Standardizing Bacterial Suspensions. 



39 



the specimens of squid be immersed in nicotine solution for i 

 minute. They are then put into the solution of camphor. Cam- 

 phor spasms take place, after which the animals lie quiet for several 

 minutes. At the end of the required latent period 1 nicotine 

 spasms occur just as typically as in animals treated with nicotine 

 alone. 



22 (1482) 



A method of standardizing bacterial suspensions. 



By Frederick L. Gates. 



[From the Department of Pathology and Bacteriology of the Rocke- 

 feller Institute for Medical Research, New York.] 



If a wire loop is thrust down into a suspension of bacteria in a 

 test tube, and viewed by looking down into the mouth of the tube, 

 the depth at which the loop disappears will be determined by the 

 opacity of the supervening suspension. If, however, a second 

 suspension of the same organism containing half as many bacteria 

 per cubic centimeter is similarly examined, or if an equal amount of 

 the diluent is added to the original suspension, and the " depth of 

 disappearance" again measured it will be found to be less than 

 twice as great as in the original suspension. In other words, the 

 observed depths of disappearance are not in proportion to the 

 bacterial concentrations or the corresponding volumes. 



This discrepancy is due to the presence in each reading of a 

 constant which is apparently related to the size and opacity of the 

 individual organisms. It is found that this constant may be elimi- 

 nated, thus bringing the opacity observations into inverse ratio 

 with the corresponding bacterial concentrations, and a corrected 

 reading (the observed reading minus the constant) for any sus- 

 pension may be obtained by making two readings at different 

 dilutions of the suspension, and substituting the observed values 

 in the following equation: 



^ _ vol a (b — a) 

 vol b — vol a 1 



^oore, A. R., J. Gen. Physiol, 1919, I, 505. 



