PREPARATION OF PLATE CULTURES 



49 



EXERCISE 12. PREPARATION OF PLATE CULTURES, 

 LOOP OR STRAIGHT-NEEDLE DILUTION METHOD 

 (QUALITATIVE) 



Plate cultures are a valuable asset to the microbiologist, 

 as they offer a means by which pure cultures of micro- 

 organisms may most easily be obtained; they also allow 

 a quantitative and qualitative 

 study of the micron 1 ora of differ- 

 ent substances. 



Their preparation consists in, 

 (1) inoculating a liquefied solid 

 culture medium with micro- 

 organisms, (2) mixing them well 

 throughout the medium, (3) 

 pouring the inoculated medium 

 into a sterile Petri dish and, 

 when it has solidified, (4) placing 

 the Petri dish or plate culture 

 at a constant temperature. 



The culture medium in so- 

 lidifying fixes in situ the micro- 

 organisms introduced, and well- 

 separated organisms develop into 

 more or less well-separated 

 " colonies " which become visible 

 to the naked eye after twenty- 

 four to forty-eight hours. From FlG - 13. -Water-bath for Melt- 

 these isolated colonies usually 

 pure cultures may then be ob- 

 tained, or a quantitative or quali- 

 tative study may be made. 



Isolated surface colonies are most frequently round 

 (concentric in growth) and generally are quite typical for 

 each species, while isolated sub-surface colonies are lenticu- 



ing Agar or Gelatin for Plat- 

 ing, containing a Removable 

 Copper Test Tube Rack. 

 (Orig. Northrup.) 



