CHEMOTAXIS 



79 



they form a dense swarm there, and begin to enter the tube. The move- 

 ments of the bacteria in the drop become more lively as soon as the 

 diffusing peptone reaches them, and at the entrance of the tube they 

 are a whirling, ' buzzing ' mass, like hiving bees. The potential energy 

 of the food-stuff has been converted into the kinetic energy of the 

 vibrating cilia. If now a cover-glass be laid upon the drop another kind 

 of chemotaxis can be observed. The bacteria which have entered the 

 tube move upward, attracted by the air in the blind end, and in about 

 half an hour that section of the capillary immediately below the bubble 

 is plugged by a thick mass of organisms. But these phenomena, the 

 attraction by food and the attraction by air, might as well be called tropho- 

 tropism as chemotaxis, the latter being shown in 

 its purest form when solutions of salt are employed. 

 For instance, a 1-9 per cent, solutiq^ of KC1 has 

 a powerful effect, and draws the bacteria into the 

 tube just as peptone does, being still feebly at- 

 tractive even in a dilution of 0-019 P er cent. Among 

 the alkalies, potassium is chemotactically the most 

 powerful, then sodium and rubidium. The alkaline 

 earths are less effective. The influence of a salt 

 is attributable mainly to its electropositive con- 

 stituent, the acid radical acting much more weakly. 

 Further details of this interesting subject will 

 be found in the works of Pfeffer already cited. 

 Among organic substances with a high nutritive 

 value, asparagin and peptone may be mentioned as 

 being strongly chemotactic, whilst sugar, one of 

 the best food-stuffs and richest sources of energy, 

 has but little attractive power. Glycerine is in 

 all cases, as far as is known, inactive. 



Contrasting with the phenomena just described, is the power which 

 certain substances have of repelling bacteria. This is known as negative 

 chemotaxis. Free acids and alkalies have this effect, and capillaries filled 

 with their solutions invariably remain empty. Alcohol too is ' instinctively ' 

 despised by micro-organisms. In some salts the action of the acid radical 

 and that of the base neutralize each other (e.g. carbonate of ammonia 1-76 

 per cent., phosphate of potash (monobasic) 3-48 per cent.), and the bacteria 

 then take up a mean position at a certain distance from the mouth of the 

 tube. 



In the case of negatively chemotactic compounds, the poisonousness of 

 the substance is no more a criterion of its repellent power than the food 

 value is an index of the attractive power in positively chemotactic bodies. 

 For instance, a solution of 0-019 P er cent, potassium chloride plus o-oi per 



FIG. 18. Chemotaxis. A part 

 of a drop of water containing 

 Bacillus fltiorescens liquefa- 

 ctens, and a capillary sealed at 

 one end and partly filled with 5 

 per cent, weakly alkaline peptone 

 solution ; at / an air^bubble. 

 About 4 minutes after introduc- 

 tion of capillary, the bacilli have 

 collected round the open end of 

 the tube, positive chemotaxis. 

 4-J hr. later, the bacilli, driven 

 by want of oxygen, have collected 

 close to the air-bubble. From 

 nature. Magn. 50. 



