PREPARATION OF BACTERIAL EMULSION 



205 



centrif ugalized. The washing with normal saline solution is repeated once 

 or twice and as much of the clear liquid as possible is finally removed; the 

 remaining cells are thoroughly mixed and in this form are ready for use. 

 The bacterial emulsions with the exception of the tubercle bacillus are 

 made from agar cultures ; the growths of gram + cocci may be as old as 

 twenty-four hours, while the coliform organisms and 

 the gram cocci are preferably used only four to ten 

 hours old (the younger the better). A loopful of cul- 

 ture from an agar tube is thoroughly rubbed up with 

 several drops of salt solution in a watch-glass by means 

 of a small glass pestle. The salt solution is best added 

 very gradually, drop by drop, in order to make a more 

 perfect emulsion. This may then be advantageously 

 centrifugalized for a short period, to bring down the 

 large clumps. The supernatant opalescent portion is 



Mar 



FIG. 24. 



taken off for use, thoroughly mixed, and if necessary diluted. Emul- 

 sions of coliform organisms are the most easily made. Frequently it is 

 sufficient to rub up with the platinum loop a loopful of such bacteria 

 on the side of a small test-tube containing saline. The proper thickness 

 of the resulting emulsion varies. As a rule, bacillary emulsions are re- 



