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Day 4 Handout: Control of Microbial Growth Today we are going to determine the susceptibility of the bacteria you isolated from the environment to antimicrobial agents, which are chemicals that stop bacterial growth. Antimicrobial agents fall into three general categories: antiseptics, disinfectants, and antibiotics. 1. Antiseptics can be used on living tissue. We will test 3% hydrogen peroxide and 10% antimicrobial handsoap. 2. Disinfectants are used on inanimate objects and are often too toxic to be used on living tissue. We will test 100% bleach. 3. Antibiotics are chemical substances produced by certain microorganisms which inhibit other microorganisms. Often, these are specific for bacteria and can be used inside and outside living organisms. We will test 3.5 mg/ml ampicillin. Materials
Procedure 1. Plan your experiment. You have 2 plates. Use 1 bacterial culture per plate and 1 antimicrobial agent per plate. You may want to test two different bacteria with the same antimicrobial agent or two different antimicrobial agents on one type of bacteria. Label the plates with your name, the date, the bacteria used, and the antimicrobial agent used. 2. Use a cotton swab to spread bacteria over the entire surface of the plate. 3. Wipe the tweezers with alcohol. Use the tweezers to pick up one sterile disk, dip it into the chosen antimicrobial agent, (try not to drip), and carefully drop it onto the middle of the plate that you just inoculated with bacteria. Wipe the tweezers with alcohol again. 4. The plates will be incubated at 30°C and you will get them back at the next meeting. You will use a ruler to measure and record the zone of inhibition. Data Record
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