MECHANICAL METHODS 79 



(ii) Using agar as the culture medium. 



Consequently, agar plates are sometimes used, especially when pathogenic 

 micro-organisms are being investigated. The technique is essentially the 

 same as in the case of gelatin plates, but the following points must receive 

 attention. 



1. Agar only melts between 90 and 100 C. and does not set again until 

 it cools to 40 C. The agar tubes will therefore have to be melted in boiling 

 water and then allowed to cool until they can be comfortably held in the 

 hand. 



2. The tubes must be sown as above, but the experiment must be done 

 quickly otherwise the agar will begin to solidify and the plates will be 

 lumpy. 



It is a good plan to have the Petri dishes standing on a levelling apparatus filled 

 with water at 40-45 C. (see below) before pouring the agar, and to cool the plates 

 slowly in order to prevent the formation of lumps at the time of cooling in the 

 dishes. 



3. Incubate the plates at 37 C. The plates should be packed into a 

 large glass dish containing a few pieces of filter paper soared in water [or 

 perchloride solution] to prevent the medium drying up. 



Agar gelatin may be used for cultures which are to be incubated between 

 25 and 35 C. 



This agar plate method never gives very good results, and when agar has to be 

 used for isolating organisms it is much better to employ surface cultures (vide 

 infra). 



B. Koch's plates. 



The use of Koch's plates constitutes an ingenious method of isolating 

 micro-organisms, but the technique is complicated and difficult to carry 

 out under strictly aseptic conditions for the following reasons : 



1. The plates must necessarily be exposed to the air for a few seconds while 

 being manipulated, and so are liable to contamination ; but if they be prepared 

 quickly and in a still atmosphere, with no dust blowing about, this exposure is 

 not of much moment. 



2. In examining the plates, it is also necessary to lift the cover of the moist 

 chamber and so again expose the medium to contamination from the air ; the 

 experiment is thus open to error. 



The technique of the method is as follows : 



Apparatus required. 1. Three glass plates (9 x 12 cm.) each wrapped up separately 

 in paper and sterilized in the hot air sterilizer (a number of these plates should 



FIG. 70. Glass support for plate cultures. 



always be at hand ready for use). [As in the case of Petri dishes, some bacteri- 

 ologists prefer to sterilize the plates in metal cases.] 



2. Three glass supports on which to rest the plates (fig. 70). 



3. Two large circular glass dishes, each about 20 cm. in diameter but one rather 

 larger than the other so that they can be fitted together to form a box. 



4. A cooling table consisting of a flat metal box the top of which is well polished 



