CFU/mL stands for what?

• A. Colony Forming Units per milliliter
• B. Cell Form Units per milliliter
• C. Colony Form Units per milliliter
• D. Colonization Form Units per milliliter

Answer: (A)

The concept here is understanding what CFU/mL measures in microbiology. CFU stands for Colony Forming Units, which is a way to quantify viable microorganisms that can grow into colonies on an agar plate. The “per milliliter” part indicates the concentration in a liquid sample, so CFU/mL tells you how many viable cells (or clumps of cells) are present per milliliter that could form colonies under the given conditions.

This term is used because a single cell or a handful of cells can give rise to a colony, so counting colonies provides an estimate of viable cells rather than simply counting all cells under a microscope. The phrasing “Colony Forming Units per milliliter” is the standard wording, which is why it’s the correct choice.

Why the other phrasings aren’t standard: they don’t reflect the established terminology for this measure. “Cell Form Units per milliliter” would misname the unit, as “CFU” specifically stands for Colony Forming Units. “Colony Form Units per milliliter” breaks the conventional wording, and “Colonization Form Units per milliliter” uses a term (colonization) not used in this context.