List five direct methods of counting microbes.

Verified step by step guidance

1. Understand that direct methods of counting microbes involve physically counting the number of microbial cells or colonies, rather than estimating based on indirect measures like turbidity or metabolic activity.
2. Identify the first direct method: Microscopic count, where a known volume of a microbial suspension is placed on a counting chamber (such as a Petroff-Hausser chamber) and cells are counted under a microscope.
3. Recognize the second method: Viable plate count, which involves spreading a diluted sample on an agar plate, incubating it, and counting the number of colony-forming units (CFUs) that grow.
4. Consider the third method: Membrane filtration, where a known volume of liquid is filtered through a membrane that traps microbes, then the membrane is placed on a nutrient medium to grow colonies for counting.
5. List the fourth and fifth methods: Most probable number (MPN) method, which uses statistical estimation based on growth in multiple tubes, and Direct cell count using flow cytometry, which counts cells as they pass through a laser beam.

Key Concepts

Direct Microscopic Count

This method involves counting microorganisms directly under a microscope using a specialized counting chamber, such as a Petroff-Hausser chamber. It provides a quick estimate of cell density but cannot distinguish between live and dead cells.

Viable Plate Count

Also known as the colony-forming unit (CFU) method, this technique counts only living microbes by spreading a diluted sample on agar plates and incubating them to allow colony formation. Each colony represents one viable cell or group of cells.

Membrane Filtration

In this method, a liquid sample is passed through a membrane filter that traps microbes. The filter is then placed on a nutrient medium to grow colonies, allowing for the enumeration of viable microorganisms, especially useful for samples with low microbial counts.