PATHOGENIC BACTKRIA. 201 



narily encountered in the throat, a property which to a cer- 

 tain extent sifts it out, as it were, from them, and makes its 

 recognition with the microscope easy in most cases. The 

 growth, furthermore, is quite characteristic, and its nature 

 can be predicted with considerable accuracy, even without 

 microscopical examination, by one who has had much practice. 

 Colonies of streptococci frequently look very hke those of 

 the bacillus of diphtheria, but those two are easily distin- 

 guished from each other with the microscope. The diagnosis 

 of the diphtheria bacillus, then, is made from the character of 

 the growth upon the blood-serum and the microscopical exami- 

 nation, taking into account the size and shape of the bacilli, 

 with the frequent occurrence of irregular forms and the pe- 

 cuKar irregularities in staining. In doubtful cases a sec- 

 ond culture should be made from the throat. 



The very large number of examinations that have been made 

 by various boards of health have shown that the diphtheria 

 bacillus may persist in the throat for a long time — occasionally 

 several weeks after the patient has apparently recovered ; also 

 that diphtheria bacilli are occasionally found in the throat 

 when there is an inflammatory condition without any pseudo- 

 membrane, and that they sometimes appear in an apparent 

 healthy throat, especially in children who have been associ- 

 ated with cases of diphtheria. It has been found that bacilli 

 sometimes occur in the throat which have all the morphologi- 

 cal and cultural properties of the diphtheria bacillus, but which 

 are devoid of virulence when tested upon animals. Such 

 diphtheria bacilli have frequently been called pseudodiphtheria 

 bacilli. A bacillus closely resembling the diphtheria bacillus, 

 but without virulence, has been found in xerosis of the con- 

 junctiva. It is called the xerosis bacillus. If not a transformed 

 diphtheria bacillus, it is at least closely related. The diph- 

 theria bacillus is subject to wide variations in morphology, 

 so that, in dealing with unknown cultures where the forms are 

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