BIOLOGICAL METHODS 83 



4. Incubate the plates at 37 C. 



Chantemesse adopts this method in isolating the typhoid bacillus, using a special 

 medium instead of agar (Chap. XXI.). 



[4. Burri's method. The aim of the method is to grow a colony from a 

 single organism. A dilute emulsion of the organism is made and further 

 diluted in an emulsion of indian ink ; small drops of the latter are then laid 

 on the surface of gelatin, covered with a cover-glass and examined under the 

 microscope. Those cover-glasses which cover only a single organism are then 

 transferred to other media and incubated. 



[Apparatus required. 1. A sterile emulsion in water of commercial indian 

 ink, 1 or better a 1 in 9 emulsion in water of a colloidal compound known as 

 Pelikan Tusche No. 54. 2 



2. A number of sterile Petri dishes, slides and cover-glasses. 



3. Half a dozen ready poured gelatin plates in large Petri dishes. 



4. Sterile dissecting forceps. 



5. Two or three fine drawing pens also sterilized. 



[Technique. 1. Prepare a dilute and homogenous emulsion in normal 

 saline solution of the culture or material under examination. 



2. Place a sterile slide in one of the Petri dishes ; with a small platinum 

 loop put four drops of the indian ink emulsion in a row on the slide and 

 replace the cover of the dish. 



3. With a straight platinum wire transfer a small drop of the bacterial 

 emulsion to the left-hand drop (No. 1) of the indian ink on the slide and mix 

 intimately. Transfer similarly a small drop from No. 1 to No. 2 and mix ; 

 from No. 2 to No. 3 and so on. 



4. With one of the sterile drawing pens take up the right-hand drop of 

 bacterial-indian ink emulsion and lay it in a series of very minute droplets 

 on the surface of one of the gelatin plates. Cover each drop separately with 

 a sterile cover-glass. 



5. Disseminate similarly drop No. 3 on another plate and cover as before. 



6. Examine the droplets under the microscope using a dry lens and an 

 high eyepiece. If necessary an oil immersion lens may be used ; in that 

 case place a drop of oil on the upper surface of the cover-glass. The 

 organisms will be seen as bright objects on a dark background. 



7. When a droplet is found in which only a single organism is suspended, 

 raise the cover-glass with a pair of sterile forceps the indian ink and 

 organism will be found to adhere to the cover-glass and transfer it to 

 another plate of gelatin or some other suitable medium laying the cover- 

 glass drop side downwards. Incubate.] 



SECTION n. BIOLOGICAL METHODS. 



The methods now to be described are only available when the detection 

 and isolation of a given organism is in question, and depend upon a know- 

 ledge of one or more properties of the organism ; this knowledge is applied 

 to facilitate the growth of that organism while at the same time hindering 

 the growth of any others which may be present. 



The separation of anaerobic from aerobic organisms may be quoted as 

 an example of the principles involved. Aerobic organisms cannot grow in 

 the absence of free oxygen ; so that by sowing the material in an atmosphere 

 free from oxygen cultures of anaerobic organisms alone are obtained. 



The methods most generally in use will be described. 



1 Giinther, Vienna. 2 Grtibler, Leipzig. 



