378 
Micro-organisms and their Action 
tended, as far back as thirty years ago, that lactic fermentation 
is caused by an organized ferment which enters the milk after 
it has left the udder, that this micro-organism is the only cause 
for the formation of lactic acid, and that in its absence no such 
decomposition of the milk-sugar takes place. 
In order to decide the question in a practical way, Lister 
and others made experiments, in which they tried to get milk 
samples under conditions excluding as much as possible external 
influences, especially guarding them from the possibility of 
coming in contact with germs from outside. This seems perhaps 
easier than is really the case, and success was at first very 
doubtful. But after some practice in conducting the experi- 
ments, many samples were obtained which did not undergo 
decomposition, thus proving that a ferment common to milk 
could not have been present, but that the cause of lactic 
fermentation certainly enters the milk from outside. When the 
different samples of milk were examined microscopically, all 
those which had kept well were found to be free from micro- 
organisms, whilst those which had undergone decomposition 
invariably contained them. 
Every drop of milk obtained in the ordinary way, when 
examined under the microscope, is found to contain a number 
of micro-organisms of different forms which are by no means 
similar, but vary in accordance with the locality and other 
surrounding circumstances. If the milk be kept until it turns 
sour, it is observed that one species of organism multiplies 
more rapidly than all the others, thus making it most probable 
that this special kind stands in close relation to the progressing 
lactic fermentation. Lister tried to separate the different kinds 
of organisms, and to study their individual effects in the fol- 
lowing way : — 
In a very small but definite quantity of sour milk the number of micro- 
organisms was ascertained by the aid of the microscope. A certain quantity 
of this milk was then diluted with sterilized water to such an extent that on 
an average rather less than one organism was present in each drop of the 
liquid. Five glasses each containing sleiilized milk were then inoculated 
with one drop of the diluted milk and kept guarded from outside influences. 
The milk in four out of the Ave glasses underwent neither lactic nor any other 
kind of fermentation, and no organisms could be detected. The fifth sample, 
however, turned sour, and was found upon exaniiuatiou to contain the lactic 
ferment in abundance. Starting from this latter sample, which was presumed 
to cotitain the pure hictic ferment unmixed with others, a furtlier experiment 
was made, by inoculating sterilised milk to that one sanii)le was supjiosed to 
contain four organisms, five samples two organisms, and ten samples one 
organism each. 
The results were that the first and second samples curdled 
within a few days, but of the third lot, viz. tlie ten samples 
