GRAVITY, OSMOTIC PRESSURE, AGITATION, CHEMOTAXIS 45 
as is evidenced by a diminution in virulence, decreased pigment produc- 
tion, 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 aflFect the volume of the liquid in which bacteria are 
suspended, nor does this pressure afJ'ect the amount of gas dissolved 
in the liquid. Thus, Larson, Hartzell and Diehl^ found that spore- 
containing bacilli may not be killed even after fourteen hours' exposure 
to 12,000 atmospheres direct pressure: non-spore-containing bacteria 
survive 3000 atmospheres direct pressure for several hours. If, how- 
ever, 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 CO 2, which is feebly 
germicidal at ordinary pressures, are rapidly killed if the pressure is 
gradually increased; that is, CO2 under these conditions becomes 
strongly bactericidal. iVccording to Berghaus,- twenty-four hours' 
exposure to 16 atmospheres' pressure of carbon dioxide will kill cholera 
vibrios on agar plates. Anthrax bacilli and B. alcaligenes grow under 
similar conditions, provided the plates are subsequently placed in the 
air. B. typhosus, paratyphosus and coli will grow in this atmosphere 
of carbon dioxide. Cholera vibrios, B. alcaligenes and anthracis will 
grow in a pressure of 2 atmospheres of oxygen. B coli will grow in 75 
atmospheres of oxygen. All the organisms mentioned grew in 75 
atmospheres' pressure of hydrogen. 
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 
• Jour. Infec. Dis., 1918, 22, 271. 
2 Arch. f. Hyg., 1907, 62, 172. Roger: Action des hautes pressione sur quelques 
bacterieg, Compt. rend. Acad. Sci., December 3, 1894. 
