OXYGEN SUPPLY 57 



It is evident that the maximum, minimum and optimum 

 oxygen pressures for anaerobic bacteria are the same, namely, 

 mm. Hg. This is true only for natural conditions, since 

 a number of anaerobic organisms have been gradually accus- 

 tomed to increasing amounts of O, so that by this process 

 of training they finally grew in ordinary air, that is, at an 

 oxygen pressure of about 150 mm. Hg. (Normal air pressure 

 is 760 mm. Hg. and oxygen makes up one-fifth of the air.) 

 The minimum O pressure for facultative anaerobes is also 

 mm. Hg. Some experiments have been made to deter- 

 mine the limits for aerobes, but on a few organisms only, 

 so that no general conclusions can be drawn from them. 

 To illustrate: Bacillus suhtilis (a common "hay bacillus") 

 will grow at 10 mm. Hg. pressure but not at 5 mm. Hg. 

 It will also grow in compressed oxygen at a pressure of three 

 atmospheres (2280 mm. Hg.) but not at four atmospheres 

 (3040 mm. Hg.), though it is not destroyed. 



Parodko has determined the oxygen limits for five com- 

 mon organisms as follows: 



In atmospheres. 

 Pseudomonas fluorescens 1 .94 to 2.51 

 Sarcina lutea . . . 2.51 to 3. 18 

 Proteus vulgaris . . 3 . 63 to 4 . 35 

 Escherichia coli . . 4 . 09 to 4 . 84 

 Serratia marcescens . 5 . 45 to 6 . 32 



These few instances do not disclose any general principles 

 which may be applied either for the growth or for the dis- 

 tinction of aerobes or facultative anaerobes. 



It has been shown that compressed oxygen will kill some 

 bacteria, but this method of destroying them has little or 

 no practical value. Oxygen in the form of ozone, O3, is 

 rapidly destructive to bacteria, and this fact is applied prac- 

 tically in the purification of water supplies for certain cities 

 where the ozone is generated by electricity obtained cheaply 

 from water power. The same is true of oxygen in the 

 "nascent state," as illustrated by the use of hypochlorites 

 for the same purpose. 



Certain thermophil bacteria in the soil have an optimum 



