40 BACTERIOLOGY. 



of proteid substance for its development. Certain 

 of them bring about most profound alterations in the 

 media in which they exist, while others produce but 

 little apparent change. In one case alterations in the 

 reaction of the media will be conspicuous, while in 

 another no such variation can be detected. With the 

 growth of some forms products resulting from specific 

 processes of fermentation appear. Other varieties pro- 

 duce poisons of remarkable degrees of toxicity, while the 

 growth of others may be accomijanied by the evolution 

 of compounds characteristic of putrefaction. 



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 mate- 

 rials to which they have access, and by the character 

 of the 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 

 accumulated 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 the inhibiting conditions that are coincident with 

 bacterial growth are alterations in the chemical reaction 

 of the matters in which the bacteria are developing. Since 

 the majority of them grow best in media of a neutral or 

 very slightly alkaline reaction, any excessive development 

 of alkalinity or acidity, as a result of growth, arrests 

 development, and no evidence of life or furtlier multi- 

 plication can be detected until this deviation from the 

 neutral (or the suitable) reaction has been corrected. 



