54 Lectures on Bacteria. [^ vi. 



Oxygen is not equally necessary in all cases. Two extreme 

 cases are distinguished in Pasteur's terminology as aerobia 

 and anaerobia. The first require an abundance of air con- 

 taining oxygen, as well as a good supply of nutrient sub- 

 stances for luxuriant vegetation and growth ; of this kind are 

 Micrococcus aceti, Bacillus subtilis, B. Anthracis, and Koch's 

 Spirillum of cholera. The other kind does well on good food 

 without oxygen ; free access of air reduces their vegetation to a 

 minimum or to zero, as for example in Bacillus Amylobacter. 



Intermediate cases, however, are found between the two 

 extremes, as is well shown by Engelmann's beautifiil example 

 which will be referred to again presently ; and according to the 

 investigations of Nencki, Nageli, and others. Bacteria which 

 excite fermentation, like the Sprouting Fungi which give rise to 

 alcoholic fermentation, grow luxuriantly without oxygen, when 

 they are in a suitable fluid capable of fermentation with them. 

 If these forms are placed in a less favourable nutrient fluid in 

 which they cannot incite fermentation, they will not grow with- 

 out a supply of oxygen. 



Oxygen may impede and even destroy vegetation even in the 

 case of aerobiotic forms if it takes place under high pressure. 

 Bacillus Anthracis, for example, remained alive for fourteen days 

 in oxygen under a pressure of fifteen atmospheres, but was dead 

 in a few months' time. Duclaux contends that the germs even 

 of aerobiotic forms, when withdrawn from the conditions re- 

 quired for growth, lose their power of development more quickly 

 under the continued effect of atmospheric oxygen than when 

 oxygen is excluded. The facts on which this view is founded 

 are in themselves remarkable. In some glass bottles which had 

 been used in Pasteur's researches about i860, and had been 

 kept hermetically sealed with their contents decomposed by 

 Bacteria, the germs of these Bacteria were found thoroughly 

 capable of development after twenty-one and twenty-two years. 

 Plugs of cotton-wool full of germs of all kinds, which had been 

 kept dry and protected from dust during the same time, but not 



