Page 76 - MNU microbiology theoritical 2025

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

3- Filtration

• Filtration is used when the numbers of organisms in a sample are relatively
low, as might occur in dilute environments such as natural waters.

• A specific volume is filtered through a membrane which is aseptically

transferred onto an agar plate. Colonies are counted after overnight incubation

B. Indirect cell count

1. Turbidity

2. Metabolic Activity

3. Total weight

1- Turbidity
- Turbidity is an indication for the presence of microbial growth

- The laboratory instrument used to measure turbidity is called a spectrophotometer

- In a spectrophotometer, a light beam is transmitted through a bacterial suspension,

the light passing through the suspension is measured by a detector.

- As the numbers of bacteria in a suspension increase, turbidity also increases and

causes less light to reach the detector.

General Microbiology & Immunology (PM 401) theoretical handbook Page 65