PHYSIOLOGY OF THE MICROBES 



81 



drop of water and the neighbourhood of air bubbles. They 

 can be grown well under shelter of a pellicle of aerobic 

 bacteria which prevent the passage of air, this being the best 

 method of cultivating the tetanus bacillus. There is no 

 necessity to suppose, as does Kedrowsky, that the aerobes 



d B C 



FIG. 35. Engelmann's spectrum. Bacteria seeking oxygen swarming 

 round an algal filament lying on a spectrum. The grains of chloro- 

 phyll are not represented. The lines of the spectrum mark out the 

 regions on which the bacteria collect, i.e., the points where most 

 oxygen is being liberated. 



secrete a special ferment which permits anaerobic growth ; it 

 is sufficient that the anaerobes are cut off from free oxygen. 



The addition to a tube of ordinary broth, aerated and hence 

 unsuitable for the culture of anaerobes, of sterile animal or 

 vegetable tissue, e.g., a fragment of flesh or a piece of banana, 

 allows the anaerobes to grow, the tissue acting as a reducing 

 agent. It is not at all correct to say, however, that the 

 anaerobes live without oxygen. They only live, as Pasteur 

 said, without free oxygen gas. They use up the oxygen which 

 is present in combination in the nutrient fluid and decompose 

 the food-stuffs to procure oxygen from them. 1 



1 Pasteur (1861) : "There exist, besides the living beings already known 

 which without exception, at least in the general opinion, live and breathe 

 only on condition of being able to assimilate free oxygen gas, others whose 

 respiration is so powerful that they can live cut off from the air by 



G 



