832 PATHOGENIC MICROORGANISMS 



section they are less easily stained, but may be demonstrated by 

 staining with alkaline methylene blue. 



Cultivation. The cholera spirillum grows easily upon all the 

 usual culture media, thriving upon meat-extract as well as upon 

 meat-infusion media. Moderate alkalinity of the media is prefer- 

 able, though slight acidity does not prevent growth. 



In gelatin plates growth appears at room temperature within 

 twenty-four hours as small, strongly refracting yellowish-gray, pin- 

 head colonies. As growth increases the gelatin is fluidified. Under 



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" "* / V > **- : 



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-gf^-Wr'- 



^ V /Pflt CB*^! ^ / 'A 



FIG. 87. CHOLERA SPIRILLUM. (After Frankel and Pfeiffer.) 



magnification these colonies appear coarsely granular with margins 

 irregular because of the liquefaction. Liquefaction, too, causes a 

 rapid development in such colonies of separate concentric zones of 

 varying refractive power. Old strains, artificially cultivated for 

 long periods, lose much of their liquefying power. 



In gelatin stab cultures fluidification begins at the surface, rapidly 

 giving rise to the familiar funnel-shaped excavation. 



Upon agar plates, within eighteen to twenty-four hours, grayish, 

 opalescent colonies appear, which are as a rule easily differentiated 

 by their transparency from the other bacteria apt to appear in feces. 

 Agar plates, therefore, are important in the isolation of these 

 organisms. 



