Pathogenic Bacteria 337 



laryiix, and trachea in human cases. The cause of diphtheria in 

 man, but also pathogenic for guinea pigs, kittens, and rabbits. 



Diphtheria is potentially one of the most dangerous o£ the 

 childhood diseases. It begins as a sore throat and the organisms 

 tend to remain localized while the powerful exotoxin is carried 

 through the bloodstream to the peripheral nervous system and to 

 the heart, kidneys, and adrenals. Because the poison is a true 

 toxin, effective antitoxins are available to combat the toxin. The 

 diphtheria organisms themselves, however, are not affected by in- 

 jections of antitoxins. Active immunity can be provided by the 

 injection of toxoid. 



This disease is spread chiefly through the agency of droplets 

 expelled from the mouth and nose of carriers and active cases. 

 About 1% of the population is estimated to harbor diphtheria 

 organisms in their respiratory tract without showing any clinical 

 symptoms of the disease. By the time that persons reach the age 

 of fifteen, most of them have had subclinical attacks of this disease, 

 and measurable amounts of antitoxin in their blood circulation can 

 be demonstrated by employing the Schick test. This test is con- 

 ducted by injecting a minute amount of diphtheria toxin intra- 

 dermally (into the skin) on the foreaiTn. If the person has 

 sufficient antitoxin circulating in his system to neutralize this in- 

 jected poison, no marked reaction at the site will be apparent. In 

 the absence of such circulating antibody, however, the injected 

 toxin will produce a reddened, swollen, inflammatory condition 

 known as a Schick positive reaction. During impending epidemics 

 all Schick positive persons should be immunized. 



Different strains of Corynebacteriwn diphtheriae have been 

 foimd to exhibit degrees of virulence, and a classification scheme 

 is based upon these findings, coupled with differences in cultural 

 and physiological reactions. The three groups of organisms are 

 designated gravis (most severe), iiiitis (least severe), and iiiter- 

 mediiis (those in between the most and the least virulent strains). 

 A number of diphtheria-like saprophytic organisms closely resemble 

 pathogenic corynebacteria both morphologically and biochemically, 

 but they are non-virulent when subjected to the usual guinea 



