MICRO-ORGANISMS IN RELATION TO MAN. 105 
The cholera-bacillus is very greedy of oxygen, and methods 
for its separation are based upon this fact; in milk it grows 
with great readiness and gives rise to no noticeable alteration. 
Acids are deadly poison to it, and the organism cannot grow 
in acid media—hence it is killed by the gastric juice of the 
healthy stomach. 
The cholera bacillus does not, so far as is at present known, 
form spores, but the ordinary vegetative cells are somewhat 
retentive of life under favourable conditions of moisture and 
temperature. Thus the organism was found alive eighty-one 
days after it had been placed in the water of the harbour of 
Marseilles, and Koch found that it lived for between five and 
six months in laboratory cultures. 
In cases of suspected death from cholera, the dejecta are 
examined for the bacillus and pure cultures made of the or- 
ganisms found. These cultures are then examined by the 
methods outlined above, and according to their behaviour 
in the nutritive media, and the production or otherwise of 
the appearances described, the case is pronounced to be one 
of cholera or the reverse. The test mentioned as_ being 
absolutely necessary before any micro-organism can with 
certainty be associated with any specific disease as cause 
and effect, has been satisfactorily carried out with the cholera- 
bacillus. Animals, guinea pigs chiefly, have been successfully 
inoculated with the cholera bacillus, and the disease thereby 
produced ; and several cases are on record in which workers 
with the organism have accidentally ingested some of the 
bacilli, with the consequent production of cholera, terminating 
in some instances fatally. 
Hitherto I have treated of micro-organisms as the enemy of 
man—secret foes who strike before we are aware of their 
presence ; but this is not the only character in which they are 
known. There are species of a friendly and_ beneficent 
nature, so beneficent in fact that it is absolutely correct to” 
say that we could not live without their aid. 
