INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 



26l 



It appears that oxygen is one of the most important factors in the life of 

 microorganisms. For some organisms oxygen is essential, others can exist a 

 long time without it, and still others can reproduce only under conditions where 

 it is entirely absent (see Part I, Chapter VIII). Microorganisms may thus be 

 separated into aerobes and anaerobes, according to their oxygen requirement. 

 Aerobes require oxygen for their development, while anaerobes can develop 

 in the complete absence of this gas. Anaerobes are either obligate or facultative. 

 Obligate anaerobes reproduce only when oxygen is entirely absent; it acts upon 

 them as a poison. Facultative anaerobes are not seriously injured by oxygen 

 and they also thrive in its absence. Acetic acid bacteria may be mentioned as 

 an illustration of aerobes; yeasts, of facultative anaerobes; and the bacteria of 

 butyric acid fermentation are obligate anaerobes. 



Motile bacteria may be used as an indicator of the relative amounts of 

 oxygen present in different regions of a mass of nutrient medium. In Fig. 106, 



a 



an 



I n 



Fig. 106. — Respiration figures of motile bacteria. 



m 



{After Beijerinck.) 



respiration figures for three different kinds of bacteria are shown. In each case a 

 drop of the culture was placed upon a slide and covered, the circular cover glass 

 being raised at one edge by a bit of platinum wire. The drop of liquid thus came 

 to lie under the half of the cover that was nearest to the slide. The first figure 

 (I) shows the behavior of typhus bacteria, which are aerobes. The moving 

 cells are most numerous in the region of the drop that contains the most oxygen. 

 Those in the zone r have ceased moving because of deficiency of oxygen, while 

 the zone/ is free from bacteria. The next figure (II) represents the distribution 

 of spirillum bacteria which require a small amount of oxygen. The cells have 

 collected in the zone sp, a certain distance from the free surface of the liquid. 

 The third figure represents the activity of anaerobes in this sort of mounting. 

 All the cells have collected in the central zone of the drop, an, where oxygen 

 is least plentiful. 



In the culture of anaerobes it is essential that precautions be taken to pre- 

 vent the access of oxygen to the nutrient medium. For this purpose Pasteur 

 employed a layer of oil over the nutrient solution. The air may also be 



c The effect of an oil layer in lowering the rate of oxygen supply to the liquid below depends 

 upon the kind of oil used as well as upon the thickness of the layer. It must not be assumed 

 that such an oil layer cuts off the supply of oxygen entirely. Of course the culture may be 

 placed in a chamber of nitrogen, helium, or hydrogen, etc. — Ed. 



