Which unit is used for fecal coliform and E. coli counts in water samples?

• A. Colony forming units per mL
• B. Colony counts per 100 mL
• C. Most probable number per 100 mL
• D. Colonies per liter

Answer: (B)

Counting colonies that grow from a defined water volume is the practical basis for reporting fecal coliform and E. coli in water. The standard reporting unit is colony counts per 100 mL because methods like membrane filtration or plate counts filter or inoculate 100 mL of sample and then count the colonies that form. This directly expresses how many viable organisms were present in that fixed volume and aligns with regulatory reporting, which commonly uses 100 mL as the reference volume. While there are methods that yield an estimate called the most probable number per 100 mL, those are based on dilution series rather than direct colony counting. Other units, such as per mL or per liter, are not the typical reporting standard for these tests.