342 MANUAL OF BACTERIOLOGY. 



of the plug that goes into the tube touches neither the finger nor 

 any other object. The principles, in fact, are the same as 

 those laid down in general for the inoculation of culture-tubes 

 with bacteria (see page 78). In board of health work these 

 tubes are returned to the office. When it is desirable, a second 

 tube may be inoculated from the swab. The tubes are placed 

 in the incubator, where they remain for from twelve to 15 hours 

 and a microscopic examination is then made of smear prep- 

 arations stained with Loffier's methylene-blue. After use the 

 tubes and swabs should be most carefully and thoroughly 

 sterilized. 



On Loffler's blood-serum kept in the incubator the bacil- 

 lus of diphtheria grows more rapidly than the other organ- 

 isms which are ordinarily encountered in the throat, a 

 property which to a certain extent sifts it out, as it were, 

 from them, and makes its recognition with the microscope 

 easy in most cases. The appearance of the bacilli under 

 the microscope is quite characteristic. Colonies of strepto- 

 cocci frequently look very like those of the bacillus of diph- 

 theria but those two are easily distinguished from each other 

 with the microscope. The diagnosis of the diphtheria bacillus 

 in practice is made from the character of the growth upon the 

 blood-serum and the microscopical examination, taking into 

 account the size and shape of the bacilli, with the frequent oc- 

 currence of irregular forms and the peculiar irregularities in 

 staining, and this usually suffices; but in doubtful cases a sec- 

 ond culture should be made from the throat, and if necessary 

 corroborates by the inoculation of a guinea-pig. 



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 pesudo- 



