54 



GENERAL MICROBIOLOGY 



stance to be studied, might cause either plasmolysis or 

 plasmoptysis as the concentration was respectively too 

 great or too weak. 



Microorganisms of different species differ markedly 

 in their susceptibility to osmotic pressure. This cannot 

 be determined, however, unless studies are made of pure 

 cultures of each, therefore the percentage of salt in the 

 diluting liquid should approximate 

 that of the substance whose micro- 

 flora is to be studied. 



The method below is applicable 

 to substances in the liquid condi- 

 tion only. Modifications of this 

 method may be utilized to apply to 

 nearly every class of substances. 



Plates are generally made from 

 three different dilutions, so that 

 well-separated colonies may be ob- 

 tained on at least two plates. 



Apparatus. Sterile 1 c.c. pipettes 

 (graduated to 0.1 c.c.); sterile 

 10 c.c. pipettes (graduated) ; sterile 



FIG. "16. -Koch's Safety Erlenmeyer flasks of 200 c.c. ca- 

 pacity containing 90 c.c. and 99 c.c. 

 of sterile water or salt solution; 

 three sterile Petri dishes; three tubes of sterile agar or gelatin. 



Burner for Incubator or 

 Water Bath. 



Note. Use only freshly prepared dilution flasks, otherwise evapora- 

 tion takes place so rapidly that accuracy is not possible. 



Culture. Substance under investigation. 



Method. 1. With a sterile 1 c.c. pipette, transfer 1 c.c. 

 of the original sample or culture to a flask containing 99 

 c.c. of sterile water or salt solution. The flask now con- 

 tains 100 c.c. of liquid containing 1 c.c. of the original 

 sample, giving a dilution of 1 in 100. 



Note. A sterile pipette must be used for each separate operation. 



