METABOLIC PRODUCTS 41 



The opposite type of bacteria is represented by the obligate or 

 strict anaerobes. These cannot exist and multiply in the presence of 

 free oxygen, hence when raised in artificial cultures the air must 

 be excluded or at least its oxygen. The pure nitrogen present does 

 not, as a rule, interfere with the growth of the bacteria. Among 

 the pathogenic bacteria the most important obligate or strict anae- 

 robics are the bacilli of tetanus, black-leg, malignant edema, and 

 the Bacillus necrophorus. Although strictly anaerobic bacteria die 

 sooner or later in the presence of free oxygen, their spores may be 

 exposed a long time before being killed. Strict anaerobes may, how- 

 ever, exist, grow, and multiply in the presence of free oxygen when they 

 are intimately associated with aerobic bacteria, which in their growth 

 use up and remove the oxygen present. 



Facultative aerobes or anaerobes are those bacteria which can 

 thrive in the presence or absence of free oxygen. To these belong 

 many saprophytes and most pathogenic bacteria, such as the organ- 

 isms of anthrax, of typhoid, of cholera, the pus-producing cocci, etc. 



FIG. 23 



- c 





Structure of bacterial Plasmolysis: a, spirillum undula; 6, bacillus solmsii; c, vibrio 



cell: a, endoplasm; 6, choleras. (After A. Fischer.) 



ectoplasm, or cell mem- 

 brane; c, central, less in- 

 tensely staining parts; d, 

 chromatin granules. (After 

 Butschli.) 



Metabolic Products. Bacteria in their growth and metabolism excrete 

 certain gaseous and soluble waste products: among them are carbon 

 dioxide (CO 2 ), volatile compounds of nitrogen, such as NH 3 , also free 

 nitrogen, hydrogen sulphide (H 2 S), etc. The latter is formed in all 

 putrefactive processes depending uponbacteria intheabsence of oxygen. 

 Some bacteria in the presence of peptone form indol, for instance 

 the colon bacillus, and this product may be used in differentiating 

 nearly related bacteria from each other (the typhoid from the colon 

 bacillus). Indol may be formed in consequence of putrefactive 

 processes or by chemical decompositions of a different type. Other 

 chemical reactions brought about by pathogenic bacteria are the 

 reduction of reducible substances with oxygen absorption on the part 

 of the bacteria, reduction of nitrates to nitrites, the formation of 



