Culturing Microorganisms

Bacteria are grown in a "culture medium" which contains the carbohydrates, minerals, proteins and vitamins they need to grow. The culture medium can be a nutrient broth solution or solid agar jelly. Bacteria grown on agar plates will form visible colonies on the surface of the jelly, or will spread out to give an even covering of bacteria. To make an agar plate, hot agar jelly is poured into shallow round plastic dishes called Petri dishes. When the jelly's cool and set, inoculating loops (wire loops) can be used to transfer microorganisms to the culture medium. Alternatively, a sterile dropping pipette and spreader can be used to get an even covering of bacteria. The microorganisms then multiply.

1) Place paper discs soaked in different types of antibiotics on an agar plate that has an even covering of bacteria. Leave some space between the discs.

2) The antibiotic should diffuse into the agar jelly. Antibiotic-resistant bacteria will continue to grow on the agar around the paper discs, but non-resistant stains will die. A clear area will be left where the bacteria have died-this is called an inhibition zone.

3) Make sure you use a control.

4) Leave the plate for 48 hours at 25°c.

5) The more effective the antibiotic is against the bacteria, the larger the inhibition zone will be.

Contamination by unwanted microorganisms will affect your results and can potentially result in the growth of pathogens. In order to avoid this, the petri dishes and culture medium must be sterilised before use by heating to a high temperature to kill any unwanted microorganisms that may be lurking on them. If an inoculating loop is used to transfer the bacteria to the culture medium, it must be sterilised. After transferring the bacteria, the lid of the petri dish should be lightly taped on to stop microorganisms from the air getting in. The petri dish should be stored upside down to stop drops of condensation falling onto the agar surface.