56 



GENERAL BACTERIOLOGY 



b. Also make a "blank" plate from an uninoculated gelatin tube, 

 observing all precautions to prevent contamination. This will serve 

 as a control or check on your other plates. If any colonies develop 

 on this it will indicate carelessness. 



EXERCISE 31. AGAR PLATE CULTURES. 



General Directions. These are made in the same way as the 

 gelatin plates except that the high melting point (96° C.) of agar 

 makes it necessary to use boiling water to melt it. Inasmuch as the 

 vitality of vegetative bacteria is destroyed at a temperature much 

 above 42° C. it must be cooled down before it is inoculated, but as 

 agar solidifies at 39-40° C. it must not, therefore, be cooled below 

 that point. It is best to keep the melted agar at about 45° C. for 10 

 minutes before it is inoculated. For this purpose a water-bath 

 should be so arranged that the temperature can be controlled by 

 means of a thermo-regulator. A cheap and 

 yet satisfactory arrangement is represented 

 in Fig. 17. Inoculate, make dilutions and 

 pour as in case of gelatin, except that be- 

 fore the agar is poured, it is well to slightly 

 warm the Petri dishes by placing them in 

 the incubator at 38° C. for a few minutes, 

 otherwise the agar may solidify in lumps in 

 the plate. In cooling, agar shrinks some- 

 what, and in doing so water is expressed 

 from the solid jelly. In the incubator this 

 condenses on the under side of the cover of 

 Fm. 17. Wat^«-imth for cool- the Petri dish to such an extent that drops 

 run down on to the culture surface, thus 

 causing the developing superficial colonies to "run." To obviate 

 this the Petri dishes, when placed in the incubator, should be in- 

 verted 



References. A. 135 ; H. 68 ; M. & R. 57 ; N. 285 ; P. 225 ; P. B. C. 

 28. 



Special Directions, a. Make three agar plates of B. coli; use 

 one loop of bouillon culture (13) for tube No. 1 and proceed as in 30. 

 h. Invert and place in incubator at 28° C. 



EXERCISE 32. ROLL CULTURES (Esmarch). 



General Directions. These are essentially plate cultures in 

 which the medium instead of being poured out into dishes is solidi- 



