26o ANIMAL AGGREGATIONS 



Burnet (1925, 1925a) has extended and verified certain aspects of 

 the work just reported. He found that "a batch of nutrient agar 

 plates which had been allowed to remain in the light for some time, 

 while still capable of growing staphylococci when heavily inoculated, 

 did not allow the growth of isolated organisms. If, however, a 

 broth culture, diluted in saline so as to give discrete colonies, were 

 spread on such a plate, and an adjacent area were inoculated heavily 

 with staphylococci, growth of isolated colonies occurred within a 

 range of approximately one centimeter from the region of heavy 

 growth." 



The inhibiting agent in the plates exposed to light was found to 

 be hydrogen peroxide, whose effects are destroyed by substances 

 produced by an active and heavy growth of staphylococci, so that 

 in regions reached by the diffusion of such growth-products, colonies 

 can develop from isolated organisms. The growth-facilitating sub- 

 stances are in part thermolabile and appear to have some of the 

 properties of enzymes; in part they are thermostable, non-enzyme 

 materials which Burnet calls "thermostable X." This substance will 

 neutralize growth-inhibiting powers of hydrogen peroxide and will 

 prevent its accumulation when plates are exposed to the action of 

 light. There is a definite quantitative relation between the amount 

 of thermostable X and the amount of peroxide that can be neutral- 

 ized, and it seems probable that the interaction is a simple reduction 

 of the peroxide, both substances being destroyed. 



An inhibition of growth due to the presence of potassium cyanide 

 can also be neutralized by these substances. Burnet regards the 

 mechanism of bacterial mass resistance to KCN to be essentially 

 the same as that which gives protection from peroxide introduced 

 into cultures directly or by the action of light on culture plates. He 

 thinks that the KCN is not destroyed by the thermostable-X sub- 

 stance as is the peroxide, but that it is inactivated more indirectly, 

 thus: •KCN acts to inhibit the normal mechanism for removing 

 peroxide, which accumulates until it is present in toxic amounts. 

 The diffusible substances produced by bacterial growth remove the 

 peroxide as shown above, and by so doing render the KCN innocu- 

 ous. Burnet gives evidence which indicates that the effect of certain 



