MILK AS A MEDIUM FOR BACTERIA 19 



In addition to carbon and nitrogen, salts are requisite in a 

 favourable food for bacteria. Sulphates, phosphates, sodium 

 chloride, potassium, and calcium salts all seem necessary for full 

 development These substances are presented to bacteria in 

 solution in water. 



. Briefly, then, we may summarise the full diet of bacteria as 

 nitrogenous matter (proteids) ; non-nitrogenous matter containing 

 carbon and hydrogen (carbohydrates) ; calcium, potassium, phos- 

 phates, etc. (salts) ; and, for some species, oxygen. 



When we turn our attention to milk as a medium for bacteria, 

 what do we find ? We find a complete bacterial diet — proteids 

 represented by casein and lactalbumin, 4 per cent in total — carbo- 

 hydrates 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 ampho- 

 teric, 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. To 

 this, however, we shall have occasion to refer again. 



In the healthy condition of animals we have, generally speak- 

 ing, 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 demonstrated 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 discovering."^ Subsequent experiment has only 

 confirmed the general truth of this statement^ With efficient 

 precautions (which will be set forth in detail at a later stage) 



^ Transactions of Pathological Society of London, 1878, p. 440. 

 ^ See also Rotch, Pediatrics, the Hygiene and Medical Treatment of Children^ 

 London, 1896. 



