ABILITY OF BACTERIA TO PRODUCE DISEASE 99 



to attenuate cultures in the preparation of vaccines to be used 

 for purposes of active immunization. 



Ordinarily, the passage of an organism through an animal 

 increases its pathogenicity only for that particular species of 

 animal. Thus the passage of certain bacteria through a guinea 

 pig increases their virulence for guinea pigs and not for rabbits 

 nor rats. A method of increasing the virulence of a given culture 

 is to inclose it in a collodion capsule of suitable thickness and 

 place the capsule within the abdominal cavity of the chosen ani- 

 mal. The body fluids are able to transfuse through the sac, de- 

 stroying such bacteria as are unable to withstand their injurious 

 influence. In this way only the strongest survive and a race 

 of more virulent organisms results. 



An exception occurs in the passage of the smallpox virus through 

 the calf, where it loses forever its power of producing smallpox. 



Certain bacteria increase their resistance against the body defen- 

 sive forces, and consequently their virulence by the formation of 

 a capsule. The capsule may be quickly lost when the organism 

 is grown on artificial culture medium, and its virulence correspond- 

 ingly lowered. Repeated passage through animals will again re- 

 store it and produce a race of capsulated virulent bacteria. 



It is thought by some authorities that bacteria actively secrete 

 substances that are able to paralyze the protective forces of the 

 body, especially the leucocytes. Little is known of them or their 

 action. Their existence, nevertheless, may explain the statement 

 made by Metchnikoff, that a virulent microorganism is not so 

 readily taken up by the leucocytes as a non-virulent one. Thus 

 it would seem that in an infection a tremendous struggle is carried 

 on between the bacteria and the body cells, each side provoked 

 by the other to manufacture forces that will either attack the 

 enemy or protect itself against counter attacks. 



Mixed and Secondary Infections. Several different microor- 

 ganisms may invade the tissues at the same time and produce a 

 mixed infection, or one may follow another or others and give rise 

 to secondary infection. Associated organisms are often influenced 

 by the activities of each other. Thus the presence of pus-produc- 



