PRIXCIPLES OF BACTERIOLOGY 



in the bactericidal properties of the blood from day to day we are not 

 surprised that they do not find the body of the same animal always 

 equally suitable. The study of bacteria in the more simple and known 

 conditions of artificial culture media has shown us how extremely 

 sensitive many bacteria are to slight chemical, and other changes. In 

 media conditions favorable to growth may still be unfavorable for toxin 

 production. 



If we take specimens of diphtheria bacilli from three different cases 

 of diphtheria, we sometimes find that on growing them for several days 

 in suitable bouillon one will have produced poison in the culture fluid 

 to such a degree that a single drop suffices to kill a large guinea-pig; 

 the second, grown in a similar manner, will kill another animal of the 

 same size with half a drop; while the third will kill with one-tenth 

 of a drop. This illustrates the important fact that different varieties 

 of the same bacillus have different toxin-producing powers under the 

 same conditions that is, that the conditions that are suitable for the 

 full development of the functions of one strain are not so for another 

 strain. 



Let us now cultivate these same strains in bouillon which is a little 

 too acid or a little too alkaline for their maximum development, and 

 we shall find that while all of them will grow, only one and probably 

 that one which produced the most toxin under favorable conditions will 

 continue to develop it, while the others will fail to produce any specific 

 poison. This illustration makes clear one reason for the variation in 

 severity among different cases in an epidemic, since the conditions in 

 one throat may favor growth but not toxin production, while in another 

 throat both are favored. The fact that growth of bacteria may occur in 

 the body and yet no specific poison be produced, and that, of the same 

 species of bacteria, some varieties are capable of producing toxin under 

 less favorable circumstances than others, is very important to remember. 



The cultivation of the tetanus bacillus also furnishes some interesting 

 facts which illustrate the complicated ways in which the growth of 

 varieties of bacteria are hindered or assisted. The tetanus bacillus, 

 when placed in suitable media, will not grow except in the absence of 

 oxygen; but place it under the same conditions, together with a micro- 

 organism which actively assimilates oxygen, and the two in association 

 will grow in the presence of air. As a rule, when tetanus bacilli are 

 driven into the flesh by a dirty nail or blank cartridge plug/ aerobic 

 bacteria are driven in also and so help to further infection. 



The influenza bacillus is a striking example of the special require- 

 ments of certain bacteria. On culture media it will thrive only in the 

 presence of haemoglobin. 



It is evident, therefore, that for each variety of organism there are 

 special conditions requisite for growth, and that a temperature, degree 

 of acidity, kind of food, supply of oxygen, etc., suitable for one may be 

 utterly unsuitable for another; that, 'still further, when two organisms 

 grow together one may so alter some of these conditions as to render 

 unsuitable ones suitable, and vice versa. 



