typical mor- 

 phology of the species is to be ascertained by use of the gram stain 

 or by examination of hanging drop preparations. 



METHODS FOR DETERMINING TOTAL BACTERIAL NUMBERS 



Whereas many bacteriological procedures are aimed at de- 

 termining the number of living cells present in a given material, 

 there are occasions when total counts— living plus dead cells— are 

 of interest and importance. For example, a sample of pasteurized 

 milk may give a relatively low colony count, which is a reflection 

 of the viable cells present, but unless technics were available for 

 determining total cell counts, it would be difficult to say with any 

 assurance what the quality of that milk was before pasteurization. 

 Heating of milk is designed to make it safe for consumption; it is 

 not an attempt to make good milk out of a poor raw product. 

 Pasteurization may kill better than 90% of the bacteria present in 



