64 BACTERIOLOGY FOR NURSES 



the tube and produce air bubbles. Three loopfuls of this mixture 

 are transferred to a second tube and the mixing process repeated ; 

 five loopfuls from the second tube are carried to a third and mixed. 

 The necks of the tubes are then flamed and the contents of each 

 poured into a Petri dish previously marked with the number one, 

 two, or three according to the dilution it is to receive. In pouring 

 the inoculated medium into the Petri dish the cover should be 

 raised along one margin just high enough to permit the entrance 



of the neck of the tube, care 

 being taken that the sides of 

 the tube do not touch either 

 the top or bottom part of the 

 dish (Fig. 18). The cover is 

 immediately replaced and the 



FIG. 18. Method of pouring Media ,. , ., , , .. 



into a Petri Dish. dish gently rotated if necessary 



to insure the even distribution 

 of the medium over the bottom of the dish before it solidifies. 



It is advisable to make a stained film preparation of the material 

 to be examined before plating as above, in order to have some idea 

 of the number of organisms present. If they are relatively few, 

 more material should be inoculated into the first tube ; if numerous, 

 then a further dilution will be necessary. 



When agar has been used for plating, the Petri dishes are inverted 

 as soon as the medium has solidified and placed in the incubator 

 at a temperature of 37 C. ; gelatin plates are kept in a dark place 

 at room temperature. Colonies will develop in from one to three 

 days according to the species, and in the second or third dilution 

 they will usually be found sufficiently far apart to be " fished." 



Surface Streaking. When it is desired to isolate bacteria 

 which are particularly sensitive to their surroundings, such as the 

 gonococcus, and which require special medium, surface streaking 

 instead of the dilution method is employed. The technique is as 

 follows : Pour plates are made of suitable medium and when hard- 

 ened the material containing the organisms to be isolated is smeared 

 over the surface of several plates in succession. If the organisms 

 are removed from the nose or throat by means of a swab, the swab 



