228 PHYSICAL INFLUENCES 



Some data have been given already in the chapter on oxygen require- 

 ments. It was mentioned in that connection that Bad. buiyricum can- 

 not tolerate more than 0.65 per cent of the total oxygen content in air 

 (0.2 atmosphere); in other words, an oxygen pressure higher thano.0013 

 atmosphere will kill the organism. The maximum pressure for B. 

 prodigiosus was found to be about 5.4 to 6.3 atmospheres. Very few 

 experiments have been made with other gases. Carbon dioxide at a 

 pressure of 50 atmospheres retards the growth of bacteria in water anfl 

 will sterilize it in twenty-four hours. Suspensions of pure cultures of 

 B. typhosus and Msp. comma are killed by 50 atmospheres carbon dioxide 

 pressure in three hours. Milk cannot be sterilized by his pressure but 

 bacteria do not multiply. Carbonated milk has been recommended as 

 a refreshing drink by several investigators. The ordinary market milk 

 will keep about two days longer under the pressure of 10 atmospheres 

 (150 pounds) than without pressure. If pasteurized it is said to keep 

 for a week. 



Gravity, — Gravity would have a great influence upon the growth of 

 microorgaiusms in Uquids if their specific gravity were much greater 

 than that of water. This does not seem to be the case however. It has 

 been estimated by accurate weighing to vary between 1.038 and 1.065. 

 Very much higher results (1.3 to 1.5) have been obtained by centrifuging 

 bacteria in salt solutions of varying specific gravity, but these data are 

 not exact since the salt solution will diffuse into the cells and thus in- 

 crease their weight. The specific gravity being very nearly that of the 

 culture medium, it is plainly seen that gravity has but Uttle influence. 

 The microorganisms will live suspended in the liquid and sediment out 

 very slowly. The slightest current in the Hquid wiU carry them 

 around and distribute them through the medium. ThelmotiUty is of 

 minor importance; the actual distance covered by motile bacteria has 

 been measured, and under the most careful exclusion of currents in the 

 liquid has been found to be about a millimeter in a minute for B. subtilis. 

 This is very slow compared with the speed of the circulating water 

 moved by changes of temperature or other incidental agents. 



Yeast cells and other gas producers use the carbon dioxide as a ve- 

 hicle. The gas bubbling up in the fermenting hquid keeps it constantly 

 in motion and moves the yeast cells against gravity toward the surface 

 where the gas escapes and lets the cells fall back to the bottom. 



The production of scums and peUicles on the surface by organisms 



