BACTERICIDAL METHODS 127 



1'2 are to be reckoned as unity that is to say, that no deduction 

 can be drawn from indices falling between these limits. In the 

 case of such organisms as those of the coli-typhoid group and 

 cholera, which are susceptible to bacteriolytic influences in the 

 serum, it may be necessary to heat the sera of the patient and 

 observer for half an hour at 55 C. This destroys any com- 

 plement present and prevents bacteriolysis occurring. In the 

 case of the b. typhosus the virulence of the strain employed has 

 been shown to be an important factor. 



Several modifications of Wright's technique have been 

 suggested. Thus Klien, instead of enumerating the bacteria 

 ingested, takes a series of dilutions of the serum and estimates 

 the dilution with which capacity for opsonising bacteria dis- 

 appears (or at any rate the dilution with which the phagocytic 

 index falls below *5). The content of the patient's serum and 

 of that of the observer may be thus compared, or the course of 

 an immunisation may be followed by making daily observations 

 of the content in opsonin. In another modification of Wright's 

 technique Simon compares not the numbers of bacteria ingested, 

 but the percentages of cells containing bacteria and those not 

 containing bacteria. This he calls the " percentage index," and 

 he states that the figure thus obtained corresponds very closely 

 to the ordinary opsonic index ; he claims that the method 

 eliminates some of the errors which may arise in the use of the 

 ordinary technique if only a relatively small number of phago- 

 cyting cells, such as 50, be examined. 



BACTERICIDAL METHODS DEVIATION OF COMPLEMENT. 



The Estimation of the Bactericidal Action of Serum. This 

 may be carried out by various methods, of which those of 

 Neisser and Wechsberg and of Wright may be given as examples. 

 In the former, the effects of varying amounts of serum on the 

 same amounts of bacteria are observed by means of plate 

 cultures ; in the latter, the number of bacteria which can be com- 

 pletely killed off by a given quantity of serum is ascertained. 

 In carrying out experiments of this kind it is convenient to have 

 a number of small test-tubes sterilised and plugged with cotton- 

 wool. We can then make any required dilution of a young 

 bacterial culture in bouillon as follows : To each of a number 

 of tubes we add '9 c.c. of *8 per cent, solution of sodium chloride. 

 To the first tube (a) we add '1 c.c. of the bacterial culture, and 

 thoroughly shake up the mixture; to the second (b) we add 



