Page 73 - MNU microbiology theoritical 2025

General Microbiology & Immunology (PM 401) Second level Semester 4 2024/2025

Measurement of Bacterial Growth

• Estimating the number of bacterial cells in a sample, known as a bacterial count,
is a common task performed by microbiologists.

• The number of bacteria in a clinical sample serves as an indication of the extent
of an infection. Quality control of drinking water, food, medication, and even

cosmetics relies on estimates of bacterial counts to detect contamination and

prevent the spread of disease

Two major approaches are used to measure cell number:

• The direct methods: involve counting cells

• The indirect methods: depend on the measurement of cell presence or activity

without actually counting individual cells.

A- Direct cell count

1- Direct microscopic cell count (Petroff-Hausser Chamber Slide)

2- Plate Count
a. Spread Plate
b. Pour Plate
3- Filtration

1- Direct microscopic cell count

• The method involves transferring a known volume of a culture to a calibrated slide

and counting the cells under a light microscope.

• The calibrated slide is called a Petroff-Hausser chamber

• It is possible to estimate the concentration of cells in the original sample by

counting individual cells in a number of squares and determining the volume of

the sample observed

General Microbiology & Immunology (PM 401) theoretical handbook Page 62