Microbiology Lab: Control of Microbial Growth

Why is sterile water tested in this experiment? Water has a hypotonic effect that inhibits growth of microbes. It serves as a negative control for the other chemical agents. Water and soap make an effective antibacterial combination. Water has mild antibacterial properties.

It serves as a negative control for the other chemical agents.

What is the correct way to measure the zone of inhibition? Measure the diameter of the agar plate and divide by the number of discs placed on the plate. Measure the complete diameter of the zone from one side across the disc to the other side of the clear area, in millimeters. Measure the radius from the center of the disc to the edge of the clear zone, in millimeters. Measure from the edge of the disc to the edge of the zone, in millimeters.

Measure the complete diameter of the zone from one side across the disc to the other side of the clear area, in millimeters.

What does a zone of inhibition around a chemical-saturated disc indicate? The chemical is not effective at inhibiting growth of the test organism. The chemical is effective at inhibiting growth of the test organism.

The chemical is effective at inhibiting growth of the test organism.

Why does the lid of the agar plate have to be removed? Removing the lid allows one to see if the UV light is shining on the specimen. Removing the lid prevents the microbe from overheating during UV radiation. Removing the lid prevents contamination of the agar surface from contaminants in the air. UV light cannot easily penetrate the plastic lid so it has to be removed.

UV light cannot easily penetrate the plastic lid so it has to be removed.

What is the purpose of this paper card? You can change the length of exposure with the card to determine the shortest length of time as the control. UV light cannot penetrate the card so half of the plate is covered and acts as a control. UV light is attracted to the visible light spectrum so the damaging effect is limited to the bacteria underneath the card, leaving the other half as a control.

UV light cannot penetrate the card so half of the plate is covered and acts as a control.