168 ANTAGONISMS BETWEEN NONSPECIFIC PATHOGENS 



as ascorbic acid and thiamin, and certain other organic compounds, such 

 as cysteine, inhibit the formation of necrosis produced by tobacco-mo- 

 saic virus. This reaction is reversible, since necroses begin to develop 

 when the tobacco leaves thus treated are placed in pure water (815). 



The ability of bacterial phages to interfere with the development of 

 other phages has been studied in detail by Delbriick and Luria (175, 

 586). They have shown that a certain phage, after inactivation by ultra- 

 violet radiation, retained its ability to interfere with the growth of an- 

 other phage acting upon the same host. The partly inactivated first 

 phage is adsorbed by the sensitive bacteria and inhibits their growth 

 without producing lysis. The partly inactivated phage interferes also 

 with the growth of the active phage. This interference between bac- 

 terial phages was explained as due to competition for a "key-enzyme" 

 present in limited amount in each bacterial cell. This enzyme was also 

 believed to be essential for bacterial growth. 



In order to explain the "mutual exclusion effect" of one virus by 

 another, a "penetration hypothesis" was proposed ( 1 74) . According to 

 this hypothesis, the penetration of one virus into the cell renders the 

 cell membrane impermeable to any other virus j each virus has a char- 

 acteristic penetration time, and a change of permeability occurs at the 

 end of this time. The depressor effect consists in competition between 

 the two viruses for the same substrate. 



The function of a co-factor, like tryptophane, was considered to be 

 either that of a cement substance acting in a specific combination be- 

 tween virus and host receptive spots or as a coenzyme which enables 

 the virus particles, during their encounters with the host cells, to be- 

 come attached to them and attack them (21). 



MICROBES AND TUMORS 



The ability of certain microbes to bring about hemorrhage in tumors 

 (455a, 1044) may also be classed among the antagonistic phenomena. 

 The hemorrhage-producing agent is a polysaccharide and is isolated 

 only from gram-negative bacteria. 



Laszlo and Leuchtenberger (549) described a rapid test for the de- 

 tection of tumor-growth inhibitors. Inhibition was judged by comparing 



