47 



Method 1. Barely touch the liquid in the preliminary enrichment 

 tube, showing gas, with the point of a platinum needle, wash off this 

 needle in a tube of melted litmus lactose or Endo agar and pour into a 

 petri dish. Incubate at the body temperature for 24 hours. 



Method 2. Pour L. L. A., Endo or other differential agar into petri 

 dishes and allow them to solidify and dry in the incubator. Dip a small 

 loop into a positive preliminary enrichment tube and wash it off in a 

 tube of sterile salt solution. Place a loop of this wash water in the 

 center of a plate containing the differential medium and spread it over 

 the surface with the aid of a sterile glass rod bent at right angles. Incubate 

 20 to 24 hours at body temperature. 



Method 3. Dip a platinum needle, whose end is bent at an angle 

 of about 120, into the positive preliminary enrichment tube, stab into a 

 plate containing the differential medium (to remove the excess organisms) 

 and then make a series of streaks on this plate about a quarter of an inch 

 apart, taking care to always streak in the same direction and to lift the 

 needle at the end of each stroke. With a little practice, very excellent 

 isolations may be obtained by this simple method. This was first demon- 

 strated to the author by Mr. Greenfield of the Illinois Water Survey and 

 it has proved very satisfactory. Incubation is, as before, at body tem- 

 perature for 24 hours. 



TABLE XXI. RELATIVE GROWTH OF BACT. COLI AND SEWAGE STREPTO- 

 COCCI FROM POLLUTED WATER IN GLUCOSE BROTH. 

 (After Prescott and Baker 1904) 



If characteristic colonies are present on these agar plates the test is 

 regarded as partially confirmed for the colon group. For further con- 

 firmation, it would be necessary merely to fish a well isolated colony and 



