236 PHYSIOLOGY OF BACTERIA 



again and again a normal crop from the same amount 

 of medium (Table 64) . After sterilization of the culture 

 with ether, no growth occurred; growth in the ether- 

 sterilized culture became possible after steam heating, 

 or after adding pieces of a Chamberland filter to adsorb 

 the inhibiting substance. 



Avery and Morgan's (1924) inhibiting substance of pneumo- 

 coccus culture finally proved to be hydrogen peroxide, a well- 

 defined cell product. Hajos (1922) obtained an inhibiting substance 

 from cultures of Bad. coli and related bacteria by repeatedly centrif- 

 uging the bacteria out of the culture and re-inoculating the medium 

 until no further growth occurred. The chemical constitution 

 remained unknown so far. Since a few drops of it prevented growth 

 in 10 c.c. of fresh broth, the toxic compound seems to be quite con- 

 centrated. But this substance is heat-resistant and is perhaps a 

 fermentation product. 



Rogers and Whittier (1928) appear to have established a growth- 

 inhibiting factor in Street, lactis which is a cell product, but not acid. 

 They grew Strept. lactis in double strength broth in a collodion sac 

 suspended in a flask of the same medium, the volume of the outside 

 medium being about ten times that in the sac. After two days of 

 growth, the outside liquid was also inoculated with Strept. lactis, 

 and the bacteria were counted from hour to hour. The cells devel- 

 oped to about 70-71 millions per c.c. while in a normal check, they 

 grew to 300 millions. Repeated addition of concentrated broth 

 brought the number of cells in the double culture up to 98 millions, 

 while the normal culture increased to 550 milHons. Rogers and 

 Whittier beheve this compound to be identical with, or similar to, 

 the agent by which Strept. lactis inhibits Lactobacillus bulgaricus; it 

 is also believed to be the substance controlling the final amount of 

 growth in solid or liquid media which contain but little carbohydrate. 



SUMMARY 



There are various reasons why growth of a bacterium 

 will cease in a given volume of culture medium. It has 

 been shown that certain streptococci in milk cease to 

 grow because of too high acidity. It has also been 



