CHAPTER 4 



THE GROWTH AND DEATH OF BACTERIA 



Lest the title of this chapter should prove misleading, it may be pointed out 

 that we are not here concerned with the broader aspects of growth, such as the 

 temperature and food requirements of bacteria, or their metabolism and respira- 

 tion. These have already been briefly dealt with in Chapter 3. In the present 

 chapter we restrict ourselves to what we may describe as the dynamic aspects 

 of growth — a study that deals essentially with the rate of change in a bacterial 

 population. 



Technique of Counting Bacteria 



Bacteria may be counted in such a way as to obtain an estimate either of the 

 total number of organisms alive and dead, or of the number of living organisms 

 only. The first we shall refer to as the Total Count, the second as the Viable 

 Count. Each method is suited to various purposes, and the choice of which to 

 employ must depend on the type of information desired. For many purposes, 

 such as calculation of the generation time of bacteria, and the quantitative study 

 of bacterial metabolism, both methods should be used in conjunction. 



The general principles underlying the counting of bacteria may be briefly 

 mentioned. [For further details the reader is referred to textbooks on practical 

 bacteriology, and for a critical review of the different methods that may be employed, 

 with the main sources of error involved, to articles by Wilson (1922) and Wilson 

 et al. (1935).] 



(1) Total Count. 



(o) Direct Counting under the Microscofe of a Stained Preparalion on a Slide. — First 

 described by Eberle (1896), this method has been used fairly extensively, and forms the 

 basis of the Breed (1911) method for counting bacteria in milk. A drop of known vokime 

 is spread over a known area on a sUde, dried, fixed, stained, and examined under a micro- 

 scope. The organisms in a given number of fields are counted, and knowing the area of a 

 given field with a particvilar combination of objective, tube length and ocular — this can 

 be obtained by means of shde and eyepiece micrometers — it is possible to calculate the 

 total number of organisms present in the original suspension. This method, though 

 valuable for certain purposes, is open to a number of technical objections, one of the most 

 important of which is that not all organisms — particularly when dead — stain sufficiently 

 deeply to be visible under ordinary illuminating conditions. The method, therefore, 

 affords an estimate of the number of stainable bacteria, not necessarily of the total bacteria. 



(6) Wright's Method (1902). — A known volume of the bacterial suspension is mixed 

 with a known volume of normal human blood. A smear preparation is made on a slide, 

 dried, fixed, stained, examined under the microscope, and the number of bacteria and red 

 cells in a given number of fields is counted. Since, in the blood of the normal adult male 

 there are about 5-5 miUion red cells per c.mm., and since the numerical relationship of 



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