72 LABORATORY COURSE IN SERUM STUDY 



entire contents of the pipette into the first watch glass ; mix by drawing 

 the solution in and out of the pipette several times. Take up five units 

 of salt solution into the pipette in similar manner, then five units of the 

 first dilution, and place this mixture in the second watch glass. Place 

 five units of salt solution and five units of the second dilution in the third 

 watch glass. Remove five units from the third watch glass and discard. 

 Place five units of salt solution alone in the fourth glass. Then with this 

 same pipette measure five units of typhoid broth into each of the watch 

 glasses. Prepare hang-drops and incubate and observe as above. 



Another method commonly used is to prepare the dilutions of serum 

 in one of the two ways given above, and then place on a series of cover 

 glasses with a platinum loop a drop of typhoid broth, then on each glass 

 with the same loop a drop of diluted serum, after which the two drops 

 are stirred together by means of a capillary pipette sealed at the end. 

 This method offers no advantages over the preceding. 



The method sometimes used for clinical diagnosis, of diluting with 

 counted loopfuls of saline drops of dried blood sent to the laboratory 

 on glass slides, is exceeding inaccurate. 



II. EFFECT OF SALT ON AGGLUTINATION 



The agglutination of bacteria which occurs in the presence of 

 immune serum has many analogies to the flocculation of colloidal 

 suspensions by electrolytes and is dependent on the presence of 

 electrolytes in the solution. Bacteria sensitized with immunized 

 serum agglutinate in the presence of traces of sodium chloride 

 and other inorganic salts, but not if suspended in distilled water. 

 These salts may also cause unsensitized bacteria to agglutinate, 

 and false clumping due to their action is sometimes observed in 

 the control tubes of agglutination tests, especially if the bacteria 

 are grown in media which have become concentrated by drying. 

 This effect is more easily demonstrated with certain salts of the 

 heavy metals than with sodium chloride. The following experi- 

 ment will serve to illustrate the effect of electrolytes on the agglu- 

 tination of sensitized and unsensitized bacteria : 



Reagents : 



1. Serum of animal immunized to B. typhosus. 



2. B. typhosus 24 hour agar slant. 



3. Distilled water. 



