180 BACTERIOLOGY FOR NURSES 



culture has been grown on ascitic broth a small portion of the 

 pellicle is removed on a platinum loop and lightly streaked over 

 several of the prepared plates; if instead of broth the mixed 

 organisms are removed from serum medium the portion showing 

 colonies which most resemble those of the diphtheria is chosen. 

 The plates are incubated for about sixteen hours at 37 C., after 

 which the most characteristic colonies are " fished " and trans- 

 ferred either to Loeffler's serum tubes or to the ascitic broth. 



Resistance. In cultures the bacilli will live for a long time 

 at room temperature. They may survive for two months or more 

 without transplanting. They are particularly resistant to cold 

 at temperatures just below freezing; they will remain alive for 

 weeks. In a moist condition, whether in cultures or a membrane, 

 their resistance to heat is comparatively low. Ten minutes' ex- 

 posure to a temperature of 60 C. is sufficient to destroy them. 

 On the other hand, in a dry condition they possess a much greater 

 power of endurance. In a membrane which is perfectly dry they 

 can resist a temperature of 98 C. for one hour. Vigorous toxic 

 diphtheria bacilli have been found on dried membrane four months 

 after its removal from a throat. On toys, pencils, paper money, 

 etc., they may live for several weeks. Their resistance to dis- 

 infectants is much the same as that of other non-spore-bearing 

 bacteria. They are killed in a solution of mercuric chloride 

 1 to 1000 in from one to five minutes and in carbolic 1 to 100 in 

 from five to ten minutes. 



Pathogenesis. With the exception of rats and mice most 

 of the lower animals are susceptible to the toxin of the diphtheria 

 bacillus, yet it is extremely rare that the disease appears in them. 

 In fact the cat is the only animal known to have contracted diph- 

 theria from contact with the disease. False membranes similar 

 to those produced by the diphtheria bacillus in human beings may 

 occur in animals, but only when the membrane has been first 

 abraded and then virulent organisms either rubbed on to it or in- 

 jected into it. 



Very small quantities of a virulent broth culture injected sub- 

 cutaneously will produce symptoms of toxemia in a guinea pig 



