VIII. 



ACTION OF GASES AND OF THE HALOID ELEMENTS 

 UPON BACTERIA. 



Oxygen. Free oxygen is essential for the development of a large 

 number of species of bacteria aerobics ; and it completely prevents 

 the growth of others anaerobics. Many bacteria, even when freely 

 exposed in a desiccated condition to the action of atmospheric oxygen, 

 retain their vitality for a long time. The gradual loss of pathogenic 

 power which Pasteur has shown occurs in cultures of the anthrax 

 bacillus and the micrococcus of fowl cholera, is ascribed by him to 

 exposure to oxygen, and as proof of this he states that cultures kept 

 in hermetically sealed tubes do not lose their virulence in the same 

 degree. But other circumstances may influence the result. Thus 

 some of the products of growth which accumulate in culture fluids 

 have an injurious effect upon the vitality of the bacteria which pro- 

 duced them, and in time may cause a complete destruction of vitality. 

 In cultures exposed to the air these products would be in a more 

 concentrated solution from the gradual evaporation of the culture 

 liquid. It must also be remembered that light in the presence of 

 oxygen is a germicidal agent. 



The experiments of Frankel show that the aerobic bacteria grow 

 abundantly in the presence of pure oxygen, and some species even 

 more so than in ordinary air. Micrococcus prodigiosus, however, 

 appeared to be unfavorably affected by pure oxygen, inasmuch as it 

 did not produce pigment so readily as when cultivated in ordinary air. 



Nascent oxygen is a very potent germicidal agent, as will be seen 

 in our account of such oxidizing disinfectants as potassium perman- 

 ganate and the hypochlorite of lime. 



Ozone. It was formerly supposed that ozone would prove to be 

 a most valuable agent for disinfecting purposes ; but recent experi- 

 ments show that it is not so active a germicide as was anticipated, 

 and that from a practical point of view it has comparatively little 

 value. 



Lukaschewitsch found that one gramme in the space of a cubic 

 metre failed to kill anthrax spores in twenty-four hours. The cholera 

 spirillum in a moist state was killed in this time by the same amount, 

 but fifteen hours' exposure failed to destroy it. Ozone for these ex- 

 periments was developed by means of electricity. 



