162 TEXT-BOOK OF BACTERIOLOGY. 



air; it liquefies the gelatin and, somewhat later, generates a special 

 pigment which is at first pink, but afterward of a deep blood-red. 



With the naked eye one sees, at first, only pale, saucer-like hol- 

 lows in the culture medium, the bottom of which are occupied by 

 whitish central masses of bacterial growth. Colonies which are 

 further advanced show clearly the full red color, and the micro- 

 scope also displays the central mass as granular and deep red, 

 while the color appears paler or dark brown toward the borders. 

 The solid gelatin does not everywhere present an equally definite 

 border dividing it from the liquefied part; it often shows a marked 

 waviness and has a collar-like edge. The great rapidity of growth 

 which the prodigiosus develops on gelatin, as well as on all other 

 artificial media, is particularly noticeable. In test-tube cultures, 

 too, a very quick and equable liquefaction of the gelatin all along 

 the puncture early makes its appearance; it soon becomes so ex- 

 tensive that it reaches the walls of the tube. The color develops 

 at first only on the surface, but gradually sinks to the bottom in 

 crumb-like portions and granules. As it is continually produced in 

 the upper part, the whole culture at last appears saturated with 

 color throughout its entire depth. 



The pigment develops particularly well on agar-agar, and on 

 the obliquely-hardened surface a massive deep-red covering forms, 

 the color of which does not penetrate into the medium. 



Blood-serum is liquefied by the prodigiosus, though less quickly 

 than gelatin, and also shows the development of color. That it 

 grows very rapidly on the potato, forming large blood-red blotches, 

 is already known. Older cultures have a peculiar play of metallic 

 color which strongly reminds us of the appearance of crystalline 

 undissolved fuchsin. 



The chief peculiarities of the prodigiosus are, therefore, the lique- 

 faction of gelatin and the production of pigment. The former is 

 caused by the action of a peculiar ferment, which dissolves glue 

 and fibrin, which can be separated from the bacteria, and the qual- 

 ities of which have recently been experimented upon by Fermi. As 

 a rule, the softening of the stiff gelatin by the prodigiosus is, as 

 already noted, very considerable. From this circumstance we may 

 take the hint to prepare at least three or more dilutions, instead 

 of the two ordinarily made, in order to get plates with well-dis- 

 tanced colonies. 



Under some circumstances, the peptonizing power of the pro- 

 digiosus may be lost to a certain extent. Thus if it be cultivated for 

 a long time in the acid solutions already mentioned and trans- 

 planted to ordinary food gelatin, the liquefaction at first displays 



