1 8 GENERAL CONDITIONS AFFECTING BACTERIA IN MILK 



acid (CO2) of the atmosphere (or in case of submerged plants that 

 which is held in solution by the water), which is decomposed under 

 the influence of sunlight by the chlorophyll-containing cells. 

 Vegetable cells not furnished with chlorophyll are unable thus to 

 decompose carbonic acid. Hence they are compelled to derive 

 their carbon, hydrogen, and nitrogen directly, and at first hand, 

 from more complex organic compounds. These elements are 

 chiefly derived by bacteria from proteids and carbohydrates, which 

 form therefore their staple diet. We might roughly express the 

 favourable diet of bacteria in three words, a neutral (or alkaline) 

 proteid solution. But, as a matter of fact, their nutrition cannot be 

 arranged quite so readily as this. ,For, in regard to their diet, 

 bacteria reveal very great differences; indeed — differences not only 

 between species but between individuals of a species, and even 

 these individuals differ under various extrinsic circumstances. 

 What suits one species will not suit another species ; what suited one 

 species some hours or days ago will not suit it now. These differ- 

 ences are due, not only to the specific myco-protein of various 

 species, but often to the effect which the organism has had upon 

 its special medium — e.g. the production of lactic acid and pro- 

 teolytic enzymes. Again, while some bacteria are capable of grow- 

 ing and multiplying upon simple nitrogenous compounds, such as 

 ammonia or urea, many other show no growth whatever under such 

 conditions. Common saprophytic bacteria are examples of the 

 first class, and pathogenic bacteria (parasitic) are examples of 

 the second, these latter thriving only in the presence of albumen 

 or albuminoid nitrogenous material. As regards the amount of 

 albuminous material, some, like the common water bacteria, flourish 

 on little ; whereas others, accustomed to live on more copious 

 nitrogenous soil, if transferred to ordinary water would diminish 

 and eventually die. The greater the amount of albuminoid 

 material present, the greater will be found, cceteris paribus, the 

 capability of multiplication.^ 



As regards the lowest limit of proteid matter possible to sustain 

 bacterial life no exact figures can be quoted. Most bacteria 

 flourish in media containing | per cent, yet some will not grow if the 

 proteid matter is much below 2 per cent. B. tuberculosis and B. 

 diphtherice require a comparatively large amount of albuminoid 

 material ; and some organisms can grow well on no higher organic 

 material than starch (Prazmowski). 



' Pathology and Etiology of Infectious Diseases (Stevenson & Murphy), 

 vol. ii., p. 17, E. Klein, M.D., F.R.S. 



