378 
Bacteria, their Nature and Function. 
ances takes place in the latter. When a plate cultivation of such 
milk is made it is seen that a large number of colonies, all of the 
same character, are developed, which colonies are made up of the 
Bacterium lactis ; through however numerous generations this 
organism is cultivated in artificial cultivations — it grows well on 
nutritive gelatine to which whey or only lactic sugar has been added 
— if it is then transferred to fresh milk, it always produces this 
souring and curdling ; that is to say, it changes lactic sugar into 
lactic acid, and as this is being formed it coagulates and precipitates 
the casein of the milk. With a trace of milk that has gone naturally 
sour— that is to say, to which the Bacterium lactis has found entrance, 
and in which by its multiplication it has produced curdling, any 
amount of normal milk can be successively turned sour and curdled. 
Bacterium lactis is not by any means a rare organism ; it is widely 
distributed, and can at any moment, in dairies and other places, 
through impurities of the utensils, by dust, &c., find access to milk, 
which would soon succumb to its attacks. When, for instance, in 
dairies or in one or another locality, the milk has a frequent tendency 
to turn sour, this means that the Bacterium lactis has taken firm 
footing in such a locality. It is well known that only extreme 
measures of cleanliness, thorough boiling of all utensils and vessels, 
cleaning of walls and floors, can banish or reduce it. In this the 
analogy with an epidemic of an infectious disease is obvious. Just 
as in an epidemic, every susceptible individual to which the con- 
tagium has had access becomes smitten by infection, and just as in 
an epidemic the contagium of the disease, being of wide distribution, 
and having taken a firm hold of the locality, attacks an increasing 
number of individuals, and thus causes the epidemic — so also 
in the case of the Bacterium lactis : when this has taken a firm 
hold of, and has acquired a great distribution in, any locality, any 
sample of milk ( i.e . susceptible individual) may take the infection, 
either by coming in contact, directly or indirectly, with a trace of 
the milk already infected, e.g. by being placed in vessels in which 
infected milk has been kept previously, or becoming infected through 
dust charged with Bacterium lactis, or coming in contact with 
water poured from a vessel in which traces of the microbes were 
still left. All this finds its complete analogy in the case of an 
epidemic infectious disease. The fermentative processes due to 
microbic activity, and playing an important part in industries 
(alcoholic and other fermentations), illustrate in a very striking 
manner some of the essential features observed in the nature, in the 
production, and in the spread of infectious diseases in man and 
animals. The fermentative processes, thoroughly established as 
b iing due to microbic activity by the researches of Pasteur, were by 
Pasteur, and others after him, used as illustrations of the way in 
which infectious disorders in man and animals arise, and it was 
exactly these considerations which led Pasteur to his brilliant 
studies of these diseases, the results of which studies have been of 
such signal service in sanitary science in general, and in the pre- 
vention of infectious diseases in particular. 
