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BIOL 2240 Lab 3 Practical - Microbiology lab test 3 study...

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Microbiology lab test 3 study guide 1) Antibiotics/antiseptics & disinfectants To make a lawn of bacteria, you have to use aseptic technique. First, you need to suspend your organism by gently swirling or shaking the tube that it is in. Then you need to remove the cap of the tube and hold it in your pinky (never put on table) and then flame the neck of the tube. After this, stick your swab in the tube to get a sample of your organism. Then flame the neck of the tube again and put the cap back on and put it away. Then you are going to keep a low angle and gently roll the swab without digging in the agar of a TSA plate. There should be no uncovered agar when you are done. The purpose of a lawn is to spread the bacteria all over the agar so that you can see if it will be allowed to grow or if it will be inhibited by different antimicrobial agents. The zone of inhibition is the “kill zone.” It is the area where no growth occurs if the agent being tested inhibits or kills the test organism. They form because the chemotherapeutic agent in the filter paper disk diffuses out forming a concentration gradient. If the agent kills or inhibits the organism, then the organism will not be able to grow where the agent has diffused out of the disk. Size of the zone of inhibition is proportional to the percent of the agent’s effect on bacteria; therefore the greater the zone, the stronger the compound and vice versa. Antibiotics: i. Broad spectrum antibiotics should be effective against Gram positive and Gram negative organisms. If it was a narrow spectrum antibiotic then it would only be effective against one of these. ii. To evaluate the effectiveness of an antibiotic & sensitivity of a bacterium by performing the Kirby-Bauer method and measuring the zones of inhibition. After you have measured the diameter of the zone of inhibition, you would look at Table 33 in the manual to determine if the bacteria are resistant, intermediate, or sensitive to a given antibiotic. For example, the zone of inhibition for Streptomycin with E.coli was 17mm. On the table, this indicates that E.coli is sensitive to Streptomycin.
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iii. Penicillin is supposed to be narrow spectrum against Gram positive cells; it inhibits cell wall synthesis. In our experiment it Staphylococcus epidermidis was sensitive to Penicillin, and E.coli and Pseudomonas putidi were resistant to it. Therefore, it was only effective against the gram positive bacteria in our experiment. Erythromycin is supposed to be narrow spectrum against gram positive cells; it inhibits protein synthesis. In our experiment, Staphylococcus epidermis was sensitive to it and so was Pseudomonas putidi. E.coli was resistant to it though. Tetracycline is a very broad spectrum antibiotic; it is restricted for young children (8 yrs or less) and pregnant women. Staphylococcus epidermidis was sensitive to it and E.coli and Pseudomonas putidi were intermediate. Streptomycin is primarily effective against Gram negative and Mycobacterium; it inhibits protein synthesis. E.coli,
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