152 



VETERINARY BACTERIOLOGY 



or it may be made by naked-eye observation of the reaction in 

 the test-tube, or macroscopically. 



Microscopic Widal, or Agglutination Test. Dilutions of the 

 serum are generally made 1 : 10, 1 : 20, and 1 : 40, or even more. 

 To prepare a 1 : 40 dilution for a hanging drop, place 19 loops of 

 physiological salt solution, separately, upon a clean microscopic 

 slide, then add one loopfulof serum to be tested, and mix thoroughly 

 with the diluent. Place one loopful of a suspension of the organ- 

 ism (broth culture or suspension from the surface of an agar slant 

 in physiological salt solution) upon each of two clean cover-glasses ; 

 to one add one loopful of the serum dilution, to the other a loopful 

 of sterile physiological salt solution. Invert over the cavity of a 



A B 



Fig. 75. The Widal or agglutination test of the typhoid bacillus, using 

 serum from a typhoid patient: A, Check showing the uniform distribution 

 of the bacilli; B, clumps of bacteria in the positive test (Jordan). 



hollow-ground slide and examine microscopically. The check 

 should show the organisms uniformly distributed over the field, 

 and moving about actively, if the organism is motile. The organ- 

 isms in the other may show no change, but if the serum has 

 come from a patient infected with the organism, the bacteria will 

 soon begin to clump together, and in the course of a few minutes 

 to an hour practically all of the organisms will be found so 

 clumped, very few or none remaining isolated in the field. Motility 

 is lost in motile forms. The test is a very delicate one, as is evi- 

 denced by the fact that the agglutination may sometimes be secured 

 in dilutions as great as 1 : 100,000. The higher the dilution 



