MILK AS A MEDIUM FOR BACTERIA 



17 



elsewhere. The chemical variation is shown in the following 

 table :— 



Boussingault, Heaton, Dyer, and Richmond have given other 

 illustrations of this point.^ 



In the second place, though it is well known that cream from 

 different herds yields a varying percentage of fat, it is probably not 

 so well known that the cream of the same herds and same cows 

 varies from day to day in such percentage. For example, it has 

 been found that a cow will yield a cream containing 6 per cent of 

 fat on one day and 8 per cent, on the next day. 



Milk as a Medium for Bacteria 



The most elementary definition of bacteria describes them as 

 living vegetable cells of microscopic size, and composed of two chief 

 parts, a body or cell substance, and a capsule or cell wall. It is gener- 

 ally held that they contain no nucleus. But in some species internal 

 differentiation of protoplasm has been demonstrated. Bacteria 

 certainly contain no chlorophyll. The cell substance consists of 

 protoplasm, 84 per cent, which is composed of a nitrogenous body 

 called viyco-protein, which in its turn is half composed of carbon, and 

 contains about 15 per cent of nitrogen and half as much hydrogen. 

 It differs from ordinary proteid, according to Nencki, in not being 

 precipitated by alcohol, and in not containing sulphur. 



The protoplasm of some species of bacteria, however, contains 

 in addition to myco-protein other substances, such as starch, 

 sulphur, or pigment. The cell membrane is in some species com- 

 posed of cellulose (CgHioOg), but in others it is believed to be formed 

 by a concentration of the body protoplasm. 



Hence it is clear that to maintain their vitality and fulfil their 

 functions like other vegetable cells, bacteria require carbon, hydro- 

 gen, nitrogen, oxygen, certain salts, and water. Parasitic and 

 saprophytic bacteria will, of course, differ from each other in many 

 [of the conditions of their nutrition. The source from which all 

 [plants containing chlorophyll derive their carbon is the carbonic 

 1 Dairy Chemistry^ H, Droop Richmond (1899), pp. 128-135. 



B 



