FITZGERALD AND FRASER 819 



Since two suspensions, prepared in apparently the same way, from two single 

 colonies derived from the same stock culture may evidence wide differences in agglu- 

 tinability,' it is often desirable for purposes of uniformity to make up a large amount 

 of suspension at one time. Dreyer^ has controlled this variable factor by standardiz- 

 ing the agglutinability of each suspension prepared. The utmost care in pipetting 

 must be observed to insure even a reasonable degree of accuracy. The Committee on 

 Pathological Methods of the Medical Research Council points out that an error in 

 carrying out six to eight doubling dilutions by the pipette method may commonly 

 be in the order of magnitude of 8-10 per cent. Information and instruction regarding 

 the selection and use of pipettes and the measurement by the drop method are con- 

 tained in its "Special Report Series," No. 51. 



In selecting tubes for carrying out the agglutination reaction it is important to take 

 cognizance of uniformity of size and the fact that the narrower the diameter of the tube the 

 better and more quickly is the agglutination brought about. The optimum temperature for 

 flocculation lies between 50° and 55° C. At a temperature of 37° C. the reaction proceeds 

 more slowly. In addition, at this temperature the growth of micro-organisms may interfere 

 with the interpretation of the reaction. Certain micro-organisms are agglutinated more 

 rapidly than others, one-half to two hours for B. typhosus at 55° C; dysentery bacilli, four 

 to six hours; meningococci, twenty-four or even forty-eight hours. In recording the results, 

 complete agglutination with clear supernatant in the highest dilution of serum may arbi- 

 trarily be chosen as the titre of the serum. It is preferable, however, to indicate the degree 

 of agglutination in each tube of the test in some manner as illustrated in the manual by 

 Wadsworth.3 Dreyer's method incorporates the drop method for all measurements and 

 formalinized cultures of known and constant agglutinability. The agglutinating power of a 

 serum he expresses in definite units. Glynn,^ Krumbhaar and Smith,s and Fennel^ have re- 

 ported favorably upon this method. Dryer's technique undoubtedly makes for delicacy and 

 accuracy. 



Mention has already been made of absorption of agglutinins. Much confusion 

 has arisen as to the application of this phenomenon in the identification of bacterial 

 species. The matter is further complicated by the introduction of the factor of 

 "variation" later to be discussed. If, to an agglutinating serum, is added a suspension 

 of its homologous micro-organism in appropriate amounts and under suitable condi- 

 tions, the power of this serum to agglutinate a fresh suspension of its homologous 

 micro-organism will have been reduced almost to zero. The amount of suspension 

 required to effect this reduction is called the "absorbing dose." Commonly this is ex- 

 pressed as the number of micro-organisms, as gauged by opacity standards, per cubic 

 centimeter of undiluted agglutinating serum. In practice it is usual before adding 

 the suspension to dilute the serum i/io to i/ioo, depending on its titre. Approxi- 

 mately 25,000-100,000 million organisms per cubic centimeter of serum will remove 

 all but the last traces of agglutinins. The minimum absorbing dose of the homologous 



' Andrewes, F. W.: op. cit., 25, 506. 1922. 



2 Dreyer, G.: ibid., 13, 331. 1909; "Medical Research Council, Spec. Rep. Series," No. 51. 1920. 



3 Wadsworth, A. B.: op. cit., p. 162. 1927. 1 Glynn, E.: Lancet, 191, 877. 1916. 

 s Krumbhaar, E. B., and Smith, W. B.: /. Infect. Dis., 23, 126. 1918. 



* Fennel, E. A.: J.A.AI.A., 70, 590. 1918. 



