148 PRINCIPLES OF BACTERIOLOGY 



other hand, cannot produce sufficient numbers in less than two weeks. 

 The virulence of the septicaemic class of bacteria is not at all the same 

 when measured in different animals, and it is largely for this reason 

 that the virulence in test animals does not usually correspond with the 

 severity of the case from which the organism was derived. We should 

 remember in this connection the varying power of resistance in dif- 

 ferent animals to the same organism and of the same individual at 

 different times. 



Mixed Infection. The combined effects upon the tissues of the prod- 

 ucts of two or more varieties of pathogenic bacteria, and also of the 

 influence of these different forms on each other, are of great impor- 

 tance in the production of disease. The infection from several different 

 organisms may occur at the same time, or one may follow the other or 

 others so-called secondary infection. Thus, an abscess is often due 

 to several forms of pyogenic cocci. If a fresh wound is infected from 

 such a source the inflammation produced will probably be caused by 

 all the varieties present in the original infection. Peritonitis following 

 intestinal injuries must necessarily be due to more than one organism. 

 Thus, whenever two or more varieties of bacteria are transferred to 

 a new soil, mixed infection takes place if more than one variety is 

 capable of developing in that locality. 



Forms of infection which are allied to both mixed and secondary 

 infection are those occurring in the mucous membranes of the respiratory 

 and digestive tract. In these situations pathogenic bacteria of slight 

 virulence are always present even in health. Thus, in the upper air 

 passages there are usually found streptococci, staphylococci, and pneu- 

 mococci. When through a cold, or the invasion of another infective 

 agent, as the diphtheria bacillus, the virus of smallpox or scarlet fever, 

 the epithelium of the mucous membrane of the throat is injured or 

 destroyed, the pyogenic cocci already present are now enabled in this 

 diseased membrane to grow, produce their poison, and even invade 

 deeper tissues. The intestinal mucous membrane is invaded in a 

 similar way by the colon bacilli and other organisms after injury by 

 the typhoid or dysentery bacilli or cholera sgirilla. Generally speaking 

 all inflammations of the mucous membranes and skin contain some of 

 the elements of mixed infection. Blood infection, on the other hand, 

 is usually due to one form of bacteria, as even when several varieties 

 are introduced, only one, as a rule, is capable of development. The 

 same is true to a somewhat less extent of inflammation of the connec- 

 tive tissue. The additional poison given off by the associated bacteria 

 aid infection by the primary invaders by causing a lowering of the vital 

 resistance of the body. In some cases the secondary infection is a 

 greater ^danger than the primary one, as pneumococcic bronchopneu- 

 monia in laryngeal diphtheria or streptococcic septicaemia in scarlet 

 fever and smallpox. 



The bacteria are also at times directly influenced by the products 

 of associated organisms. These may affect them injuriously, as, for 

 example, the pyogenic cocci in anthrax; or they may be necessary to 



