FOOD OF MICROORGANISMS. 95 



amounts of oxygen; others are less sensitive. There is enough free 

 oxygen in the atmosphere to prevent the development of certain bacteria 

 having purple pigment, but they multiply easily wherever there is a more 

 limited supply of oxgyen. Their maximum of oxygen is only a little 

 smaller than the concentration of oxygen in the atmosphere. Even the 

 aerobic bacteria have their maximum of oxygen which is many times the 

 amount of oxygen of the atmosphere. The following table gives the 

 maximum concentration of oxygen for a few species. The oxygen content 

 of the atmosphere is taken as 100. 



Maximum Oxygen Tolerance 



B. (Clost idium) butyricux, 1.35 per cent of atmospheric oxygen. 



B. chauvei, . 5- 2 



B. cedematis maligni, 3- 2 5 



Purple bacteria (Molisch), about 90 



Thiosulphate bacteria (Nathansson), about 400 



Pink yeast, 9 



Penicillium glancum, about 1700 



B. prodigiosus, 3000 



These numbers mean that B. (Clostridium) butyricus cannot live unless the 

 atmosphere is diluted to about i per cent of its original content of 

 oxygen, either by evacuation or by other gases. B. prodigiosus is able 

 to live even if the air is compressed thirty times, but not more. The 

 table demonstrates also that there is no natural line between anaerobic 

 and aerobic bacteria, and a classification founded upon oxygen supply 

 is as arbitrary as all classifications of organisms; but like all classifica- 

 tions, it is necessary for the designation of certain qualities of bacteria 

 though a few species merge into the well-defined groups at both 

 extremes. 



The obligate anaerobic bacteria which tolerate only a very small 

 amount of oxygen can adapt themselves to higher oxygen concentrations 

 by a very slow increase of the oxygen pressure. It has also been claimed 

 that occasionally the obligate anaerobic bacteria in pure culture will grow 

 in air. It is known that these bacteria, if cultivated in an atmosphere 

 with small amounts of oxygen, will use up the oxygen. In what form 

 and how the oxygen is bound cannot be stated because the quantities in 

 question are too small to be traced in the various products of metabolism. 



The chemical changes instituted by anaerobic organisms are partly 

 hydrolytic decompositions, partly intramolecular changes. Carbon 



