14 VIRULENCE OF BACTERIA 



and (2) the virulence of the infecting bacterium. A third the 

 number of bacteria which gain access is also of importance, 

 especially under experimental conditions, for it is found that, 

 within limits, lack of virulence can be compensated for by an 

 increase in the dose given. It is, however, one which we can 

 rarely estimate in natural disease ; besides which the growth of 

 bacteria is so rapid that, if not checked by the resisting power of 

 the body, a single organism would multiply in a very few hours 

 to an enormous extent, and render it a matter of but little impor- 

 tance whether one or a hundred bacteria had gained access at 

 first. The number of bacteria is probably of more importance in 

 connection with the occurrence or non- occurrence of infection, 

 rather of the severity of the disease when once infection has 

 occurred. Thus we find in epidemics of typhoid fever due to 

 water or milk that the disease is most prevalent in those who 

 take a large amount of the infective material, but it is not neces- 

 sarily more severe in them than in the patients who have appa- 

 rently become infected with a small dose. This is, however, not 

 the case with artificial infection of animals, for there the severity 

 of the disease (in animals as similar as possible in age, weight, 

 etc.) is fairly proportional to the dose given. But the conditions 

 are somewhat different in the two cases, and in the artificial injec- 

 tion of animals we eliminate altogether the steps by which, e.g., 

 the typhoid bacillus passes the natural barriers, and gains access 

 to the tissues. 



The question of virulence is of much greater importance, and 

 is one which must be more fully discussed subsequently, after we 

 have seen the methods in which the host immunizes itself against 

 the bacterium. Some general points must be mentioned here. 



Cultures of the same organism, identical in all respects in 

 morphological, cultural, and chemical characters, may differ 

 enormously in this respect : thus a culture of streptococci may be 

 entirely devoid of virulence to rabbits, or may be so potent that a 

 minimal dose, containing probably but a single coccus or short 

 chain, may be inevitably fatal. Similar facts hold for pneumo- 

 cocci. According to Eyre, a virulent culture may kill when 20 to 

 200 cocci are injected, whereas an avirulent one may fail to do 

 so in massive doses. In most organisms there is, perhaps, not 

 such a marked difference, but all pathogenic bacteria vary greatly 

 in this respect, and cultures from different sources show marked 

 variations in pathogenicity. 



