SURFACE PLATES 



2 3 I 



various conditions, but when the main object of the 

 separation of the bacteria is to obtain subcultivations 

 from a number of individual bacteria, "surface" plates 

 must be prepared, since here colony formation is 

 restricted to the surface of the medium. The method 

 adopted varies slightly according to whether the 

 medium employed is gelatine or agar, or one of the de- 

 rivatives or variants of the latter. 



(a) Gelatine Surface Plates. 



1. Liquefy three tubes of nutrient gelatine. 



2. Pour each tube into a separate Petri dish and al- 

 low it to solidify. Then turn each plate and its cover 

 upside down. 



3. When quite cold . 

 raise the bottom of plate 



i , revert it and deposit a 

 drop of the inoculum 



/ , ,1 n 1 i, FIG. 126. Surface plate spreader. 



(whether a fluid culture 



or an emulsion from solid culture) upon the surface 

 of the gelatine with a platinum loop close to one side 

 of the plate ; replace the bottom half of the Petri dish 

 in its cover. 



4. Take a piece of thin glass rod, stout platinum wire 

 or best of all a piece of aluminium wire (say 2 mm. 

 diameter) about 28 cm. long. Bend the terminal 4 cm. 

 at right angles to the remainder, making an L-shaped 

 rod (Fig. 126). Sterilise the short arm and adjacent 

 portion of the long arm, in the Bunsen flame, and allow 

 it to cool. 



5. Now raise the bottom of the Petri dish in the left 

 hand, leaving the cover on the laboratory bench, and 

 holding it vertically, smear the drop of inoculum all 

 over the surface of the gelatine with the short arm of 

 the spreader by a rotatory motion, (Fig. 127). Replace 

 the dish in its cover. 



6. Raise the bottom of plate 2 and rub the infected 



