CHEMOTAXIS. 



53 



versely, motile bacteria may also be employed as a delicate reagent 

 for oxygen. BEYEUINCK (I.), to whom we are indebted for a very 

 useful treatise on this question, examined more closely the methods 

 of topographical arrangement adopted in liquid media by various 

 motile bacteria under the influence of oxygen, which arrangement 

 he called respiration figures. Some of these are shown in Fig. 1 1. 

 From the researches of Stahl and De Bary, and especially 

 those of W. PFEFFER (I.), we learn that not only oxygen, but 

 also various other substances, are capable of attracting or repelling 

 bacteria and other micro-organisms, a faculty to which the name 

 of positive or negative chemotaxis has been given. Use is made 



FlG. ii. Respiration figures of motile bacteria. (After Beyerinck.) Natural size. 



The three figures are horizontal projections of bacterial preparations, each in a large 

 drop of water. The three large circular cover-glasses only are shown, the slides not being 

 reproduced. A small platinum wire (not shown in the figures) is placed at the part 

 represented by the top of the drawing, between the cover-glass and slide, so as to form a 

 wedge-shaped space, which is occupied by the drop of water, the base of which lies in 

 m (= meniscus). 



The three figures represent : 



I. Respiration figure of the aerobic type. The roving individuals collect in the 

 oxygenated border zone (a), whilst the quiescent ones (r) remain in the interior, leaving 

 a vacant space (/) between the two. 



II. Respiration figure of the spirillum type. These organisms require and tolerate 

 only traces of oxygen. They therefore collect, not at the circumference of the drop, but 

 at a little distance therefrom (sp), where the tension of the penetrating gas is lower. 



III. Respiration figure of the anaerobic (air-shunning) type. These migrate to the 

 centre (an) of the drop, as being the place of lowest oxygen content. 



of this property in order to capture the motile species in a 

 bacterial mixture by introducing therein a capillary tube filled 

 with a solution of a substance which exerts an attractive influence 

 on one or other of the motile species. 



Among the inorganic compounds the salts of potassium have 

 the greatest power of attraction, and are therefore most frequently 

 employed for this purpose. Of the organic compounds, asparagin 

 is particularly effective. The sap or juice of raw potatoes con- 

 tains both these lures, and is therefore highly efficacious. More 

 particular information on this method of isolation is given by 

 ALI-COHEN (II.). 



The attractive power of such agents is not limited to motile 



