88 LABORATORY GUIDE IN BACTERIOLOGY 



a) Small quantities of the serum are diluted with 

 sterile salt solution (0.85 per cent.) so as to represent 

 dilutions of 1:5, 1:25, and 1:50. 



b) A suspension of a 24-hour-old agar-culture of B. 

 typhosus in salt solution is prepared. This suspension 

 should be faintly turbid and uniformly so. 



c) Three hanging-drop preparations are made by 

 mixing a loopful of this suspension, with a loopful of the 

 three serum dilutions, respectively. The final dilutions 

 then are: 1:10, 1:50, and 1:100. 



d) Examine with the high power (dry lens), and 

 observe the clumping of the bacilli, preceded by the loss 

 of motihty. 



e) Tabulate the results as to time and completeness 

 of reaction. 



/) Make a control hanging-drop without serum to 

 test the motility and the absence of clumps. 



Blood may also be obtained by puncturing the lobe 

 of the ear and collecting it in a capillary glass tube with 

 a small bulb. By holding the bulb down, fill three- 

 fourths full with blood, and seal the ends in the flame. 

 In 45 minutes the serum will have separated, and is 

 to be tested in the above manner. 



The above-described method of agglutination test is 

 known as the microscopic test. Another method, in 

 which larger amounts of serum and suspension' are 

 required, is known as the macroscopic method. Small 

 test-tubes are used, and definite amounts of bacterial sus- 

 pensions are introduced by means of finely graduated 

 sterile pipettes. The serum is then added in varying 

 amounts so as to effect the desired dilutions (see table, p. 

 117). The tubes are then incubated at 37°, usually for 



