LAVERAN'S METHOD 



867 



for distributing the medium at the end of the experiment, is drawn out and 

 sealed in the flame. 



Thirty c.c. of water are poured into the flask and then sterilized in the 

 autoclave. To use the apparatus connect the plugged tubulure with an 

 aspirator, remove the glass cap from the central tube, and start the aspirator : 

 the air now bubbles through the water in the flask and when sufficient air 

 has been drawn through, wash out the central tube several times by blowing 

 gently through the lateral tubulure. Then break off the sealed end of the 

 other lateral tube and distribute the contents of the flask into a number 

 (30 or 40) of flasks containing broth ; these flasks are then incubated. The 

 method is not accurate because the mere dipping of the tube into the liquid 

 is insufficient to entrap all the organisms and conse- 

 quently many escape detection. 



III. Laveran's method. Method recommended. 

 Laveran employs a piece of apparatus which while < M 

 giving very accurate results, is simple in construction 

 and not easily broken. Two glass tubes closed below 

 are connected together at the junction of the middle 

 and upper thirds by an horizontal tube. Each of the 

 vertical tubes is plugged at its upper end with an india- 

 rubber stopper through which a pipette reaching to the 

 bottom of the tube is passed. One of the tubes has a 

 10 c.c. mark on the glass and one of the pipettes is 

 graduated in tenths of a cubic centimetre. The upper 

 end of each pipette is plugged with wool. 10 c.c. of a 

 1 per cent, solution of sugar in water are poured into 

 the graduated tube. The apparatus is then autoclaved. 



For use, remove the wool plug from the pipette 

 dipping into the sugar solution and connect the other 

 with an aspirator. The aspirated air bubbles through 

 the solution, passes into the first tube through the horizontal connecting 

 tube, descends in the other limb and escapes through the pipette connected 

 with the aspirator. A very large volume of air can thus be aspirated. 



When sufficient air has bubbled through gently aspirate the sugar solution 

 into the entry pipette to wash it, then run the fluid into the second tube 

 and so into the second pipette, and repeat this several times in order to collect 

 all the organisms which have been deposited on the glass. It only remains 

 now to remove the liquid by means of the graduated pipette and to distribute 

 it into different culture media (gelatin and agar plates, etc.). 



Suppose, for example, that 200 litres of air have been aspirated and twelve colonies 

 have developed on a gelatin plate sown with one cubic centimetre of the sugar 

 solution, it follows that : 



200 litres of air contain 12 x 10 aerobic organisms. 



1 cubic metre therefore contains 12 x 10 x 5 aerobic organisms. 



The advantages of the method are that it is available for large volumes of 

 air and provides plenty of material for sowing cultures : thus it fulfils the 

 requirements of special investigations for the detection of pathogenic micro- 

 organisms. 1 



1 For details reference must be made to the chapters devoted to the consideration of 

 the various pathogenic micro-organisms. For the detection of the tubercle bacillus it 

 will, of course, be necessary to inoculate animals. 



FIG. 416 

 Laveran's tube. 



