62 



Prof. A. E. Wright. On the Measurement of [Aug. 5,, 



I have found it convenient to allow the serum to remain in contact 

 with the culture for a period of 18 to 24 hours at 37° C. 



(b.) Method of Testing the Continued Vitality or otherwise of the Bacteria 

 which have been in Contact with the Serum. 



The sterile broth which has been filled into the capillary pipette 

 furnishes, as we have seen, the means for determining whether the 

 bacteria which have been brought in contact with the serum have or 

 have not retained their vitality. If the serum has failed to kill the 

 bacteria, this will be evidenced by the development of turbidity in the^ 

 broth which will follow upon the aspiration of the column of fluid in 

 the capillary stem into the bulb of the pipette. If, on the other hand, 

 the serum has killed all the bacteria with which it has been mixed,, 

 the nutrient broth will, under the circumstances, remain clear. 



The steps of the procedure are as follows : — 



The tubes having been taken in hand singly, the lower portion of the 

 capillary stem is in each case drawn out, and after heating in a peep- 

 flame, into the finest possible filiform tube. 



A condition of negative pressure is now established in the interior 

 of the pipette by fitting over its upper end a collapsed rubber teat. 

 While carefully regulating this negative pressure by keeping* the 

 finger and thumb in position on the teat, the finely-drawn-out end of 

 the capillary stem is gently snapped across. The column of fluid will 

 then be very quietly carried up into the bulb of the pipette. 



The determination of the continued sterility or otherwise of the 

 broth may generally be made after incubation by mere naked eye in- 

 spection. Where a doubt arises either as to the existence of a growth, 

 or as to the nature of the cultivation obtained, a drop of the culture 

 may be microscopically examined or cultivated on nutrient agar. 



III. Method of Expressing the Results obtained by the Method of Bactericidal 

 Estimation here in question. 



The question which is investigated by the method described above 

 is, as has been seen, the question as to what is the lowest dilution of 

 the particular enumerated culture employed which is completely 

 sterilised by digestion with an equal volume of serum. No attempt 

 is made to determine what reduction in number of living bacteria, 

 and what subsequent increase occurs in the case of those tubes which 

 are not completely sterilised. 



It is claimed that by narrowing down the issue, as is here done, we 

 escape from a fallacy which consistently arises in connection with 

 estimations of bactericidal power arived at by a comparison of th& 

 * Vide note on previous page. 



