124 THE BACTERIAL CONTENT OF MILK 



certified milk. We admit the reasonableness, on the whole, of 

 these figures, although we should not necessarily condemn milk 

 which contained a larger number. The fact is that numerical 

 estimation of organisms is not, by itself, a sufficient criterion. All 

 the circumstances must be taken into consideration, including the 

 condition of the farms, the presence of preservatives, and the 

 species of the bacteria. 



There are three chief special natural conditions affecting 

 bacteria present in milk, which call for careful consideration : — 



1. The influence of temperature. 



2. The influence of time. 



3. The inter-relationship of different species of bacteria, and the 

 germicidal effect of the milk. 



1. The Influence of Temperature 



It is now well known that the vitality of bacteria depends upon 

 the fulfilment of a number of external physical conditions- 

 medium, moisture, oxygen, temperature, etc. One of the chiefj 

 among these is temperature, and in few media, if any, does it 

 exert a more powerful or more permanent influence than in the 

 case of milk. 



Every species of micro-organism has, generally speaking, its three 

 degrees of temperature : {a) The mininium temperature, varying as 

 a rule from 10° to 14° C, though under exceptional circumstances 

 growth may take place in some species at a very much lowei 

 temperature. {b) The optimum temperature, usually about the 

 temperature of the natural habitat of the micro-organism it 

 question. Ordinary saprophytes — those able to live upon deac 

 organic matter — engaged in putrefactive functions flourish betweei 

 18° and 24" C. ; whilst the parasitic, living in the blood or body oij 



' Within recent years various workers have shown that ahhough multipHca^ 

 tion practically ceases at zero, the vitality of micro-organisms is maintained 

 much lower temperatures. For example, MacFadyen and Rowland have mad^ 

 experiments with organisms possessing varying degrees of resistance, tej 

 organisms altogether being used and cooled down to — I90°C., in the first'^ 

 instance for twenty hours, and eventually for seven days. These exposures did 

 not produce any appreciable impairment in the vitality of the organisms, either 

 as regards their growth or their characteristic physiological properties, such as 

 pigment and gas production, pathogenicity, etc. Amongst the organisms 

 tested were photogenic bacteria, and these likewise preserved their normal 

 luminous properties. An exposure to the temperature of liquid hydrogen (about 

 -252°C.), a temperature which was as far removed from that of liquid air as 

 was that of liquid air from the average summer temperature, for ten hours had 



