56 Biology of Micro-organisms 



development of the frozen cultures takes place from the few surviving 

 organisms. 



Bacteria usually grow best at the temperature of a comfortably 

 heated room (i7°C.), and are not affected by its occasional slight 

 variations. Some, chiefly the pathogenic forms, are not cultivable 

 except at the temperature of the body (37°C.); others, like the tu- 

 bercle bacillus, grow best at a temperature a little above that of the 

 normal body. 



The temperature endurance of the molds resembles that of the 

 bacteria. The mycelia are killed at temperatures of 6o°C. and over, 

 but their spores endure ioo°C. The yeasts and oidia, that have no 

 resisting spores, are killed at about 6o°C. The protozoa are still 

 more sensitive to heat variations than the plant organisms and are 

 killed by less extreme variations. Here again, however, the encysted 

 protozoa endure greater variations than the active organisms. 



Efifect of Chemic Agents. — The presence of chemic agents, espe- 

 cially certain of the mineral salts, in an otherwise perfectly suitable 

 medium may completely inhibit the development of bacteria, and 

 if added to grown cultures in greater concentration, destroy them. 

 Such substances are spoken of as antiseptics in the former, germi- 

 cides in the latter case. Bichlorid of mercury and carbolic acid are 

 the most familiar examples of germicides. 



Though these agents are supposed to operate in definite concentra- 

 tions with almost unvarying result, Trambusti* found it possible to 

 produce a tolerance to a certain amount of bichlorid of mercury by 

 cultivating Friedlander's bacillus upon culture-media containing 

 gradually increasing amounts of the salt, until from 1-15,000, which 

 inhibit ordinary cultures, it could accommodate itself to 1-2000. 



The various chemic agents act in different ways upon the micro- 

 organisms. Thus, they may combine with the protoplasm to make 

 a new and no longer vital compound; or, they may coagulate or 

 dissolve or dehydrate or oxidize the protoplasm to a destructive 

 extent. 



The addition of chemic agents to solutions containing micro- 

 organisms also changes the osmotic pressure. When an active 

 organism is living in its normal environment, it contains within its 

 plasm a greater concentration of solutes than are to be found in 

 the surrounding fluid. Under these circumstances the pressure 

 on the inside of the ectosarc or other cell membrane is greater than 

 that on the outer side, and the cell is in a state of turgor. If now salts 

 are added so that the solutes on the outside exceed those on the 

 inside, water is drawn out and the protoplasm is made to shrink or 

 condense. According to the degree of this change the organism 

 will be embarrassed, made impotent, or destroyed. 



On the other hand, when micro-organisms have enjoyed a con- 

 centrated medium like blood-serum and are suddenly transferred to 

 * "Lo Sperimentale," 1893-94. 



