190 



BACTERIA PATHOGENIC TO MAN 



FIG. 82 



probably have been obtained for a much longer period. On slate- and 

 lead-pencils, as well as on paper money, they may live for several weeks, 

 while on coins they die in twelve to thirty-six hours. In culture media, 

 when kept at the blood heat, they usually die after a few weeks; but 

 under certain conditions, as when sealed in tubes and protected from 

 heat and light, they retain their virulence for years. The bacillus is 

 not sensitive to cold, for we found it to retain its virulence after exposure 

 for two hours to several hundred degrees below zero. 



Growth on Culture Media. BLOOD SERUM, especially coagulated in the 

 form of Loeffler's mixture, is the most favorable medium for the growth 

 of the diphtheria bacillus, and is used particularly for diagnostic pur- 

 poses in examining cultures from the throats of persons suspected of 

 having diphtheria. For its' preparation, see p. 51. If we examine 

 the growth of the diphtheria bacillus in pure culture on blood serum 

 we shall find at the end of from eight to twelve hours small colonies 



of bacilli, which appear as pearl- 

 gray, whitish-gray, or, more rarely, 

 yellowish-gray, slightly raised points. 

 The colonies when separated from 

 each other may increase in forty- 

 eight hours so that the diameter 

 may be one-eighth of an inch. The 

 borders are usually somewhat un- 

 even. The colonies lying together 

 become confluent and fuse into one 

 mass when the serum is moist. 

 During the first twelve hours the 

 colonies of the diphtheria bacilli 

 are about equal in size to those of 

 the other pathogenic bacteria which 

 colonies of diphtheria bacim. x 200 diameters. ar e often present in the throat; but 



after this time the diphtheria col- 

 onies become larger than those of the streptococci and smaller than 

 those of the staphylococci. The diphtheria bacilli in their growth never 

 liquefy the blood serum. 



GROWTH ON AGAR. On 1 per cent, slightly alkaline, plain nutrient 

 or glycerin-agar the growth of the diphtheria bacillus is less certain 

 and luxuriant than upon blood serum; but the appearance of the colo- 

 nies when examined under a low-power lens, though very variable, 

 is often far more characteristic. (See Fig. 82, and Fig. 44, page 62.) 

 For this reason nutrient agar in Petri dishes is used to obtain diph- 

 theria bacilli in pure culture. The diphtheria bacillus obtained from 

 cultures which have developed for some time on culture media grows 

 well, or fairly well, on suitable nutrient agar, but when fresh from 

 pseudomembranes one prevalent type of bacilli grows on these media 

 with great difficulty, and the colonies develop so slowly as to be 

 frequently covered up by the more luxuriant growth of other bacteria 

 when present, or fail to develop at all. 



