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

• A. A direct microscopic count of the total number of cells present, including live and dead cells
• B. A measure of viable microorganisms, defined as one cell or a clump of cells capable of forming a visible colony on solid media
• C. A unit describing the number of viral particles required to lyse a bacterial lawn (plaque-forming units)
• D. The mass of bacterial biomass produced per milliliter of culture, typically measured by dry weight

Answer: (B)

Verified step by step guidance

1. Understand that a colony-forming unit (CFU) is a microbiological term used to estimate the number of viable microorganisms in a sample.
2. Recognize that CFUs represent either a single viable cell or a group (clump) of cells that can multiply and form a visible colony on a solid growth medium.
3. Differentiate CFUs from other counting methods, such as direct microscopic counts, which include both live and dead cells and do not distinguish viability.
4. Note that CFUs are distinct from plaque-forming units, which relate to viral particles, and from biomass measurements, which quantify total mass rather than viable cell numbers.
5. Conclude that CFUs specifically measure the number of living microorganisms capable of reproduction, making them a key metric in assessing microbial growth and viability.