In the context of measuring microbial growth by plate counts, what is a colony-forming unit (CFU)?

• A. An estimate of the number of viable microorganisms in a sample based on the assumption that each visible colony arose from one cell or a clump of cells
• B. A unit of turbidity that reflects optical density and directly equals cells per milliliter without calibration
• C. The total number of cells in a sample, including both live and dead cells, determined by microscopic direct count
• D. A measure of microbial biomass based on the dry weight of cells collected from a culture

Answer: (A)

Verified step by step guidance

1. Understand that a colony-forming unit (CFU) is a term used in microbiology to estimate the number of viable microorganisms in a sample.
2. Recognize that when performing plate counts, each visible colony on an agar plate is assumed to have originated from a single viable cell or a clump of cells capable of growth.
3. Note that CFU counts do not measure total cells, but only those that are alive and able to reproduce under the given culture conditions.
4. Differentiate CFU from other measurements such as turbidity (optical density), total cell counts (including dead cells), and biomass (dry weight), which are distinct methods of assessing microbial presence or growth.
5. Conclude that CFU provides an estimate of viable microbial numbers based on colony formation, making it a practical and widely used method in microbiology for quantifying live cells.