158 



LABORATORY EXERCISES IN BACTERIOLOGY. 



aerobic bacteria (obligate aerobes), and facultative anaerobic bacteria (facultative anaerobes'). 

 The first group are unable to vegetate or sporulate except in the absence of free oxygen ; 

 the second, except in the presence of free oxygen ; the third group, while usually develop- 

 ing in the presence of the atmospheric oxygen, are able to maintain their activities 

 in its absence. Probably this division should not be accepted in an absolute sense, 

 since in case of many of the known aerobes there is the power of adaptation to atmos- 

 pheres containing less and less oxygen, and the same is true of many anaerobes grown 

 in atmospheres of nitrogen or hydrogen to which are gradually added small proportions 

 of oxygen. It has been noted, too, that when living in a culture with aerobic bacteria 

 it is sometimes possible that development of associated anaerobes will occur in the 

 ordinary atmosphere, as seen clinically in the association of the bacillus of tetanus 

 with the pyogenic cocci in superficial wounds. So, too, it has been noted that the 



FIG. 47. LARGE CLASS INCUBATOR WITH INTERIOR WARM-WATER TANK, AND WIRE- 

 CAGE DRAWERS, WALL OF WOOD, COVERED WITH FELT. 



A. Side section. /. Water-bath. 2. Wire-cage drawers. ?. Partition between rows of 



drawers, perforated for proper circulation of warm air. 



B. Front view, open door. /. Water-bath. 2 and j. Wire-cage drawers ; front of partitions 

 removed above to show insertion of (4} thermometer and (5) thermostat. 



addition of certain substances, as sodium sulphide (one or two drops of a ten per cent, 

 solution to ten cubic centimeters) , to ordinary bouillon or gelatine will enable many 

 anaerobes to develop in such media in the ordinary atmosphere. On the other hand, 

 an excessive proportion of oxygen in the atmospheres of cultures of aerobes may 

 modify their vital phenomena (as in the reduction of the virulence of Bacillus anthracis 

 by excess of oxygen) or eventually destroy them. 



Among the anaerobic organisms are many of the soil bacteria, that of lockjaw- 

 being of much pathologic importance; among the aerobes are most of the common 

 saprophytes, while in the group of the facultative anaerobes bacteria pathogenic and 

 parasitic to animal life are found. 



Atmospheres of nitrogen or hydrogen are well suited for the grow r th of anaerobic 



