RATE OF GROWTH OF BACTERIA 39 



lieved that the limitation of growth of pneumococci was due to 

 the accumulation of a thermolabile toxic substance which they named 

 "bacterocidin," but in later papers M'Leod and Gordon showed that 

 this substance was hydrogen peroxide and that it was produced by 

 several other organisms, notably the streptococci. This was con- 

 firmed by Avery and Morgan, and Morgan and Avery concluded 

 that the limitation of growth in pneumococcus cultures is due to 

 three factors, the accumulation of acid, the production of peroxide, 

 and the exhaustion of the nutrients. These findings would indicate 

 that here again we are dealing with a special case not generally appli- 

 cable to all bacteria. 



Hajos grew various members of the colon-typhoid group in broth 

 until a maximum growth was attained, centrifuged out the bacteria, 

 re-inoculated, and incubated again, and repeated this process until 

 no further growth occurred. This exhausted medium was distinctly 

 inhibitory to bacteria; a few drops added to 10 c.c. of fresh broth 

 was sufficient to produce a definite inhibition of growth. The toxic 

 substance was merely inhibitory, failing to kill bacteria suspended 

 in it, was not specific, was not destroyed by heating to boiling, and 

 passed through a Bechold ultrafilter. This substance is quite dif- 

 ferent from those previously described. Hajos considers it to be a 

 product of metabolism rather than a specific "autotoxine." This 

 substance can, however, hardly be of great importance in the cessa- 

 tion of growth in normal cultures, since it could only be obtained 

 in quantities sufficient for demonstration after the medium had been 

 completely exhausted by repeated subcultures. 



Curran, observing that growth occurred in media in which bac- 

 teria had previously reached a maximum growth, and from which 

 they had been removed by heating or by filtration, also concluded 

 that the cessation of growth is due to a thermolabile substance 

 readily adsorbed by bacterial filters. To test the latter theory more 

 carefully, he performed the following experiment. A three-day old 

 culture of B. coli was passed through a filter, the first and last 50 

 c.c. portions of the 200 c.c. culture being retained. These two por- 

 tions were tested separately for their ability to support growth; the 

 culture in the last portion grew more slowly, only half as many 

 organisms being present after ten hours as in the first portion, though 



