192 A MANUAL OF BACTERIOLOGY 



begin to aggregate together into clumps, and by the end of the half 

 hour there will be very few isolated bacilli visible. In less marked 

 cases the motility of the bacilli does not cease for some minutes, 

 while in the least marked ones the motility of the bacilli may never 

 be completely arrested, but they are always more or less sluggish 

 as compared with the control hanging drop made from the culture, 

 while clumping ought to be quite distinct by the end of one hour 

 (with a 1 in 30 to 1 in 50 dilution). 



The central portions of the drop should be examined, not the 

 margins. With blood which has been dried and dissolved, organisms 

 may become entangled in debris, and must not be mistaken for 

 clumps. 



In all cases two or three different dilutions should be made to exclude 

 the possibility of a " zone of no reaction " with some particular dilution 

 (see p. 188). 



Macroscopic, or sedimentation method. The serum, having been 

 diluted by means of a pipette with four times its volume of salt 

 solution, is mixed with five to twenty times its volume of culture 

 suspension containing plenty of micro-organisms in the same manner 

 as described in the previous section. The mixture is sucked up into 

 a fine, but not capillary, bore tube. This is sealed at the lower end 

 and allowed to stand in the upright position for eight to twenty-four 

 hours at 20 C., or six hours at 37 C. ; the reaction is often distinct 

 within an hour at 37 C. When the reaction is posit i ve t he organisms 

 become agglutinated, and form flocculi, which are easily seen wilh 

 the naked eye or with a hand-lens and stick to the sides or sink to 

 the bottom of the tube. The dilution usually employed is 1 in 30 

 to 1 in 50. Whole blood is not suitable for the sedimentation test ; 

 clear serum should always be used. It is well to set up at the same 

 time a control tube with saline solution, or, preferably, with normal 

 serum. 



If sufficient serum is available the mixture may be put up in 

 little test-tubes, such as the inner tubes of Durham's culture-tubes 

 (p. 83). 



B. For the Recognition of Bacterial Species 



1. Bordet-Durham reaction. This is carried out in much the 

 same manner as for clinical diagnosis, but an immune serum of 

 high agglutinating value or high " titre " (at least 1 : 1000) is 

 required, and the serum from a patient is not applicable. The 

 immune serum may be obtained from a horse or other animal 

 immunised with killed cultures (and living also if a high titre is 

 required). In the laboratory the serum may be prepared by giving 



