Widal Reaction of Agglutination 5 1 1 



tically no risk in making a positive diagnosis. But for absolute cer- 

 tainty, and, above all, in cases in which the result of the reaction is 

 not prompt, complete, and unmistakable, higher dilutions should be 

 employed; if the amount of serum permits only one such, it may be 

 1:50; but preferably intermediate dilutions should also be made, and 

 it is desirable, if not absolutely necessary, to try dilutions higher than 

 1 : 50." 



A time limit must also be fixed, and with the standard dilution 

 of 1 : 50 a reaction should come on in an hour. 



The condition of the culture must be carefully considered from 

 several standpoints, else the validity of the test may be called into 

 question. 



Young cultures of the typhoid bacillus are actively motile, and are 

 apt to contain rather elongate individuals, both of which are condi- 

 tions essential to success. The more actively motile the bacilli, the 

 greater the contrast when this motility ceases ; and the longer the ba- 

 cilli, the more typical the agglutinations. Cultures less than twenty- 

 four hours old are best. In an emergency, however, they can be em- 

 ployed so long as they remain motile (forty-eight hours). 



Johnston has shown that virulent bacilli frequently transplanted 

 from culture medium to culture medium eventually developed an 

 abnormal activity in which reactions were apt to occur with normal 

 blood. The best method of avoiding this error seems to be to keep a 

 regular stock culture growing in the laboratory, transplanting it from 

 agar to agar only as often as is necessary to keep it in good condi- 

 tion say, once in three or four weeks. A culture from this to what- 

 ever medium is preferred is made about twenty-four hours before 

 testing. 



Johnston is emphatic upon the use of attenuated rather than virulent 

 i.e., freshly isolated cultures, stating* that "with virulent cul- 

 tures the presence of agglutinative substances in non-typhoid bloods 

 may lead to pseudo-reactions." These reactions are characterized by 

 clumping without loss of motion. Kuhnan,f however, says that the 

 reaction with non-virulent cultures is nearly twice as marked as that 

 obtained with virulent ones. Foerster J did not observe that the 

 difference between the reaction obtained with virulent and attenuated 

 bacilli was great. He experimented with nine different bacilli, and 

 expresses the difference observed as 5 : 8. 



Johnston found that when the bacilli are cultivated upon acid media 

 they may entirely lose their ability to agglutinate. Care should be 

 taken, therefore, to insure that the culture medium employed always 

 has the same degree of alkalinity. On the other hand, the media must 

 not be too alkaline, as this condition will also cause erroneous results. 



Except for the fact that dead bacilli are not motile, and hence cannot 

 show loss of motility as a part of the reactive phenomenon, they are 

 useful for making the test. In fact, the absence of any danger of 

 infection, and the convenience with which the sterilized cultures can 

 be sent from place to place to be used by physicians who are unac- 

 quainted with bacteriologic technic, have made Wright and Semple 

 recommend their use for the purpose. 



It is interesting to note the different forms of reaction, which may 

 be described as follows: 



(a) Cessation of motion (atypical). 



* "Montreal Med. Jour.," March, 1897. 

 t "Berliner klin. Wochenschrift," May 10, 1897, No. 19. 

 J "Zeitschrift fur Hygiene," 1897, vol. xxiv, p. 500. 

 "Brit. Med. Jour.," May 15, 1897. 



