MEASUREMENT 'OF GROUP AGGLUTININS 119 



four hours at 37° C. in ordinary veal peptone bouillon in au Erlenmeyer 

 flask. At the end of this time the flask is shaken and there is added to 

 it O'l per cent, of commercial formalin ; it is again shaken and placed at 

 once in a cool chamber at about 2° C. in the dark. The shaking is 

 repeated at intervals for 4 to 5 days, the flask always being replaced in 

 the cold chamber. At the end of three or four days the culture will be 

 found to be sterile and will keep practically indefinitely. Such killed 

 cultures are very suitable for sedimentation tests. 



Each culture is standardised by (a) its opacity being brought, by dilution 

 with normal saline containing O'l per cent, formalin, as nearly as possible 

 identical with that of a "standard agglutinable culture," and (b) by measur- 

 ing its agglutinability as compared with that of the standard culture. 1 



Measurement of Group Agglutinins. — In the case of certain 

 groups of allied organisms, — notably the b. coli and its allies, — 

 it has been found that when a serum clumps one member of the 

 group it may also clump some of the allied forms. If the 

 greatest dilution with which agglutination is obtained be esti- 

 mated, the end-points for the different strains affected will be 

 found to differ. The determination of the end-point is important, 

 as the disease condition from which the serum is derived is 

 generally caused by the organism which is clumped in highest 

 dilution. In comparing the effect of a serum on different bacteria, 

 the sedimentation method is usually employed. A series of 

 emulsions of the different bacteria to be tested is prepared by 

 scraping off the growth on an agar tube, and suspending in 

 normal saline. Each of these should contain approximately the 

 same number of bacteria per unit volume. This is attained by 

 using emulsions of equal opacity, as judged of by noting the 

 point at which transparency to some arbitrary standard such as 

 a particular type or set of parallel lines ceases. A given amount 

 of each emulsion is now mixed with different dilutions of the 

 serum to be tested, the mixtures are all made up to the same 

 volume, say 1 c.c, and the tubes placed at 55° C. for two or 

 three hours. The results are then read. Dreyer takes as 

 standard agglutination the highest dilution in which marked 

 agglutination without sedimentation can be detected by the 

 naked eye. It is often, however, an advantage to examine the 

 results with a hand lens. Further details will be given in deal- 

 ing with individual organisms. 



The Absorption Method of testing Agglutinins. — This 

 method is applied under circumstances similar to those of the 

 last, namely, when several agglutinins acting on allied organisms 

 are present in a serum. The principle is to remove all the 

 agglutinins acting on one organism, and to study the properties 



1 Full details as to the use of standard cultures and sera may be had from 

 the Department of Pathology, University of Oxford. 



