116 DISINFECTANTS 



The use of pathogenic species for disinfection studies 

 'offers no advantages. Saprophytes can be handled more 

 easil}^ The strain of Bacterium coli used by us has a 

 phenol coefficient almost identical with that of the Hop- 

 kins strain of Bad. typhosum. 



DETAILS OF THE PROCEDUEE 



It is advisable to make a strong solution of the disinfectant 

 some time before the test, to be certain of a sterile start. Di- 

 lutions are made, as was said above, either with water, or with 

 a mineral solution which keeps bacteria viable for a long time. 

 For Bacterium coli we used a solution containing 0.05% NaCl, 

 0.02% KCl, 0.05% CaClo and 0.05% MgCl,, although Brooks 

 and Winslow (1927) have shown that this species can remain 

 alive in distilled water for a long time. All dilutions of dis- 

 infectants, and all salt solutions must be prepared with glass- 

 distilled water. Small Erlenmeyers containing about 50 cc. 

 of this salt solution, and others containing skimmed milk di- 

 luted 1 :10 are sterilized and kept in stock. 



The test culture to be used is a 21-hour culture grown in 

 standard nutrient broth at 37°. It is ceutrifuged, and the bac- 

 teria are re-suspended in the sterile salt solution to the volume 

 of the original culture. 



The actual exposure of the bacteria to the disinfectant is 

 done in 18 mm. test tubes which permit thorough mixing. The 

 final volume is always 5 cc. If the concentrations 0.2%, 0.1% 

 and 0.05% are desired, they can be made from a 0.5% solution 

 of the disinfectant according to the following chart: 

 0.2 % = 2 cc. of 0.5% + 2.5 cc. salt sol'n -f 0.5 cc. bact. susp'n 

 0.1 % = 1 cc. of 0.5% -j- 3.5 cc. salt sol'n + 0-5 cc. bact. susp'n 

 0.05% = 0.5 cc. of 0.5% + 4.0 cc. salt sol'n + 0.5 cc. bact. susp'n 



The disinfectant and salt solution (or milk) are mixed first 

 and then the tube is shaken and placed into a water bath at 

 20°. When it has reached a constant temperature, the bac- 

 terial suspension is added. The probable death times for each 

 concentration are estimated, and arranged into a time sched- 

 ule of the following type : 



0.2%: 1/2, 1, 2, 3, 5 minutes; 

 0.1%: 2, 4, 8, 15, 30 minutes; 

 0.05%: 15, 30, 45, 60, 90 minutes; 



