BIOLOGY OF BACTERIA. 49 



as culture-media arid a variety of other substances, to a 

 temperature beyond that known to be the extreme limit 

 of bacterial endurance. 



The presence of certain substances especially some 

 of the mineral salts in an otherwise perfectly suitable 

 medium will prevent the development of bacteria, and 

 when added to grown cultures of bacteria will destroy 

 them. Carbolic acid and bichlorid of mercury are the 

 best known examples. 



It is interesting to mention in this connection the 

 results of the experiments of Trambusti, who 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 inhibited 

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



(/) x-Rays. The action of the .r-rays upon bacteria 

 has been investigated by Bonome and Gros and others. 

 When the cultures are exposed to their action for pro- 

 longed periods, their vitality and virulence seem to be 

 slightly diminished. They are not killed by the .r-rays. 



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. 



According to their products of metabolism, bacteria 

 are often described as 



4 



