Difference Between  Solid Media and Semi Solid Media

Key Difference – Solid Media vs Semi Solid Media
 

The culture medium can be defined as a solid or liquid formulation that contains nutrients and other necessary conditions for the growth of microorganisms and cells. A cultural medium is used to grow microorganisms under laboratory conditions for various purposes such as research, identification, classification, drug development, recombinant DNA technology, enzyme extraction etc. There are different types of culture media. Based on the consistency, culture media are three types; solid media, semi solid media and liquid media. Solid media are prepared using an inert solidification agent (agar) at a concentration of 1.5 to 2.0 %. Semi solid media are prepared using a solidifying agent (agar) at 0.2 to 0.5 %. The key different between solid media and semi solid media is that solid media contain a high concentration of agar and are used for the  identification and characterization of the colony morphologies of microorganism while semi solid media contain a low concentration of agar and are basically used to determination of the motility of bacteria.

CONTENTS

1. Overview and Key Difference
2. What is Solid Media
3. What is Semi Solid Media
4. Similarities Between Solid Media and Semi Solid Media
5. Side by Side Comparison – Solid Media vs Semi Solid Media in Tabular Form
6. Summary

What are Solid Media?

Solid media are a type of growth or culture media that are used for the growing microorganisms or cells in laboratories. Medium is prepared by mixing necessary nutrients and materials at proper concentrations. Other than nutrients, a solidification agent is used during the preparation of solid and Semi Solid Media. The common solidification agent used in media preparation is agar. Agar is an inert substance extracted from sea algae. It does not show any nutritional value.

Solid media contain a high concentration of agar. Agar is added at 1.5 to 2.0 % concentration. Agar solidifies the medium below 40 ⁰C. Once the medium solidifies, it allows a solid surface to streak and grows microorganisms. Solid media are used to identify microorganisms. And also they are used to study characteristics of different microorganisms and study of colony morphologies.

What is Semi Solid Media?

Several techniques are used to observe and detect the motility of bacteria. Among them hanging drop method is one such method. However, it has several disadvantages such as the tedious nature of the method, uncertainty of the results, difficulty of identifying the motility when only a few cells are motile, need of active or fresh cultures etc. Hence, scientists have developed semi solid media for above purpose. Semi solid media are microbial culture media that are prepared to add less amount of agar (solidifying agent at 0.2 to 0.5 %) to observe motility of bacteria. Semi solid medium was first introduced by Hiss in 1982 for the purpose of distinguishing typhoid and colon bacilli.

The results of the semi solid media are macroscopic. When motile bacteria are inoculated to stab cultures that were prepared using Semi Solid Media, a diffuse zone of growth along the inoculation line of the stab can be clearly observed. It eliminates the overlooking of motility if only a few are motile.

What are the Similarities Between Solid Media and Semi Solid Media?

• Solid and Semi Solid Media are types of culture media based on the consistency.
• Both are used to grow bacteria.
• Both media contain nutrients.
• Both media contain a solidification agent.
• Both media are important in microbiology.

What is the Difference Between Solid Media and Semi Solid Media?

Summary – Solid Media vs Semi Solid Media 

Culture medium contains different nutrients and other materials such as water, a source of carbon and energy, the source of nitrogen, minerals and several growth factors etc. for the growth of microorganisms and cells. Solid and Semi Solid Media are two types of media that were classified based on the consistency of the medium. Solid medium contains 1.5 to 2.0 % solidification agent while semi-solid medium contain 0.2 to 0.5 % solidifying agent. When poured into plates, solid medium solidifies and provides a solid surface to grow microorganisms. Semi-solid medium is soft, and it doesn’t solidify completely as solid media. Hence, Semi Solid Media allow motile bacteria to move and grow in the medium, unlike solid media. Solid medium is used to identify and characterize bacteria and other microorganisms while the semi-solid medium is used for the determination of the bacterial motility. This is the difference between solid media and semi solid media.

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Reference:

1.Acharya, Tankeshwar,“Bacterial Culture Media: classification, types and uses.”Microbeonline, 15 Oct. 2016. Available here 
2.“Bacterial culture media.” Medical microbiology. Available here 

Image Courtesy:

1.’Agar Plate’By Y tambe – Y tambe’s file, (CC BY-SA 3.0) via Commons Wikimedia  
2.’Stab culture of Legionella pneumophila.’ by CDC-Public Health Image Library (Public Domain) via Public Domain Files