THE DIPHTHERIA BACILLUS 179 



a degree of virulence, and that by the use of Neisser's stain the 

 virulent forms of the diphtheria bacilli could thus be distinguished 

 from the non- virulent without the delay of inoculating animals. 

 Experiments have shown, however, that the variation in form and 

 staining properties has no relation to pathogenicity and that by 

 no means at present known can the virulent strains be distin- 

 guished from the non-virulent except by animal inoculations. 

 The bacilli are non-motile and do not form spores. (Fig. 27.) 



Cultivation. Growth takes place best at body temperature. 

 Development will occur at a temperature as low as 20 C. ; below 

 that, however, it usually ceases. The organism is aerobic and 

 facultative anaerobic. When freshly isolated it grows much more 

 readily on media containing serum. The mixture of Loeffler 

 has been found to be one of the best for making cultures direct 

 from suspected throats. At the end of about twelve hours colonies 

 of the diphtheria bacilli appear as pearl-gray or occasionally yel- 

 lowish gray, slightly raised points a little larger than the colonies 

 of streptococci and a little smaller than those of the staphylococci. 



On gelatin at 22 C. a stab culture shows a beaded appearance 

 along the line of inoculation, while at the surf ace 'growth forms a 

 small disk. No liquefaction occurs. Milk is an excellent medium. 

 Growth is rapid and luxuriant ; the lactose is not fermented nor is 

 the casein coagulated. In broth a cloudiness is first produced 

 which soon settles to the bottom and along the sides of the tube as 

 a fine, powdery deposit. If the broth is inoculated on the surface 

 and the tube is allowed to remain undisturbed growth is apt to occur 

 as a fine but distinct scum upon slightly alkaline nutrient agar to 

 which has been added 1 per cent dextrose. Good growth will 

 result after one or two generations have been cultivated on serum 

 media. The appearance of the colonies on agar is peculiarly 

 characteristic and for this reason it is of value for the isolation of 

 the organism. Surface colonies appear to have a dark, coarsely 

 granular, piled-up center with a thin irregular border which some- 

 times appears jagged or torn. 



Isolation. Petri plates are first prepared by pouring into them 

 nutrient glucose agar and allowing it to solidify. If the mixed 



