Bacteriology Lab report

Bacteriology Lab report - Mott 1 The Effect of...

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Mott 1 The Effect of Antimicrobial Substances on the Growth of Different Types of Bacteria Olivia Mott BSC 120-002
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Mott 2 Abstract The purpose of this experiment was to observe the effect of various antimicrobial substances on bacterial growth. To achieve this, bacterial lawns of E. coli , S. marcescens , and B. cereus were prepared. Each plate was then split into four parts. Small, paper disks were infused with different antimicrobial substances and placed on the three different plates. Each plate received a disk with tetracycline, a disk with ampicillin, a disk with Listerine, and a disk with Windex. The plates were then allowed to incubate for a week, after which the area clear of bacteria around each disk (called the zone of inhibition) was measured with a ruler and compared. Tetracycline produced the largest zone of inhibition in all three strains of bacteria. The Windex and the Listerine, generally produced smaller zones of inhibition, although the zone for ampicillin was smaller in S. marcescens . Ampicillin produced a large zone of inhibition for E. coli , and the zones for B. cereus were generally larger than the other two bacteria strains for all four substances. From this, I concluded that antimicrobial substances have more effect on Gram-positive bacteria like B. cereus , and that generally, antibiotics are more effective than disinfectants, though this may not be true for some very resistant, Gram-negative strains, like S. marcescens . Introduction Bacteria cells are very different from eukaryotic cells. They do not have the same metabolic processes nor do they have the same structure. This enables substances called antimicrobials to utilize these differences. By targeting processes or structures specific to bacteria, they can damage bacteria cells without harming the tissues of the human host (Casey 2012). An important factor in the effectiveness of these antimicrobial substances is the structure of the bacteria’s cell wall. There are two types: Gram-positive and Gram-negative, distinguished with the use of Gram staining. In Gram-positive bacteria, the cell wall surrounds the plasma membrane and is made up of peptidoglycans (Patel 2009). This layer of peptidoglycans is what the Gram stain adheres to. The cell wall of Gram-negative bacteria has an additional outer layer of lipopolysaccharides that surrounds the cell wall, and which the Gram
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Mott 3 stain cannot adhere to. This additional layer provides the bacteria protection from various antimicrobial and immunological substances (Casey 2012). In this lab, we used three strains of bacteria: E. coli , B. cereus , and S. marcescens . E. coli and S. marcescens are both Gram-negative bacteria (Gould 2009). B. cereus is Gram-positive (Yeo 2011). The antimicrobial substances we used were tetracycline, ampicillin, Listerine, and Windex. Tetracycline and ampicillin are antibiotics, which function by interfering with the growth of the cell wall, synthesis of proteins, or synthesis of nucleic acids in bacteria (Patel 2009). Both tetracycline
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