THE COLON -TYPHOID GROUP 105 



Fifteen drops of broth or salt solution are successively 

 taken up in a loop of platinum wire, and placed on a glass 

 slide; close to them is placed one drop of the serum, and 

 the sixteen drops are then thoroughly mixed. A small 

 drop of this diluted serum is mixed on a cover-glass with 

 an equal-sized drop of a broth culture of the typhoid 

 bacillus. This gives a total dilution of the serum of 

 rather over 1 in 30. 



The culture should preferably be from a specimen of 

 low virulence, but it is not an essential point; some 

 cultures agglutinate much better than others. It is also 

 better (but again not essential, if free from clumps) for 

 the culture not to be over twenty-four hours old. If no 

 broth culture is available, an agar culture rubbed up in 

 broth or salt solution will do as well, but should be filtered 

 through filter-paper to remove clumps before use. Dead 

 broth cultures (killed by heating to 65 C. for an hour) 

 may also be used. A control hanging-drop preparation 

 of the culture should always be made, and examined to 

 see that no clumps of bacilli are present in it. 



The mixed drop of diluted serum and of culture is 

 examined as a hanging-drop (p. 47) and should have very 

 little depth, as the * clumps ' have a tendency to sink 

 and get beyond the focal distance of the lens. At first 

 the bacilli may be active, but if the case is one of enteric 

 fever they soon slow down or stop, then gradually groups 

 of two or three form; these groups soon aggregate until 

 nearly all are collected into various crowds or ' clumps,' 

 with very few isolated bacilli left. If the reaction is com- 

 plete within thirty minutes, the case is certainly one of 

 enteric fever. Generally it is not complete, and there may 

 be groups of only ten or twenty, but the occurrence of 

 grouping and loss of movement are in themselves decisive. 



The dilution of the serum is necessary, because the 

 serum of normal individuals will often, undiluted, evince 

 an agglutinative power, but not, so far as experience goes, 

 in a total dilution of more than 1 in 16. More than one 

 dilution should always be examined, and it is advisable 

 that one should be high, such as 1 to 80 or 1 to 100. The 

 lower dilutions may conveniently be 1 in 30 and 1 in 50. 



The time-limit is necessary since many normal sera 

 will act, even when diluted, if left an indefinite time; 

 with a dilution of 1 in 30 to 1 in 50 an hour should be 



