34 



BACTERIOLOGY FOR NURSES 



The agar is added to the broth and dissolved by boiling the mix- 

 ture for thirty to forty-five minutes. Loss by evaporation is made 

 up by the addition of water, the reaction is adjusted, and the media 

 cleared, heated, filtered, tubed, and sterilized in the autoclave. 

 Agar medium serves a great variety of purposes ; it is perhaps the 

 most frequently used of all media. 



6. Potato Medium. Large potatoes are chosen, thoroughly 

 scrubbed with a brush and then peeled, after which they are kept 



under running water to prevent discoloration. 

 Cylindrical pieces are removed by means of a 

 large apple corer, and the cylinders in turn are 

 cut in half diagonally. 



The reaction of the potato is normally acid. 

 This is corrected by leaving the pieces over- 

 night in running water or by placing them in a 

 k I' 1 per cent solution of sodium carbonate for half 

 ML an hour. Each cylinder is placed in a large test 

 155^ tube with the slant surface uppermost (Fig. 10). 

 About one c.c. of water may be added to retard 

 drying and the tubes sterilized by the fractional 

 method. Potato is generally chosen as a medium when pigment 

 production is to be studied. 



7. Glycerin Potato is prepared by covering the potato slices 

 in the tube with a 6 per cent solution of glycerin in water and 

 steaming in the Arnold Sterilizer for half an hour. The glycerin 

 is then poured off and the tubes are sterilized for another half 

 hour. Glycerin potato is sometimes used for the cultivation of 

 the tubercle bacillus. 



8. Peptone Water. 



FIG. 10. Potato 

 Tube. 



Water . . . 

 Peptone . . 

 Sodium chloride 



100 c.c. 

 2 c.c. 

 0.5 c.c. 



The peptone and salt are dissolved in the water by heating. 

 As the fluid is generally alkaline it does not need adjusting 

 unless required for special purposes; it may be filtered, tubed, 

 and sterilized at once. Peptone water is used to test for the 



