Microbial Growth Measurement: Direct Methods

Direct methods for microbial growth measurement involve observing or counting individual cells in a microbial culture.

Cells are often counted directly under a microscope using a Petroff-Hausser chamber, a specialized slide with etched grids that help determine cell density in a defined volume.

Although this method gives immediate results, errors may arise from counting dead cells as live, debris interference, or motile cells moving out of focus.

Plate counts measure colony-forming units by assuming that each cell forms a single colony.

Samples are diluted through serial dilution, usually in factors of 10, to reduce cell numbers to a countable range.

A known volume from each dilution is then spread on solidified agar or mixed with molten agar. After incubation, the colonies are counted.

The total dilution factor is calculated by multiplying all successive dilutions, allowing accurate determination of the original cell density.

Plate counting is effective in determining the number of viable colonies in a sample. However, cell clumps may appear as one colony, causing inaccuracies.