84 MICROBES AND TOXINS 



indefinitely, i.e., in traces. Yeast cells are made to grow shut 

 off from air in a series of communicating flasks all carefully 

 sealed. The flasks are inoculated in series, the second with the 

 yeasts of the first and so on. To isolate each flask from the 

 preceding the little communicating tube is sealed in the flame. 

 Towards the tenth generation, fermentation is seen to stop and 

 only revives when a little oxygen is admitted to the confined 

 atmosphere. 



Pasteur had already observed that an air bubble about the 

 size of a pin-head was sufficient to reawaken a slackening 

 fermentation. 



All microbes require oxygen but their requirements are very 

 unequal. Betweeen the aerobes and the strict anaerobes there 

 exists every intermediate condition. Each species requires an 

 oxygen pressure suitable for itself just as do the higher animals, 

 which are of course aerobic organisms. 



By exhausting the air under a bell-jar, Khoudiakow observed 

 that the B. butyricus could still multiply at the pressure of five 

 millimetres, Clostridium butyricum at ten millimetres, the vibrion 

 septique and the tetanus bacillus at twenty millimetres ; the 

 bacillus of systematic anthrax at forty millimetres, at which 

 pressure the latter microbe behaves like an aerobe, using up 

 the oxygen in oxidations. 



An anaerobe like the B. butyricus can be trained to live under 

 an oxygen pressure greater and greater up to fifty millimetres, 

 a pressure ten times greater than that which it will bear 

 normally. This acclimatisation of anaerobes to contact with 

 air can be carried out within certain limits in the laboratory, so 

 much so that it has even been thought useful to create the 

 barbarous phrase ' aerobisation ' of anaerobes. 



Khoudiakow has made a complementary experiment by 

 modifying the pressure on aerobes. B. subtilis, grown on 

 gelatin, lives fairly well under three atmospheres, but begins to 

 suffer at four. At the other end of the scale it still grows well 

 at ten millimetres of pressure, but not at five. Aspergillus niger 

 has for minimum and maximum five millimetres and three 

 atmospheres. Spores are more resistant to the action of air 



