GRAVITY, OSMOTIC PRESSURE, AGITATION, CHEMOTAXIS 47 



mately 15 pounds to the square inch, or one kilogram per square 

 centimeter of surface). Bacteria are weakened, however, by these 

 great pressures, as is evidenced by a diminution in virulence, decreased 

 pigment production, and the partial or complete inability to multiply. 

 It is a curious fact that motile bacteria may retain their motility after 

 an exposure of several hours to 2000 atmospheres from the pressure 

 liquids, even although their powers of reproduction are quite lost. 



Liquids are practically non-compressible, consequently direct pres- 

 sure does not affect the volume of the liquid in which bacteria are 

 suspended, nor does this pressure affect the amount of gas dissolved 

 in the liquid. If, however, bacteria are exposed in liquids to gas 

 pressure in the place of direct pressure, the germicidal action of the 

 gas plays the prominent part in the final result. The amount of gas 

 dissolved in the liquid increases with increase of pressure, consequently 

 feebly germicidal gases may become powerfully germicidal as the 

 pressure is increased. Thus, bacteria suspended in water overlaid 

 by CO2, which is feebly germicidal at ordinary pressures, are rapidly 

 killed if the pressure is gradually increased; that is, CO 2 under these 

 conditions becomes strongly bactericidal. According to Certes, 1 

 600 atmospheres pressure of an inert gas, as nitrogen, will not kill 

 anthrax bacilli. 



Diminished Pressure. Diminished pressure, aside from lowering 

 the oxygen tension to a point below that necessary for the growth of 

 aerobic bacteria, does not interfere seriously with bacterial growth. 



2. Osmotic Pressure. The boundary layer, ectoplasm, of every 

 bacterial cell reacts like a semi-permeable or osmotic membrane. 

 Through this membrane must pass all the elements necessary to the 

 nutrition of the organism. A normal bacterial cell always tends to 

 maintain a greater concentration of solutes within its substance than 

 exists in the surrounding medium; hence the pressure from within 

 upon the cell membrane is somewhat greater than the pressure from 

 without upon the cell membrane, and the cell is consequently in a 

 state of continual turgor. The osmotic pressure exerted by dissolved 

 substances varies very greatly. Those of high molecular weight, as 

 albuminoses or peptones, exert little or no osmotic pressure. Crys- 

 talloids, on the contrary, may exert very considerable pressure. Thus, 

 a 30 per cent, solution of dextrose exerts a pressure of about 22 atmos- 

 pheres. A bacterial cell placed in such a solution is under a great 

 strain. If bacteria which are in a state of equilibrium with reference 



1 Compt. rend. Acad. de sc., 1884, 99, 385. 



