MEDIA MAKING AND APPARATUS 



2 73 



preparing media generally accepted are given on p. 263. The chief 

 uses to which they are put are described below. 



Plain nutrient agar — remains solid at 37 C. — 40 C. ; melts at 

 ioo° C: solidifies again about 40 C. Used for plating (in 

 10 c.c. quantities) and for characteristics of growth (in 

 5 c.c. quantities; tubes are slanted while cooling to secure a 

 wide long surface of jelly). The sizes, shapes and staining of 

 bacteria after growth on this medium for 24 hours are taken 

 as the standards in describing their morphology. Chromogenic 

 and fluorescent bacteria show their pigments well on this 

 medium. For plates to be incubated at 37 C. porous tops 

 should be used, to lessen " spreading." 



Fig. 68. 



Hardening Agar Slants. (After Conn.) 



Plain nutrient gelatin — if well made, remains solid at 24 — 25 ° C. ; 

 melts just above this point; solidifies on cooling to 24 — 25 u C. 

 Used for plating (in 10 c.c. quantities) and for characteristics 

 of growth (in 5 c.c. quantities; tubes are vertical while cooling, 

 and inoculations are made by thrusting a straight needle — not 

 a looped needle — half way to the bottom of the jelly along the 

 central axis of the tube — thus making a "stab"). Many 

 bacteria liquefy gelatin, through the action of a soluble di- 

 gestive ferment (or enzyme) secreted by them; many do not 

 liquefy it, hence a point of distinction between species is 

 yielded; the shape, size and rapidity of development of the 



