MICROS IAL DISEASES OF MAN AND DOMESTIC ANIMALS 813 



The organism is detected in the following manner: A sterile swab is rubbed gently 

 over the inflamed area or against any visible membrane, care being taken to touch 

 other parts as little as possible. The swab is then immediately inserted into a tube of 

 specially prepared medium Loeffler's inspissated blood serum over the surface of 

 which it is rubbed back and forth. The swab is returned to its own tube or left 

 against the serum and the culture and swab sent to the laboratory. After twelve to 

 eighteen hours of incubation, at 37, smears are made from the cultures and stained 

 with Loefflers methylene blue. The diagnosis is made on the morphological char- 

 acters of the bacterium. Occurring in pure cultures the form of the bacterium is 

 subject to remarkable changes according to the medium and length of cultiva- 

 tion. Its size as it appears in exudates and from serum media varies from i^ to 



FIG. 170. Bacterium of diphtheria. X 1000. (After Williams.) 



7/ji in length and 0.25^ to 2/* in width. The rods are straight or slightly curved, 

 usually not uniformly cylindrical but with swellings at the ends, or in the middle, or 

 irregularly disposed (Fig. 170). Both ends may be rounded or both pointed, or one 

 rounded and the other pointed. Branching forms are not infrequently found. 

 The bacteria may appear in pairs end to end; more frequently and typically they are 

 inclined to one another at a greater or less angle and may assume a parallel arrange- 

 ment or the form of a short zigzag chain. The arrangement is most clearly under- 

 stood and remembered by considering the peculiar "snapping" type of fission 

 characteristic of the group. There are no flagella and no spores. The cell mem- 

 brane is possessed of great elasticity as evidenced by the post-fission movements. 

 The bacteria stain readily with all the aniline dyes and retain the primary stain fairly 

 well by Gram's method. With Loeffler's methylene blue they stain in a character- 

 istically irregular manner, and show metachromatic "granular" forms, "barred" 



