DIPHTHERIA. 335 



hence forms large colonies which have a very characteristic appear- 

 ance. 



The diphtheria colonies, as they appear on serum within 24 

 hours, are relatively large, round, in outline, grayish and moist 

 in appearance. The center is thicker and more opaque than the 

 outer zone which is surrounded by a slightly wavy border. Cover- 

 glass preparations made from these colonies and stained with 

 Loffler's methylene blue, will reveal the characteristic diphtheria 

 bacillus. Care must be taken not to over-stain the specimen which, 

 moreover, should be examined with the oil immersion objective. 

 The presence of irregular, club-shaped or swollen rods, among other- 

 wise normal bacteria, is an important mark of recognition. More- 

 over, many of these rods will show irregularities in staining. Thus, 

 transverse, alternately dark and light bands may be seen. In many 

 cells, one or more bright blue, deeply stained, roundish bodies will be 

 observed. These characteristics are, as a rule, sufficient to establish 

 the identity of the organism in question. It is well to ascertain the 

 kind of organisms which are associated with the Loffler bacillus. 



In doubtful cases, Gram's stain and cultivation on 

 agar at low temperature may be resorted to. The subcu- 

 taneous injection of a guinea-pig with ^ c.c. of a 24-48 hour 

 bouillon culture will serve to distinguish virulent from non- 

 virulent forms, and from the pseudo-diphtheria bacillus. 



The pseudo-diphtheria or xerosis bacilli are found in the 

 mouth, nose, and on the conjunctiva of healthy persons, 

 and consequently may be met with in various throat and 

 lung affections. They grow better than the Lsffler bacillus 

 on agar and impart to it a dark color. One species of this 

 group forms spores. The absence of pathogenic properties 

 is an important distinction from the diphtheria bacillus. 

 It must be borne in mind, however, that attenuated diph- 

 theria bacilli may be met with. Indeed, Roux and his co- 

 workers even at the present time, regard this organism as 

 a modified, weakened diphtheria bacillus. An important, 

 though not absolute distinction, is the change in the reac- 

 tion of litmus-colored bouillon cultures. The diphtheria 

 bacillus promptly produces an acid reaction, whereas the 

 pseudo-diphtheria bacillus does not. Again, the diphtheria, 

 like the tubercle bacillus, does not readily form a homo- 

 geneous suspension when touched into a drop of water, 

 whereas the pseudo -forms do. 



