10 



DAIRY BACTERIOLOGY 



ficial, for nearly all microorganisms can live without atmospheric 

 oxygen for shorter or longer periods, provided that other sources 

 of energy are available, and towards some species oxygen acts as 

 a poison. Organisms which cannot live without atmospheric 

 oxygen are defined as aerobic, and those which can do without it 

 as anaerobic. The latter group is subdivided into facultative and 

 obligate anaerobes, which are respectively helped or hindered in 

 their growth by the presence of atmospheric oxygen. As a rule 



FIG. 16. Stab Cultures. A. The Anthrax Bacterium (aerobic). B. The Swine 

 Erysipelas Bacterium (facultative anaerobic). (After Mifjula.) C. The 

 Tetanus Bacillus (obligate anaerobe). (After Ball.) 



aerobic organisms will, sooner or later, form a film on the surface 

 of the culture solution or spread over the surface of solid media, 

 only penetrating slightly below the surface. Facultative anae- 

 robes will grow equally well at all .depths or on the surface, 

 while obligate anaerobes only thrive below a certain depth. 

 These relations are illustrated by the accompanying figures. 

 The true lactic acid bacteria generally tolerate air, but thrive 

 best in its absence ; in stab cultures, therefore, they will not 

 spread over the surface, but penetrate the medium evenly from 

 all points. 



