82 



PRINCIPLES OF BACTERIOLOGY 



very gradually lowered, while at the same time gently moving the slide 

 back and forth to the slightest extent possible with the left hand. If 

 any resistance is felt the lens should be raised, for it has gone beyond 

 the point of focus and is touching the cover-glass. 



Testing Agglutinative Properties of Serum. By agglutination is meant 

 the aggregation into clumps of uniformly disposed bacteria in a fluid; 

 by sedimentation the formation of a deposit composed of such clumps 

 when the fluid is allowed to stand; sedimentation is thus the naked-eye 

 evidence of agglutination. 



The blood serum of animals is found to acquire the clumping power 

 for almost every variety of motile bacteria, and for many non-motile 

 forms after infection with that variety. 



In order to help the student to thoroughly understand what com- 

 prises a reaction, Wilson prepared a set of drawings, which are here 

 reproduced. The culture to be tested should be of about twenty hours' 



FIG. 58 



FIG. 59 



Microscopic field, showing the top of a hanging ; Microscopic field, showing a cross-section of the 

 drop in a normal typhoid culture. drop in Fig. 58. 



growth, either in bouillon or on agar. If on the latter a suspension 

 is made in broth or normal salt solution. A loopful of the fluid con- 

 taining the bacteria is placed on the cover-glass, and to it an equal 

 quantity of the serum dilution is added. 



In making the hanging drop to be examined it is necessary to have 

 it of such a depth that it will show at least three focal planes, otherwise 

 the examination will be incomplete and unsatisfactory. The moist 

 chamber must be well sealed by vaselin so as to prevent drying, and 

 kept at a temperature of at least 20 and not over 35 C. 



Fig. 58 shows a microscopic field of the top of a hanging drop 

 of a normal bouillon culture of typhoid bacilli. The culture is twenty 

 hours old and the organisms are freely motile. This represents the 

 control drop used for comparison with the drop of the same culture, 



