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VIRULENCE 255 



in the false membranes : thus a false membrane may be dried and exposed 

 to a temperature of 95-100 C. for an hour, and yet the bacilli may be found 

 to have escaped destruction when cultures are sown. 



Drying has but little effect on the vitality of the diphtheria bacillus if 

 albuminous matter be present : thus Roux and Yersin dried a piece of mem- 

 brane and kept it away from the light at room temperature ; portions of 

 it sown 3 months or 5 months later yielded cultures of the diphtheria bacillus. 

 Bacilli from serum cultures are more easily destroyed by drying especially 

 if they be rapidly dried. Light has considerable bactericidal properties : 

 thus if a piece of false membrane be dried and then exposed to air and sun- 

 light the organisms are destroyed in a relatively short space of time. Roux 

 and Yersin, for instance, found no living bacilli in a piece of membrane which 

 had been dried and then exposed to air and sunlight for a period of 6 weeks. 

 A culture on serum which had been dried and spread in a thin layer was found 

 to be sterile after an exposure of 24 hours to diffused light (Ledoux-Lebard). 



Antiseptics rapidly sterilize cultures of the diphtheria bacillus : 1 per cent, 

 carbolic acid, 2 per cent, bichromate of potassium etc. kill cultures instantly. 



If silk threads be dipped in a culture of the diphtheria bacillus and dried, 

 it will be found that the organisms on the dried threads are more resistant 

 to the action of antiseptics and can withstand the action of 1 per cent, carbolic 

 acid, 5 per cent, salicylic acid in alcohol, etc. for several minutes (Chantemesse 

 and Widal). The resistance of the bacilli in false membranes to antiseptics 

 is even greater. 



Virulence. 



The virulence of a given diphtheria bacillus must be tested as follows : 



Sow the organism in broth, incubate at 37 C. for 24 hours, inoculate 1 c.c. 

 beneath the skin of a guinea-pig weighing 400-500 grams. One of several 

 results may follow : 



(a) In the case of a very virulent bacillus death will occur in 24^30 hours. 



(/3) In the case of a bacillus of intermediate virulence the animal will 

 succumb in 2-6 days. 



(y) In the case of a slightly virulent organism death may not take place 

 for 8-10 days. 



(5) In the case of a bacillus of very low virulence the animal will survive 

 but a local oedema followed by a slough will form at the site of inoculation. 



(e) Finally, should the bacillus be avirulent no lesion whatever will follow 

 the inoculation. 



[It will be necessary, as pointed out above, to make certain that the 

 bacillus used for inoculation is growing well in broth.] 



The virulence of the organisms isolated from false membranes is very 

 inconstant : in severe cases, virulent bacilli are very numerous : in mild cases 

 in addition to colonies of virulent bacilli, numerous colonies of non-virulent 

 organisms will be found. 1 



[The pathogenicity of different strains of the diphtheria bacillus when first 

 isolated, as tested with two-day broth cultures, varies greatly (minimal 

 lethal dose varies as 400 to 1) ; and the virulence of washed bacilli from 

 two-day broth cultures of different strains varies at least as much as the 

 pathogenicity of whole cultures (Arkwright).] 



(a) Attenuation of virulence. In old cultures the diphtheria bacillus loses 

 much of its virulence but the latter can be fully recovered by sowing the 



[ l It is not justifiable to assume that because a diphtheria bacillus is non-pathogenic 

 to laboratory animals that it is therefore non -pathogenic to man : on the other hand 

 there, is some evidence to show that " non- virulent " diphtheria bacilli are capable of 

 producing diphtheria in a susceptible human subject.] 



