MICROBIOLOGY - Coursework Example

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Microbiology Practical 1. Describe the full theory behind the catalase test. Why must plain agar be used? Catalase is an enzyme that converts hydrogen peroxide to water and oxygen. Aerobic and facultative organisms have developed various protective mechanisms against the toxic forms of oxygen (especially superoxide radicals as they can potentially inactivate vital cell components)…
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Download file to see previous pages Catalase test is used to identify organisms that are capable of producing catalase enzyme. The enzyme converts toxic H2O2 into water and oxygen and thereby prevent the formation of highly dangerous hydroxyl radicals. 2H2O2 Catalase 2H2O + O2 To test catalase activity, the culture must be of 18- 24 hour as old culture lose catalase activity and may display false negative test. The emerging bubbles in catalase positive test is due to the formation of oxygen gas. Aerobic bacteria in general display catalase positive test while most of the anaerobes are deficient in catalase enzyme indicating the sensitivity of anaerobic bacteria towards oxygen. E.g. Catalase positive organisms are Micrococcus spp. and Staphylococcus spp. Catalase negative organisms are Enterococcus spp. and Streptococcus spp. Bacto Agar or plain agar is used for the study as it contains calcium and magnesium. On the contrary, blood agar are unsuitable to carryout catalase test as blood contains catalase (Web: Catalase test, n.d.; Pelczar, 1993). 2. Explain the principle of Oxidase test. Oxidase test is a test for the presence of cytochrome oxidase (playing vital role in electron transport chain or ETC) which catalyses oxidation of reduced cytochrome by oxygen. Cytochrome oxidase transports electrons from the ETC to oxygen, the terminal electron acceptor. The principle is utilized to test the organisms for the production of cytochrome oxidase. To test this ability of microorganisms, artificial electron donors and acceptors are provided. As soon as the electron donor (1.0-1.5 percent solution of tetramethyl p-phenylene diamine hydrochloride) is poured over the colonies, it is oxidized by cytochrome oxidase. Oxidase positive colonies become maroon, purple and black in 10-30 minutes (Pelczar MJ. 1993). E.g. Members of Pseudomonadaceae and Enterobacteriaceae are oxidase positive (Pelczar, 1993). 3. Explain how Oxoid Chromogenic UTI media (Clear) works. Oxoid Chromogenic UTI media is a nonselective medium to isolate, differentiate and enumerate the pathogens prevalent in urinary tract and are responsible for urinary tract infection. The medium facilitates the differentiation and identification of E. coli and Enterococcus without undergoing confirmatory tests (Web. Dehydrated Culture Media, n.d.). The principle utilized is based on the difference in susceptibility prototypes of the microorganisms. For the efficacy of antimicrobial therapy, the organism must be identified to the species level. However, the prevalent species Enterococcus spp. and Escherichia spp. produce enzymes for the metabolism of glucosidase or lactose or both and aids identification. On the contrary other microorganisms may not produce enzyme for such substrate fermentation (Web. Dehydrated Culture Media, n.d.). The chromogen mixture present in the Oxoid Chromogenic UTI media (Clear) comprises artificial substrates or chromogens which are responsible for releasing different colored compounds when specific microbial enzymes released by Enterococcus spp. and Escherichia spp act on substrate molecules, thereby a differentiation process works for the detection of certain species or a group of microorganism (Isenberg and Garcia, 2004). For instance the chromogen X-glucoside is a substrate for ?-glucosidase enzyme of enterococci which form blue colonies while chromogen red-galactoside is utilized by enzyme ?-galactoside generated ...Download file to see next pagesRead More
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