Question
Presumptive coliform count
Answer
The presumptive coliform count is the first stage of the standard multi-stage bacteriological method for assessing the microbiological quality/potability of water, used to estimate the number of coliform organisms present as an indicator of possible faecal contamination.
Principle: coliforms (a group including E. coli and related Gram-negative, lactose-fermenting, facultatively anaerobic bacilli) 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 — testing directly for every possible pathogen in every water sample would be impractical, making a reliable, easily-tested indicator organism essential.
Method (Multiple Tube/Most Probable Number, MPN, 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 (an inverted small tube to trap gas); tubes are incubated at 37°C for 24–48 hours; acid and gas production (visible as a colour change from the pH indicator and gas bubble trapped in the Durham’s tube) in a tube is taken as a presumptive positive result for coliform presence in that tube.
Result and interpretation: the pattern of positive/negative tubes across the different sample-volume 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 unit for reporting water quality.
Confirmatory and completed tests: since the presumptive test alone is not fully specific (other non-faecal organisms can occasionally also ferment lactose with gas production, giving false positives), a positive presumptive test is followed by a confirmed test (subculturing to a more selective/differential medium, e.g., brilliant green lactose bile broth, to confirm coliform identity) and, if further certainty is needed, a completed test (further biochemical/morphological confirmation, e.g., IMViC tests, to specifically identify E. coli as confirmation of definite faecal origin) — together forming the complete standard sequence for water potability testing.
Public health significance: this test underlies water-quality standards and regulatory monitoring of drinking water supplies, with WHO/national guidelines specifying that potable water should show zero coliforms per 100 mL.