284 Dr. Edward E. Klein [Feb. 20, 



more weakened, so much so, that if this culture (premiere vaccine) be 

 injected into sheep or cattle (animals very susceptible to anthrax) 

 the effect produced is slight ; then injecting the culture which has 

 been growing only eight days at 42"^ • 5, the effect is a little more 

 pronounced, but not sufficient to endanger the life of the animal. 

 Such an animal, however, may be regarded as having passed through 

 a slight attack of anthrax, and as being now protected against a 

 second attack, however virulent the material injected. In the case 

 of swine erysipelas, Pasteur found that the microbe of this disease, 

 transmitted through several rabbits successively, yields a material 

 which is capable of producing in the pig a slight attack of swine 

 erysipelas, sufficient to protect the animal against a second attack of 

 the fatal form. Passing the anthrax virus from however virulent a 

 source through the mouse, it becomes attenuated, and is then capable 

 of producing in sheep only a mild form of disease protective against 

 the fatal disorder. Attenuation of the microbes has been brought 

 about outside the body by growing them under a variety of conditions 

 somewhat unfavourable to the microbe. 



Attenuation of the action of the anthrax microbes has been pro- 

 duced by adding to the cultures some slightly obnoxious material 

 (e.g. mercuric bichloride 1:40,000), by which the growth is some- 

 what interfered with ; or subjecting an otherwise virulent culture for 

 a short time to higher degrees of temj)erature (anthrax to 56^ C. for 

 five minutes ; fowl enteritis, twenty minutes, 55^^ C.) ; or exposing 

 them for short periods to some obnoxious chemical substance (e, g. 

 anthrax to carbolic acid, anthrax to bichloride of mercury 1 : 25,000 

 for twenty minutes) ; or the microbes are passed through, i. e. are 

 grown in the body of certain species of animals, whereby the microbes 

 become weakened as regards other species (swine erysipelas, anthrax, 

 diphtheria, and tetanus) ; finally, some microbes become attenuated 

 spontaneously, as it were, by growing them in successive generations 

 outside the animal body, e. g. the pneumonia microbe, the erysipelas 

 microbe, and others. However good the nutritive medium, these 

 microbes gradually lose their virulence as cultivation is carried on 

 from subculture to subculture ; in diphtheria the culture which was 

 virulent at first loses its virulence as the same culture becomes 

 several weeks old. 



All these facts are of considerable importance, inasmuch as they 

 enable us to understand how, in epidemics, the virulence of the 

 microbe gradually wears off and becomes ultimately nil, and because 

 they indicate the ways of attenuating microbes for the object of 

 protective inoculations. 



Another important step in the study of Bacteria was this : it was 

 shown that they have, besides their special morphological and cultural 

 characters, definite chemical characters. Specific chemical characters 

 (specific ferment actions) of Bacteria have been known for a long 

 time through the earlier researches of Pasteur — e.g. the Bacteria 

 causing the acetic acid fermentation of alcohol, the mucoid fcrmen- 



