104 OUTLINES OF BACTERIOLOGY 



excrement, and, in fact, in all those places where organic decomposition 

 in the absence of oxygen is taking place. Most of them thrive on 

 dead organic matter, and are therefore saprophytes, whilst a few 

 are parasites, inasmuch as they thrive on living organisms. Of the 

 latter class are Bacillus oedematis maligni, the cause of surgical 

 gangrene, and Bacillus tetani, which is responsible for the lock-jaw 

 disease. Among the saprophytes a few are important from an 

 economic and industrial standpoint, though these have not as yet 

 been carefully investigated. 



Anaerobes do not all behave alike in the presence of oxygen. To 

 some that gas is either fatal or a positive hindrance to growth ; others, 

 while flourishing best in its absence, are able to live and grow in its 

 presence. The former are known as obligate anaerobes and the latter 

 as facultative anaerobes. As will be presently shown, these remarks 

 apply only when oxygen is present at the normal atmospheric pressure. 



The facultative anaerobes do not show the same physiological charac- 

 teristics when growing anaerobically as when oxygen is present. This 

 is shown by the results of several investigators. Thus it has been 

 demonstrated by one observer that Bact. formicicum under anaerobic 

 conditions uses up salts of formic acid when these are presented to 

 them in the nutrient medium, but under aerobic conditions these salts 

 remain unchanged. Again, of the seven species of facultative anaerobes 

 examined by another observer all, when growing aerobically, liquefied 

 gelatine, but under anaerobic conditions all were devoid of this faculty. 

 This points to the conclusion that when oxygen is presented to faculta- 

 tive anaerobes they are able to make use of it, and this conclusion has 

 actually been verified experimentally ; for by cultivating one of these 

 organisms in an atmosphere containing a very small quantity of oxygen, 

 it was found that the whole of the oxygen was used up. 



Recent investigations, especially those of Beijerinck and Chudjakow, 

 have profoundly modified our conceptions of the anaerobic bacteria. 

 The results of the following experiment, taken from Beijerinck's 

 researches, will show the trend of these changes. He poured a 

 little sterilised nutrient-gelatine into a test-tube, added sterilised water 

 to the gelatine, and then made an inoculation with an anaerobic 

 bacillus. The water became turbid in consequence of the growth of 

 this organism, but the turbidity was greatest, not at a point furthest 

 removed from the surface where there would be least oxygen, but 

 somewhat nearer the surface. All other anaerobic bacteria experi- 

 mented with showed the same peculiarity, though the level at which 

 the turbidity was greatest was not the same for the different organisms. 



