428 



G. S. WILSON 



however, required to count tubes containing this number of 

 colonies is considerable, if several tubes have to be dealt with 

 and the resulting gain is comparatively small. Taking all factors 

 mto consideration then, it would seem that if considerable accu- 

 racy IS required the optimum number of colonies to be desu-ed 

 per tube is in the neighborhood of 200 to 400, when only three 

 tubes are put up; if a greater number are prepared, the number 

 of bacilli inoculated may be decreased to 100. This will be seen 

 to be somewhat higher than the number advocated by Park and 

 "Williams, but taking the experimental data given above as a 

 gmde, one feels justifed in regarding this recommendation as 

 more m accordance with the laws of scientific accuracy. 



Technique of the viable count 



Ha\dng dealt with these prehminary pomts, the final technique 

 of the ^^able count may now be described. Presuming a broth 

 culture of an organism is to be counted, a certain amount of fluid 

 IS withdrawal by means of a dropping pipette, and 10 drops are 

 delivered at intervals of one second between each drop into a 

 flask containing a known quantity of sterUe tap water at 18°C. 

 After shaking thoroughly, a fresh pipette is used to transfer 10 

 drops of this emulsion into a second flask, likewise containing a 

 known quantity of tap water at 18°C. The quantity of water 

 m each flask will vary according to the age of the culture which 

 is to be counted; in some cases only 5 cc. will be requu-ed, in others 

 as much as 50 or 100 cc; in any case, the quantity is dehvered by 

 means of a carefully cahbrated volumetric pipette. '\^Tien the 

 second flask has been shaken 4, 8 or 12 drops of this emulsion are 

 delivered into three test tubes, measuring 6 inches by | inch, each 

 containing about 2 cc. of melted agar at 45°C. The contents are 

 then mbced by gentle shaking, and the tubes are rolled by rotation 

 between the fingers. The tubes are incubated in an inverted 

 position for three days at 37°C., and at the end of that time are 

 counted in the way previously described. 



