LOOP MEASUREMENT. 215 



typhoid convalescent rarely fails to agglutinate in 

 these higher dilutions. It is generally held that 

 a dilution of one in forty or fifty is sufficiently 

 high to eliminate the possibility of agglutination 

 by a non-typhoid serum, and sufficiently low to 

 render the serums of all, or nearly all, typhoid pa- 

 tients agglutinating. The necessity for dilution of 

 the serum is emphasized by the additional fact 

 that infections with related organisms, as the colon 

 bacillus, cause a slight increase in the agglutinat- 

 ing power for the typhoid bacillus along with a 

 relatively large increase of colon agglutinins. A 

 test with a low dilution of this colon serum might 

 give a positive reaction with the typhoid bacillus 

 and lead to an incorrect interpretation; but 

 if a dilution of one in forty were used, the non- 

 agglutination of the typhoid bacillus would speak 

 against a typhoid infection. This will be consid- 

 ered under "group agglutination" (Chapter XIV). 



A convenient method of measuring small The LOOP 



... , , m Measurement. 



amounts of culture and serum is by means of a 

 fine platinum wire which is bent at its tip to form 

 an eyelet or "loop." 3 If one places one loop of 

 serum into a small watch glass or hollow-ground 

 slide, and adds nine loops of bouillon or of salt 

 solution, a dilution of one in ten is reached. Five 

 loops of this mixture with five of the diluent gives 

 a dilution of one in twenty. One loop of the sec- 

 ond dilution, to which is added one of the culture 

 suspension, gives the desired dilution of one in 

 forty. The last may be mixed directly on the 

 coverglass, and then inverted on a hollow-ground 



3. Pfeiffer introduced a conventional "loop" of such 

 dimensions that it holds 2 milligrams of bacterial cells as 

 they are taken from a solid surface, like that of agar. 



