232 A MANUAL OF BACTERIOLOGY 



1 in 25, 1 in 50, arid 1 in 100. A platinum loop may also be 

 employed as a rough measure ; a loopful of the serum is deposited 

 in a watch-glass, and by spotting round it nine or fourteen loops 

 of salt solution a dilution of 1 in 10 or 1 in 15 is prepared, or any 

 other dilution in a similar manner. 



1. The microscopic method. Principally employed for the diag- 

 nosis of disease, but may also be used for the recognition of bacterial 

 species. Two or three hanging-drop slides are vaselined, and 

 two or three cover-glasses cleaned. One loopful of a dilution of 

 serum is placed on each cover-glass, and to each is added a loop- 

 ful of the broth culture of the organism e.g. typhoid and well 

 mixed up, and the specimens are mounted as hanging-drops. 

 Starting with three dilutions of serum e.g. 1 in 15, 1 in 30, and 

 1 in 60 the dilutions in the specimens will be 1 in 30, 1 in 60, and 

 1 in 90 respectively. 



For the microscopic test a young (twenty-four to forty-eight 

 hours) broth culture of the organism is to be preferred. A con- 

 trol hanging-drop of it should be made before use to ascertain that 

 it is satisfactory and free from clumps. A thin suspension of an 

 agar culture may also be employed ; it should be centrifuged or 

 be filtered before use. Such suspensions may be preserved with 

 formalin (see below, " Garrow's method "). 



Care should be taken that the hanging-drop preparations are 

 quite sealed with the vaseline, so that evaporation is prevented. 

 The hanging-drops are then examined microscopically, a ^-in. 

 objective sufficing for typhoid. In the case of typhoid the fol- 

 lowing phenomena will be observed : The motility of the majority 

 of the bacilli is very quickly arrested, and in a few minutes they 

 begin to aggregate together into clumps, and by the end of the 

 half -hour there will be few isolated bacilli visible. In less marked 

 cases the motility of the bacilli does not cease for some minutes, 

 while in the least marked ones the motility of many of the bacilli 

 may never be completely arrested, but they are always more or 

 less sluggish as compared with the control hanging-drop made 

 from the culture, while clumping ought to be quite distinct by the 

 end of one hour (with a 1 in 30 to 1 in 60 dilution). 



The central portions of the drop should be examined, not the 

 margins. With blood which has been dried and dissolved 

 organisms may become entangled in debris, and must not be 

 mistaken for clumps. 



