Bacteriological examination of water

The bacteriological examination of water tests for the presence of faecal bacteria to find if the water is safe to drink. As testing for each pathogen is not practical, indicator organisms (coliform and E. coli) are used.

1. Sampling

Collect the sample in a sterile, clean glass bottle (with sodium thiosulphate if the water is chlorinated to remove residual chlorine). Flame the tap, let water run for 2 to 3 minutes, fill the bottle leaving an air space, and keep it cool (below 10 degrees C). Test within 6 hours (maximum 24 hours).

2. Tests

• Standard plate count (total bacterial count): 1 ml of sample is placed on nutrient agar and incubated at 37 degrees C for 24 hours (and 22 degrees C for 72 hours); colonies are counted. Treated water should have a low count (less than 100 per ml).
• Multiple tube fermentation test (MPN): presumptive test in lactose broth at 35 degrees C, confirmed test in BGLB or EC broth, and completed test on EMB agar. The result is given as MPN of coliform per 100 ml.
• Membrane filter technique: 100 ml is filtered through a 0.45 micron membrane and incubated on m-Endo or m-FC medium; coliform colonies are counted.
• Presence-absence test and field kits (H2S strip test, Colilert): quick tests for screening.

3. Interpretation

Zero E. coli per 100 ml must be found in drinking water (Nepal Drinking Water Quality Standards and WHO). A positive result shows faecal contamination, and the water should be treated (chlorination) and the source protected.