48 PATHOGENIC BACTERIA. 



175 C. for five to ten minutes. Freezing kills many, but 

 not all bacteria, but does not affect the spores at all. 



Most bacteria grow best at the ordinary temperature of 

 a comfortably heated room, and are not affected by its 

 occasional slight changes. Some, chiefly the pathogenic 

 forms, are not cultivable except at the temperature of 

 the animal body (37 C.) ; others, like the tubercle bacil- 

 lus, grow best at a temperature a little above that of the 

 body 40 C. 



Some forms of the bacteria are never found except in 

 the tissues of diseased animals. Such organisms are 

 called parasites. The parasitic group really is divisible 

 into the purely parasitic and the occasionally parasitic 

 bacteria. Of the first division the tubercle bacillus may 

 be used as an illustration, for, so far as is known, it is 

 never found in other places than the bodies and dejecta 

 of diseased animals. The cholera spirillum illustrates 

 the second group, for, while it produces the disease 

 known as Asiatic cholera when admitted to the digestive 

 tract, it is a constant inhabitant of certain waters, where 

 it multiplies with luxuriance. 



Bacteria which do not enter the animal economy, or if 

 accidentally admitted do no harm, but live upon decaying 

 animal and vegetable materials, are called saprophytes. 

 The parasitic organisms alone possess much interest to 

 the physician, but as in their growth the saprophytes ex- 

 hibit many interesting vital manifestations, it is not well 

 to exclude them or their products from the following 

 consideration of the 



Results of Vital Activity in Bacteria. i. Fermenta- 

 tion. The alcoholic fermentation, which is a familiar phe- 

 nomenon to the layman as well as to the brewer and the 

 chemist, is not the work of a bacterium, but of a yeast- 

 plant, one of the saccharomyces fungi. The acetic-acid, 

 lactic-acid, and butyric-acid fermentations are, however, 

 caused by bacilli. A considerable number of bacilli seem 

 capable of converting milk-sugar into lactic acid, some- 

 times associating this with coagulation of milk, some- 



