52 BACTERIOLOGY 



free oxygen is essential, for others it is harmful. In one case 

 alterations in the reaction of the media will be conspicuous, 

 while in another no such variation can be detected. As 

 shown above the growth of some species is accompanied by 

 evidence of specific fermentations; of others by the appear- 

 ance of poisonous; of others by putrefactive changes. 



In considering the normal development of bacteria we 

 must not lose sight of the fact that this is influenced both 

 by the quality and the quantity of the nutritive materials 

 to which they have access, and by the character of the 

 metabolic products that accumulate in these materials as 

 a result of their vital processes. Nitrogen and carbon 

 compounds may be present in amount and kind entirely 

 suitable to normal bacterial growth, and yet this may be 

 checked, after a comparatively short time, by the accumu- 

 lated products of bacterial metabolism, some of which 

 possess the property of inhibiting growth and ultimately 

 of even destroying the bacteria that produced them. The 

 most common and conspicuous examples of such inhibiting 

 conditions is alteration in the chemical reaction of the media 

 in which the bacteria are developing. 



In the case of a number of species there begins, coincidently 

 with retardation of normal development, a process of dis- 

 solution, self-digestion or "autolysis," which may continue 

 until the cells are unrecognizable as bacteria. This pheno- 

 menon is the result of the action of enzymes located within 

 the cells which, under normal conditions of growth, are 

 concerned in the life processes of the cell, but which, on the 

 advent of conditions unfavorable to the growth and mul- 

 tiplication of the cells, react upon them and cause their 

 actual solution. An analogous "autolysis" is often to be 

 seen with animal cells. If bits of living tissue be removed 



