46 



OUTLINES OF BACTERIOLOGY 



as shown by its predominance it will tend to become still more 

 predominant in the second culture. An inoculation is now made from 

 the second into a third nutrient tube. This still further intensifies 

 the predominance of the predominant form, so that after three or four 

 such inoculations we may reasonably hope to obtain a culture in which 

 all the other forms have been eliminated. This method has, like the 

 first, the disadvantage of permitting only one form to be isolated, and 

 it also requires a longer time, for we have to wait till the culture has 

 reached its zenith of activity, which may take two or three days, and 

 certainly not less than one, so that when often repeated much time is 

 lost. Care has to be taken that all the tubes contain exactly the same 

 nutrient medium, otherwise the new conditions may be more favour- 

 able to the species very feebly represented, in which case these latter 

 will soon become predominant, and the experiment be valueless. 



Koch's Plate Method is now universally employed. This method 

 enables us to separate several species at once, is easy to perform, and 



when done with Petri-dishes, is 

 considerably better than the other 

 methods in every respect. We 

 shall therefore describe it in detail. 

 Three sterilised tubes, each con- 

 taining about 8-10 c.c. of sterilised 

 nutrient gelatine, are gently heated 

 in order to liquefy the gelatine. 

 Let us call them A, B, and C 

 (Fig. 61). A small quantity, as 

 much as a platinum loop will hold, 

 is removed from the liquid under 

 examination, care being taken to 



B 



fj jl sterilise the platinum loop in order 



to exclude all extraneous bacteria. 

 The platinum loop is dipped into 

 the tube A, while the gelatine is 

 still liquid, taken out and again 

 sterilised by passing through the 

 flame. After A has been gently 



shaken, the platinum loop is inserted into it and the same quantity of 

 liquid removed from A to B. The process is repeated so that a loopful is 

 transferred from B to C. The effect of this process on the distribution 

 of bacteria in the three tubes is as follows : A will contain thousands 

 of bacteria, and if we are examining a liquid like sewage-water, it will 



C' 



FIG. 61. 

 (For explanation see text.) 



