CHAPTER XVIII 



ON THE TYPES OF BACTERIA COMMONLY FOUND IN MILK AND 

 THEIR THERMAL DEATH-POINTS 



THE varieties of bacteria present in milk will occur in innumer- 

 able combinations, which will evidently depend upon the source 

 from which they are derived, i.e. air, cow, milker, vessels, etc. 

 Roughly, however, they can be classified into several main groupings 

 according to the action they display upon certain substances. 

 The most usual groupings are as follows : (i) Acid-forming bacteria, 



(2) those forming acid and gas with coagulation of milk protein, 



(3) peptonising bacteria, and (4) inert bacteria. 



Milk is a good medium for the growth of the majority of bacteria 

 except where the growth of one organism is antagonised by that 

 of another variety. 



In the early hours after milk has been collected there is an 

 apparent decrease in the total number of organisms found. This 

 has already been referred to on pp. 114-20 in connection with the 

 alleged bactericidal properties of milk. It was there shown that 

 this decrease is probably apparent rather than real, and due to 

 the agglutination of the bacteria by the action of the agglutinins, 

 which are found in milk. When milk, examined in the early hours 

 after milking, is violently shaken so as to break up the clumps of 

 bacteria, the number of colonies found on plating out samples of 

 the milk does not suggest that there has been any actual decrease 

 in the number of bacteria present. Some six hours after milking 

 an increase in the bacterial content commences. The pre- 

 dominating growth appears usually, if not always, in the acid- 

 forming organisms. The acid thus produced renders the medium 

 an unsuitable one for the growth of certain other organisms, which 

 become gradually reduced in numbers. The rate of this reduction 

 will depend upon the relative proportion of the bacteria present 

 and the temperature at which the milk is kept. 



The increase in the number of acid-forming bacteria has been 

 investigated by a number of workers, but is well shown in the 

 following table, taken from the work of Conn and Stocking. Similar 

 results were obtained by Conn and Esten. 



283 



