Standard methods for the examination of water and sewage

by American Public Health Association. Laboratory Section; American Chemical Society; Association of Official Agricultural Chemists

1. Count the agar plates made on the first day.

2. Record the number of lactose broth fermentation tubes which show 10 per cent. (10%) or more of gas.

Note:—In case only the presumptive test for B. coli is required, fermentation tubes showing more than 10 per cent. (10%) of gas at this time may be discarded.

Third Day:

1. Count gelatin plates made on first day.

2. Record the number of additional fermentation tubes which show 10 per cent. (10%) or more of gas.

3. Make a lactose-litmus-agar or Endo’s medium plate from the smallest portion of each sample showing gas. Incubate plate at 37° C.

Note:—In case the smallest portion in which gas has been formed shows less than 10 per cent. (10%) of gas, it is well to make a plate also from the next larger portion, so that, in case the smallest portion gives a negative end result it may still be possible to demonstrate B. coli in the next larger dilution.

Fourth Day:

1. Examine Endo’s medium or lactose-litmus-agar plates. If typical colonies have developed, select two and transfer each to a lactose broth fermentation tube and an agar slant, both of which are to be incubated at 37° C.