Page 90 - MNUmicrobiology practical 2025

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

Learning outcomes

• Define “mixed culture” and “pure culture”
• Articulate the principles of aseptic transfer.

• How to aseptically transfer bacteria from one pure culture to another using
broths, slants, and deeps

• The proper way to prepare streak plates to isolate pure cultures from a mixed

culture.

In the real world outside the laboratory, bacteria grow in communities made of many
bacterial species. If you need to identify the types of bacteria present in environmental

or medical samples, you must have a way to separate out the different types and
produce pure cultures.

A pure culture contains a single bacterial species, whereas a mixed culture may contain
many different types of bacteria.

Principle

The sample/inoculum is diluted by streaking it across the surface of the agar

plate. While streaking in successive areas of the plate, the inoculum is diluted to the

point where there is only one bacterial cell deposited every few millimeters on the

surface of the agar plate. When these lone bacterial cells divide and give rise to

thousands and thousands of new bacterial cells, an isolated colony is formed. Pure

cultures can be obtained by picking well isolated colonies and re-streaking these on

fresh agar plates.

General Microbiology & Immunology (PM 401) Practical note Page 79