80 PATHOGENIC BACTERIA 



smaller doses of such a broth culture are lethal within the same 

 time; death may be deferred for seven or even twenty-one days 

 when the virulence is low. 



The crucial test, however, as to whether a bacillus is truly a 

 diphtheritic one is not, strictly speaking, its possession in gen- 

 eral of a pathogenic property, but of one so specific that the lo- 

 cal or general action of a broth culture is inhibited by the pre- 

 vious injection of the animal with anti-diphtheritic serum. 



The true diphtheria bacilli, again, are characterized physi- 

 ologically by producing an acid reaction in a 1 per cent, glucose 

 broth. This reaction is not given, e.g., by Hof mann's bacillus 

 a form sometimes present along with that of true diphtheria, or 

 met with in cases of sore throat where none of the true forms 

 occur (see Plate). 



Lastly, it is a fact, with much practical bearing, that bacilli 

 having the morphological characters of those of diphtheria, and 

 highly virulent, as tested upon the guinea pig, may be isolated 

 from the throats of persons, who themselves exhibit no disease 

 and have not suffered from diphtheria; since such individuals, 

 whilst themselves immune, may serve as carriers of infection. 

 This has been particularly shown in the case of outbreaks of 

 diphtheria in schools. 



And, what is equally important, after a diphtheritic patient 

 has quite recovered from the disease, that is to say, after having 

 acquired an immunity from the disease by reason of having had 

 it, he may bear about diphtheria bacilli in the throat for months, 

 harmless enough to himself, but capable of conveying the dis- 

 ease to others. It becomes, hence, strictly necessary to make 

 repeated bacteriological examinations of the throat after conva- 

 lescence is established; and if the patient is to be no longer a 



