Culturing Microorganisms (The 5 I’s)

What is the inoculum?

The small sample of an organism used to start a culture in a sterile growth medium.

What happens during incubation?

The inoculated medium is placed in a controlled environment (incubator) to allow microbial growth.

What is agar and why is it preferred over gelatin for solid media?

Agar is a gelling agent from algae. It melts at ~90°C, solidifies at ~50°C, and most microbes cannot degrade it.

What is the difference between defined media and complex media?

Defined media have all known chemical components; complex media have some unknown components.

What is the purpose of enriched media?

To promote the growth of fastidious organisms by adding specific nutrients (e.g., blood agar).

How does selective media work?

It promotes growth of certain organisms while inhibiting others (e.g., MacConkey agar promotes G⁻, inhibits G⁺).

What does differential media do?

It distinguishes between groups of microbes based on growth and metabolic products (e.g., colony color, hemolysis).

What is a colony in microbiology?

A macroscopically visible cluster of microbes originating from a single microorganism.

Describe the streak plate method in one sentence.

A loop is used to streak an inoculum across an agar plate in quadrants to isolate individual colonies.

What is examined during inspection and identification?

• Colony morphology (size, color, shape)

• microscopic examination

• biochemical/genetic testing