190 BACTERIA IN MILK AND MILK PRODUCTS 



and the experiment was repeated as at thirty days. During the 

 intervening period the flasks had been kept, hermetically sealed, at 

 the three temperatures. The result was that the flasks were found 

 to be germ-free, with the exception of an abundant growth of Oidmm 

 lactis and other moulds. 



Mr Swithinbank and the writer came to the conclusion that the 

 explanation of the results of this investigation could only be found 

 in a glance at the life-history of such a milk as that under 

 consideration. 



" At the time of milking there is, as we have seen, an introduction into the warm 

 milk of vast numbers of common saprophytic and parasitic bacteria. Finding 

 themselves in an ideal nidus, they multiply with almost incredible rapidity. Hence 

 the first rise in numbers of bacteria. Competition and exhaustion of pabulum 

 soon produce inevitable effects, and we obtain the first decline. At this stage it 

 may be said that the common extraneous bacteria, whether putrefactive or simple 

 saprophytes, practically die out, and that for a very simple reason, namely, that 

 they cannot live in the presence of the new tide of acid-forming bacteria. Although 

 the lactic acid group of organisms do not multiply as rapidly as ordinary sapro- 

 phytes, they reach a much higher maximum in the end. It is to this family of 

 bacteria that the second and maximum r ( ise is due. In time, also, the same 

 inimical conditions begin to act, and the lactic acid bacteria decline owing to the 

 acidity and to the lack of pabulum. Eventually, the medium, which twenty days 

 before was an ideal one for any organism, and mostly so for those which came 

 first, and which ten days before was favourable to lactic acid organisms, is now 

 favourable to no bacteria at all. Accordingly, bacteria of all descriptions gradually 

 die out, and the medium is eventually left in possession of Oidium lactis and the 

 common moulds. That the destruction of large quantities of solid albuminous 

 substances may occur simply through bacterial agencies has been conclusively 

 shown in the so-called septic tank method of sewage disposal. The death of 

 bacteria under these circumstances always follows shortly after their enormous 

 multiplication, and how much is due to starvation or how much to poisoning by the 

 products of their own activity it is impossible to say. It is, however, clear that the 

 decomposition of large quantities of albuminous substance is first accompanied by 

 great bacterial reproduction, and this is invariably followed by a season of speedy 

 and extreme mortality of bacteria. In a general way that represents, we believe, 

 the changes taking place as represented in the record we have considered. (That 

 there are two rises and two falls in the number of bacteria, the first rise being due 

 to extraneous organisms, and the second rise to lactic acid organisms, we believe to 

 be the almost universal rule in untreated ' natural ' milk. " ^v 



The effect of temperature and time has been illustrated by Dr 

 Buchanan Young's researches into the numbers of bacteria in milk 

 according to season, and the results of which were laid before the Royal 

 Society of Edinburgh. He estimated in the Edinburgh milk supply that 

 three hours after milking there were 24,000 micro-organisms per c.c. 

 in winter ; 44,000 in spring ; 173,000 in late summer and autumn. 

 Again, he found that five hours after milking there were 41,000 

 micro-organisms per c.c. in country milk, and more than 350,000 

 micro-organisms per c.c. in town milk. Many London milks would 



* Bacteriology of Milk (Swithinbank and Newman), 1903, p. 135. 



