Water quality testing — what does the MPN index measure? (Interpretation of Most Probable Number counts in sanitary analysis.)

Introduction / Context:
The Most Probable Number (MPN) method is a statistical, culture-based approach widely used in sanitary microbiology. Regulators apply MPN to estimate microbial contamination in drinking water, recreational waters, and treated effluents.

Given Data / Assumptions:

• Multiple-tube fermentation or enzyme-substrate methods are employed.
• Incubation targets coliform growth characteristics (gas, acid, fluorescence).
• Result reported as MPN per 100 mL.

Concept / Approach:
Indicator organisms such as total coliforms and Escherichia coli serve as surrogates for fecal contamination risk. The MPN index estimates the probable concentration of these bacteria by assessing growth patterns across serial dilutions and consulting statistical MPN tables.

Step-by-Step Solution:
1) Inoculate serial dilutions into selective media.2) After incubation, record positive tubes (gas/acid/fluorescence).3) Use the pattern of positives to read an MPN value from standard tables.4) Conclude that MPN quantifies coliform bacteria presence.

Verification / Alternative check:
Compare with membrane filtration counts (CFU/100 mL) for coliforms; both target the same indicator group.

Why Other Options Are Wrong:
BOD5 measures biochemical oxygen demand; DO is oxygen concentration; hardness reflects multivalent cations (Ca2+, Mg2+). None of these are quantified by MPN.

Common Pitfalls:
Confusing total coliforms with fecal coliforms/E. coli thresholds; misinterpreting MPN as exact rather than probabilistic.

Final Answer:
Coliform bacteria