Counting viable bacteria: which techniques yield colony-forming units (CFU)? Which methods below directly provide a viable plate count expressed as colony-forming units per millilitre (CFU/mL)?

Introduction / Context:
Microbial enumeration can measure total cells or viable cells capable of forming colonies. Clinical and food laboratories often need viable counts to assess contamination or product safety. This question distinguishes methods that yield CFU-based counts from those providing total cell estimates or indirect proxies.

Given Data / Assumptions:

• Viable plate counts depend on colony formation after incubation.
• Spread and pour plates both plate diluted samples onto/in agar to obtain discrete colonies.
• Hemocytometers and turbidity readings count total or estimate biomass, not viability per se.

Concept / Approach:

Spread plating distributes diluted cells over agar surfaces; pour plating embeds cells within molten agar. After incubation, visible colonies are counted and back-calculated to CFU/mL using the dilution factor and plated volume. Hemocytometers count cells microscopically irrespective of viability; spectrophotometry measures optical density as an indirect proxy of biomass.

Step-by-Step Solution:

Verification / Alternative check:

Standard protocols express viable counts as CFU/mL using countable plates (typically 30–300 colonies) from spread or pour methods.

Why Other Options Are Wrong:

• Hemocytometer: Counts all cells, live and dead, unless viability dyes are used, still not CFU.
• Turbidity: Indirect, affected by cell size and aggregation, not a direct viable count.

Common Pitfalls:

• Misinterpreting “CFU” as equivalent to microscopic counts; CFU requires growth to colonies.

Final Answer:

Both (a) and (b)