Question
Principle of presumptive coliform testing and its interpretation.
Answer
The presumptive coliform test is the first stage of the standard multi-stage bacteriological method for assessing the microbiological quality/potability of water.
Principle: coliforms are used as indicator organisms for faecal contamination of water, since their presence suggests the water may also harbour more difficult-to-directly-detect enteric pathogens (Salmonella, Shigella, Vibrio cholerae) from the same faecal source.
Method (Multiple Tube/Most Probable Number technique): serial measured volumes of the water sample are inoculated into multiple tubes of a lactose-containing broth medium (e.g., MacConkey broth or lactose peptone water) containing a Durham’s tube to trap gas; tubes are incubated at 37°C for 24–48 hours; acid and gas production (colour change from the pH indicator plus a gas bubble in the Durham’s tube) is taken as a presumptive positive result for coliform presence.
Interpretation: the pattern of positive/negative tubes across dilutions is compared against standard McCrady’s probability (MPN) tables to estimate the Most Probable Number of coliforms per 100 mL of the original water sample — the standard reporting unit for water quality.
Confirmatory and completed tests: a positive presumptive test is followed by a confirmed test (subculture to a more selective/differential medium, e.g., brilliant green lactose bile broth) and, if needed, a completed test (further biochemical confirmation, e.g., IMViC tests, to specifically identify E. coli), since the presumptive test alone is not fully specific.
Public health significance: this test underlies drinking-water quality standards, with WHO/national guidelines specifying that potable water should show zero coliforms per 100 mL.