BACILLUS DIPHTHERIA 583 



the human body upon the simple meat-extract media. On agar plates 

 its colonies are larger, less transparent, and whiter than are those of 

 true diphtheria bacilli. In fluid media there is even clouding and less 

 tendency to the formation of a pellicle than with B. diphtherise. A 

 positive means of distinction between the two is given by the inability 

 of B. Hoffmanni to form acid upon various sugar media. The differ- 

 entiation on a basis of acid formation was first attempted by Cobbett 44 

 and has been recently worked out systematically by Knapp, 45 and con- 



FIG. 61. BACILLUS HOFFMANNI. FIG. 62. COLONIES OF BACILLI 



HOFFMANNI ON AGAR. 



firmed by various observers. 46 The results of this work, carried out 

 with the serum-water media of Hiss, to which various sugars were added, 

 show that B. Hoffmanni forms acid upon none of the sugars used, while 

 B. diphtheria acidifies and coagulates media containing monosaccharids 

 and several of the more complex sugars, as given in the diagram in the 

 section following, dealing with B. xerosis. 



Differentiation can finally be made on the basis of animal patho- 

 genicity, B. Hoffmanni being entirely innocuous to the ordinary labora- 



44 Cobbett, Cent, f . Bakt., 1898. 



^Kuapp, Jour. Med. Kes., vii, 1904. 



46 Graham Smith, Jour, of Hyg., vi, 1906; Zinsser, Jour, Med, Res., xvii, 1907. 



