180 BACTERIOLOGICAL EXAMINATION OF WATER. 



under these conditions a longer exposure is necessary to 

 bring out the specific reaction. This is well shown by the 

 table on page 179. 



GENERAL CONCLUSIONS ON THE REACTION OF AGGLUTINATION. 



Summarising our present knowledge on the subject of agglu- 

 tination the following practical rules should be observed when 

 testing the reaction of an unknown organism to anti-typhoid 



e 



serum. 



(1) The serum from a patient suffering from enteric fever 

 must not be used, as it is possible that he may be suffering from 

 a mixed coli and typhoid infection. 



(2) The serum of an animal immunised by injections of 

 B. typhosus should be employed. It should be standardised 

 against a stock culture of B. typhosus, and if, when diluted 

 1-1000, it fails to agglutinate this bacillus, it should be rejected. 



(3) The agglutination reaction cannot be considered specific 

 unless it is produced by the anti-typhoid serum, diluted 1-500 

 and upwards. 



(4) The specific reaction must be considered as only one link 

 in the chain of evidence, and the result of the test must be 

 taken in conjunction with the cultural reactions of the micro- 

 organism under investigation. 



Several methods have been suggested for the performance of 

 the agglutination test. Widal recommended the following 

 procedures for testing an unknown serum with the B. typhosus : 



(a) Mix the serum with sterile broth in the required propor- 

 tions ; then inoculate the mixture with the B. typhosus and 

 incubate at 37 C. After four to seven hours granular masses 

 appear, and at the end of twenty-four hours the bacilli are 

 usually found in little white heaps at the bottom of the tube 

 and the fluid appears nearly clear. 



(b) Add the serum in the required proportion to a twenty- 

 four hours broth culture of B. typhosus and incubate at 37 C. 

 After several hours the culture loses its uniform opacity, 

 granular masses appear, and finally the broth clears by precipi- 

 tation of the masses to the bottom of the tube. 



(c) Add the serum in the required proportion to a twenty- 

 four hours broth culture, or an agar culture made into an 



