The following group reactions indicate the source of the culture with a high degree of probability:
| Methyl red + | B. coli of fecal origin. |
| Voges-Proskauer − | |
| Gelatin − | |
| Adonite − | |
| Indol, usually + | |
| Saccharose, usually − | |
| Methyl red − | B. aërogenes of fecal origin. |
| Voges-Proskauer + | |
| Gelatin − | |
| Adonite + | |
| Indol, usually − | |
| Saccharose + | |
| Methyl red − | B. aërogenes, probably not of fecal origin. |
| Voges-Proskauer + | |
| Gelatin − | |
| Adonite − | |
| Indol, usually − | |
| Saccharose + | |
| Methyl red − | B. cloacae, may or may not be of fecal origin. |
| Voges-Proskauer + | |
| Gelatin + | |
| Adonite + | |
| Indol, usually − | |
| Saccharose + |
12. ROUTINE PROCEDURE FOR EXAMINATION OF SAMPLES OF WATER.
First Day:
1. Prepare dilutions as required.
2. Make two (2) gelatin plates from each dilution, and incubate at 20° C.
3. Make two (2) agar plates from each dilution, and incubate at 37° C.
4. Inoculate lactose broth fermentation tubes with appropriate amounts for B. coli tests, inoculating two (2) tubes with each amount.
Note:—Where repeated tests are made of water from the same source, as is customary in the control of public supplies, it is not necessary to make duplicate plates or fermentation tubes in each dilution. It is sufficient, in such circumstances, to make duplicate plates only from the dilution which will most probably give from 25 to 250 colonies per plate.
Second Day: