8o MILK [vi 



and to them the term aerobics has been applied 

 require air, and perish if they do not have access to 

 it. Others, on the contrary, can develop without 

 air. Recent researches of a most interesting kind 

 have shown the important influence which sunlight 

 exerts on bacteria. It has been found that, for most 

 kinds of bacteria, sunlight is highly inimical. Indeed, 

 putrefactive liquids may be actually rendered sterile 

 by simply submitting them to the action of sunlight. 

 A free supply of air at the same time helps sunlight 

 to exert its full sterilising effect. Lastly, it may be 

 added that moisture, as a rule, is a necessary condition, 

 although its absence does not in all cases prove fatal. 

 With regard to the development of yeasts and moulds, 

 it may be explained that the former multiply by 

 " budding " or " sprouting," while the latter develop 

 by the formation of long threads (hypkae). 



Having thus described the general nature and 

 conditions of life of micro-organisms, we may next 

 proceed to deal with those met with in milk. And 

 before dealing with the different species, it will be 

 well to say a word or two regarding their number. 



Number of Bacteria in Milk. We have already 

 pointed out that in milk, even drawn under the most 

 careful conditions, hundreds of thousands exist in 

 every ounce. From 60 to 100,000 have been found 

 in 1 cubic centimetre (-^th f an ounce) of milk a 

 few minutes after milking. Milk investigated by 



