180 BACTERIA IN MILK AND MILK PRODUCTS 



There is ample evidence in support of the fact that not all the 

 persons partaking of infected milk suffer equally, and occasionally 

 some escape altogether. We know little or nothing of the causes of 

 such modification in the effect produced. It may be due to other 

 organisms, or chemical substances already in the alimentary canal of 

 the individual, or it may be due to some insusceptibility or resistance 

 of the tissues. Be that as it may, it is a matter which must not be 

 neglected in estimating the effects of food contaminated with bacteria 

 or their products. 



There are few liquids in general use which contain such enor- 

 mous numbers of gerins as milk. To begin with, milk is in every 

 way, physical and physiological, admirably adapted to be a favourable 

 medium for bacteria. It is constituted of all the cliief elements of 

 the nutriment upon which bacteria live. 



Briefly, we may summarise the full diet of bacteria as : — nitro- 

 genous matter (proteids) ; non-nitrogenous matter containing carbon 

 and hydrogen (carbohydrates) ; calcium, potassium, phosphates, etc. 

 (salts) ; and, for some species, oxygen. When we turn our atten- 

 tion to milk as a medium for bacteria, we find a complete bacterial diet 

 — proteids represented by casein and lactalbumin, 4 per cent, in 

 total — carbohydrates represented by lactose, the most readily affected 

 of all the sugars by bacteria; fat as palmitin and olein; salts, 

 potassium and calcium largely as phosphates, the calcium phosphate 

 being united with casein. Even the normal reaction of milk, 

 neutral or amphoteric, is favourable to the growth of bacteria, most 

 of which find a definitely acid or a definitely alkaline reaction 

 inimical to their growth. It is true that changes, mostly of a 

 fermentative nature, rapidly set in, which affect milk as a medium 

 for bacteria. But in its fresh, normal, untreated condition we have 

 theoretically an almost ideal medium for both saprophytic and 

 parasitic bacteria. Notwithstanding the truth of this general 

 statement, we must not pass over the experiments of Fokker, 

 Freudenreich, Cunningham, and others, which appear to demonstrate 

 that freshly-drawn milk possesses for certain species of bacteria a 

 germicidal power. 



In the healthy condition of animals we have, generally speaking, 

 no micro-organisms whatever in their secretions, whatever may be the 

 condition of their excretions. Hence, though milk affords, from its 

 constitution, such an ideal nidus for the growth and multiplication of 

 bacteria, it is, as secreted, a perfectly sterile fluid. This- was demon- 

 strated more than twenty years ago by Lister, who states that 

 " unboiled milk as coming from a healthy cow, really contains no 

 material capable of giving rise to any fermentative change, or to the 

 development of any kind of organism which we have the means of 



