No. 6. DEPARTMENT OF AGRICULTURE. 317 



less vigorously than when air is freely admitted; in this case the ten- 

 dency is towards an anaerobic habit, but such a habit may not be 

 fixed, and it may be changed in a measure by cultural conditions 

 under the control of the bacteriologist. That is, an organism which 

 has a slight anaerobic habit can be made to grow more and more 

 freely with the exclusion of atmospheric oxygen. The relation of 

 bacteria to atmospheric oxygen may manifest itself in a number of 

 ways. 



Thus if a fine sterile platinum needle be dipped into a bacterial 

 culture, so that Its surface becomes covered with a particular germ, 

 and if this contaminated needle be* then stabbed into a tube con- 

 taining solid nutrient gelatin, a medium in which bacteria grow 

 readily, the latter are, so far as food material is concerned, free to 

 grow^ at all points along the line of stab, and their development 

 is only limited by their ability to grow in the presence or absence 

 of atmospheric oxygen. If growth takes place as well in the depth of 

 the medium, where air is excluded, as it does at the surface where air 

 is abundantly present, such an organism is clearly indifferent to its 

 atmospheric environment. If on the other hand no growth takes 

 place in the depth of the gelatin, but only on the surface, the organ- 

 ism would be aerobic in habit, as shown in Fig. 3 A, or again if the 

 growth be confined to the deeper portion of the line of inoculation 

 with no growth at the surface, the organism w^ould show the op- 

 osite or anaerobic habit. Fig. 3 B. Between these extremes there 

 will in different bacteria be seen to be a wide range of variation. In 

 the different soil bacteria so far studied there was but little tendency 

 for them to grow in the depth of the gelatin, practically all of the 

 growth taking place at the surface, thus showing the generally aero 

 bic habit of the great majority of them. 



Another valuable method of demonstrating the relation of an or- 

 ganism to atmospheric oxygen is by means of a culture in a fermen- 

 tation tube, seen in Fig. 4, containing beef broth to which two per 

 cent, by weight of grape sugar has been added. In the fermentation 

 tube it is noticed that one end of the same is open and exposed to 

 the air while the other is closed and excluded from the air. If 

 bacteria are disseminated throughout the broth in the tube, they 

 will be free to develop in either arm as they find atmospheric oxy- 

 gen favorable or unfavorable to their growth. Thus if the growth is 

 confined to the open arm of the tube the organism concerned is 

 aerobic. Fig. 4 A, and if confined exclusively to the closed arm. Fig. 

 4 B, anaerobic in habit, while if an equal amount of growth takes 

 place in both arms it is indifferent to atmospheric oxygen and is con- 

 sequently facultative anaerobic in habit. 



Of ten of the most common soil bacteria recently studied by the 



20 



