Microbiology Microbiology / Lab. 8 / Lab. 8

o Culture (Growth) MediaCulture (Growth) Media

I. What is a medium (plural media)?

II. What is culture medium?

III.What is meant by Inoculation of Media?

IV.Classification/Types of Culture Media

o Streak Plate MethodStreak Plate Method

• What is a medium (plural media)?Any preparation that contains nutrients essential for bacterial growth, is called a medium.

• What is culture medium?A medium that has been successfully inoculated with bacteria is called a culture medium. Bacteria are capable of growing in a culture.

• What is meant by Inoculation of Media? It means introduction of infected material to the medium for the cultivation of organisms present in the material.

Classification/Types of Culture Media

1. According to physical state …

a) Liquid Media: fluid in nature, usually placed in test tubes, for example, nutrient broth.

b) Solid Media: Prepared by adding solidifying agents (1-5%) like gelatin and agar to the liquid medium, for example, nutrient agar.

c) Semi-solid Media: Reducing the amount of agar to (0.2-0.5%) makes the medium semi-solid. Such media are fairly soft and are useful in demonstrating bacterial motility and separating motile from non-motile strains.

2. According to Composition …

a) Simple Media: It contains only basic substance such as nitrogen , carbon and minerals that are essential for bacterial growth, for example, nutrient broth, nutrient agar, peptone water etc.

 

b) Enriched Media: Some nutritionally enriched material like blood or serum is added to the medium, required for proper growth of some bacteria, for example, blood agar, chocolate agar.

  

c) Differential Media (indicator media): It differentiate between two groups of bacteria (distinguish one microorganism type from another growing on the same media), for example, blood agar, MacConkey's Medium.

d) Selective Media: In this media an inhibitory substance is added to the media which prevents growth of all organisms except the one for which it is designed. for example, Mannitol salt agar.

e) Media used for biochemical reaction: This media is used to detect different biochemical reactions produced by different organisms. for example, simmon citrate medium.

f) Transport Media:

Transport media should fulfill the following criteria:

• Temporary storage of specimens being transported to the laboratory for cultivation.

• Maintain the viability of all organisms in the specimen without altering their concentration.

• Contain only buffers and salt.

• Lack of carbon, nitrogen, and organic growth factors so as to prevent microbial multiplication.

for example, Stuart transport medium.

• The purpose of this method is to isolate pure colonies of bacteria. In other words, if you had a mixture of 2 or more bacteria in a suspension, you could separate them out.

• The disadvantage is that contamination is common with this technique. It could be improved by using good aseptic techniques, no coughing on petri plate or talking above it.

The method for isolating individual strains of bacteria from a sample is called the Streak Plate procedure, begins by sterilizing a loop in a flame.

To cool the sterilized loop, touch it to sterile agar plate, then put the loop into a sample containing a mixture of bacteria then spread it over the surface of the agar.

Before continuing to streak the plate, the remaining bacteria on the loop are first killed in the flame. After cooling the sterilized loop, drags it through the previous path, picking up a small number of bacteria and spreading them into a new area of the plate .

After sterilizing and cooling the loop again, the procedure is repeated. With each new path, the loop picks up a smaller number of bacteria, and therefore can spread them farther and farther apart.

During the incubation, the bacteria multiply and produce colonies. Each colony consist of millions of cells derived from a single bacterium. The isolated colonies found in the last streak represent isolated strains.

LABORATORY CULTURE: QUADRANT STREAK METHOD

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Confluent mixture

Isolated colony

Heavy growth

light growth

"four corners"