MECHANISM OF BACTERIAL INVASION 93 



though this may not produce an infection, a large number of bacteria so 

 transported may be able to resist destruction and prove capable of 

 causing infection. 



5. Aside from the question of local conditions in the process of in- 

 fection, other factors may exert an influence. The temperature of the 

 host may be unsuitable for the growth of a certain parasite, even though 

 it has gained entrance to the deeper tissues; a particular route for the 

 introduction of the infecting agents may be necessary, as in typhoid 

 fever and cholera, which are probably always intestinal infections, and, 

 finally, even after the infecting agent has reached the deeper tissues, 

 extension is prevented by a local inflammatory reaction. In many such 

 instances the question of natural immunity is brought into intimate 

 relation with the subject of infection. 



After invasion has occurred, some bacteria can best sustain them- 

 selves against the defenses of the host at the local point of entry. Such 

 microorganisms may, however, possess unusual vitality, and indirectly, 

 through the lymphatics, find their way to the blood-stream, producing 

 a bacteremia. This is a morbid condition characterized by the presence 

 of microorganisms in the circulating blood. 



Some microorganisms may gain entrance to the general circulation 

 more readily than others, and their mode and route of entry vary in the 

 different infections. It is essential that they possess an unusual degree 

 of invasive power, and be capable of protecting themselves against the 

 manifold defensive factors contained in the blood. Kruse believes that 

 in local infections the high pressure of an inflammatory exudate may 

 force bacteria into the adjacent vessels; that they may sometimes be 

 carried into the deeper tissues, and even into the blood-stream, by 

 leukocytes is not to be denied. 



When bacteria have entered the circulation, they may act as emboli 

 in the finer capillaries, or, being unable to remain in the circulation, may 

 collect in the capillaries of less resistant tissues, proliferating and pro- 

 ducing local metastatic lesions, usually purulent in character. The 

 condition thus produced is known as pyemia. 



Saprophytic bacteria or pathogenic bacteria of feeble invasive powers 

 may be able to grow in diseased tissues, such as gangrenous areas, and 

 may assist in effecting morbid changes, producing toxic products of 

 decomposition, which when absorbed into the body, give rise to a series 

 of toxic phenomena, such as fever, rapid pulse, malaise, etc. This con- 

 dition is known as sapremia, a term that has also been applied to the 

 decomposition of relatively sterile organic material and absorption of 



