PLAXT PARASITES. 149 



tion of their nutrient material. Some of the chemical compounds set free 

 by the growing bacteria are bad-smelling or aromatic ; some are inert and 

 harmless substances; some are powerful poisons, and may, when they 

 have accumulated in the fluids where they grow, inhibit activity and 

 growth or even destroy the bacteria which have produced them. 



Fermentations and putrefactions are due to the activities of micro- 

 organisms, some to bacteria, some to yeasts, some to moulds. Putrefac- 

 tion is a form of fermentation in which nitrogenous compounds are 

 decomposed by micro-organisms setting free, especially in the absence of 

 oxygen, bad-smelling substances. 



Bacteria w r hich induce fermentation are called zymogenlc and each 

 species induces fermentation of a particular character in the presence of 

 a special substance, as glucose, or members of a certain class of substances 

 such as carbohydrates. Some of these fermentations are important in the 

 arts ; some are concerned in the changes which food products undergo 

 under natural or artificial conditions, such as the development of 

 koumyss from milk and the common butyric, lactic, alcoholic, and other 

 fermentations. 



The chemical changes which are induced by micro-organisms in the 

 process of fermentation are extremely complex and little understood. 



Bacteria may develop in their metabolic activities soluble ferments 

 or enzymes of various kinds resembling diastase, pepsin, trypsiu, rennet, 

 etc. These may remain in the bacterial cell or may be diffused into the 

 surrounding media. 



Many bacteria form pigments as they grow (chromogenic bacteria). 

 This pigment may be developed in or upon the germs themselves or may 

 be diffused through the surrounding media and may be developed only 

 in the presence of light, oxygen, etc. Gas-producing bacteria are called 

 aerogenic. Certain species when growing in masses emit a phosphor- 

 escent light photogenic bacteria. 



Certain of the basic chemical compounds resembling the vegetable 

 alkaloids, which are formed by the action of bacteria in organic matter, 

 are called ptomains. ' The chemical substances produced by certain forms 

 of bacteria are of practical importance because they induce deleterious 

 effects in many of the infectious diseases ; these are called toxins. Com- 

 plex proteid bodies may be produced during the growth of bacteria; 

 these may be in part set free, in part assimilated in the bacterial cell 

 protoplasm. These proteid bodies belong in part to the albumins, in 

 part to the albumoses, while some of them resemble the peptons. Many 

 of them seem to be most potent factors in the induction of the phe- 

 nomena and lesions of the infectious diseases. The poisonous albumi- 

 nous substances produced in the body by the growth of certain disease- 

 producing bacteria are called toxalbumins. These may also be produced 

 in culture media by certain species. 



Bacteria are widely distributed in the air, in water, and in the super - 



1 Leucomdins are basic products produced in the tissues of living animals by cell 

 metabolism. 



