62 LABORATORY COURSE IN SERUM STUDY 



similar inaccuracies in the microscopic method. For these 

 reasons the macroscopic method is the one usually employed in 

 experimental investigations. On the other hand, in testing 

 patient's sera against the typhoid bacillus (the Widal reaction) 

 considerable information is given by the cessation of motility of 

 the bacilli in the presence of immune serum. It is desirable to 

 use small amounts of serum and unnecessary to set up more than 

 three or four dilutions in one test. Consequently for diagnosis 

 of typhoid the microscopic technique is the method of choice. 



The macroscopic test may be carried out with living bacteria, 

 with bacteria killed either by exposure to heat (56 C.) or by ex- 

 posure to weak antiseptics such as phenol, 1 per cent solution, or 

 formaldehyde 0.8 per cent. Killed suspensions of bacteria may 

 be kept a long time in the ice box and still remain serviceable. 

 The flocculation is most easily observed in narrow test tubes, 

 f of an inch in diameter, but larger tubes can also be used with- 

 out difficulty. Some workers prefer to use tubes pointed at the 

 bottom, similar to the centrifuge tubes, since the sediments 

 which settle out can be readily observed in the tips of these tubes. 



The period of incubation varies. A marked reaction is visible 

 after an hour at 37, but with many organisms it is necessary to 

 incubate for two hours to obtain clear-cut results, and the floc- 

 culation often becomes more distinct when the tubes are removed 

 from the water bath and placed in the ice box for from half an 

 hour to 12 hours. Such organisms as the streptococcus and the 

 staphylococcus usually require the longer periods of incuba- 

 tion. On the other hand, for organisms of the typhoid-colon 

 group long incubation is unnecessary. 



Reagents : 



1. Serum of a rabbit immunized against typhoid. For this test the 

 serum of animals immunized by the class should be used, the members 

 in charge of the rabbits bleeding them from the carotid at the previous 

 exercises and distributing to each other member of the class 2 c.c. of a 

 1-10 dilution of the serum. 



2. B. typhosus 24 hour culture on agar slant. 



3. Normal salt solution. 



