158 MANUAL OF BACTERIOLOGY. 



is cultivated at 42'^ to 43° C. Exposure to light and to oxygen 

 tends to weaken the virulence; and also cultivation upon un- 

 favorable media, such as those containing a small proportion 

 of carbolic acid or certain other chemical germicides. 



In laboratory work the virulence is usually maintained 

 best by inoculating the bacteria from time to time into sus- 

 ceptible animals. Bacteria coming freshly from infected 

 animals are likely to be highly virulent. The virulence may 

 be increased by beginning with an especially sensitive animal 

 like a very young guinea-pig, and progressively inoculating 

 into less sensitive animals. The infection of relatively in- 

 susceptible animals may sometimes be produced by the injection 

 of a very large dose of the bacteria. The addition of the toxic 

 products of the bacteria, which may be obtained by using 

 large doses of cultures in bouillon, makes infection more likely. 

 Cultivation on a particular medium may maintain or increase 

 the virulence. 



Finally, the combination of two or more kinds of bacteria 

 may produce infection when neither one would do so alone. 

 On the other hand, it is said that the fatal effects of an inocula- 

 tion of virulent anthrax bacilli into a susceptible animal may 

 be averted if the animal be inoculated with a culture of bacillus 

 pyocyaneus shortly afterward. 



Mixed Infection. — It is not uncommon in disease to find 

 two kinds of bacteria associated together, producing a mixed 

 infection. In diphtheria, very frequently, the bacillus of 

 diphtheria is found to be accompanied in the membrane by 

 the streptococcus pyogenes. The course of the diphtheria 

 may be modified in this manner. The term secondary in feci ion 

 is rather loosely used. It is sometimes employed to designate 

 an infection occurring in an individual, the resisting power of 

 whose tissues has been weakened by some chronic organic 

 disease. Such an infection is often called a terminal infec- 

 tion. Terminal infections are very common in cases of car- 



