11Tues_Staph Strep Enteric

11Tues_Staph Strep Enteric - Lab Exercise Staphylococcal...

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Lab Exercise: Staphylococcal, Streptococcal & Enteric Unknowns OBJECTIVES 1. Identify each of the organisms worked with in this lab exercise. 2. Understand the use of differential and selective media in the identification of bacteria and the enrichment of such cultures for organisms of interest. 3. Understand the inoculations used throughout this laboratory exercise, use Table 4 as a study tool. 4. Understand the use of a dichotomous key and other data (including data tables) in determining species identification. BACKGROUND Staphylococcus The identification of bacterial pathogens, particularly from closely related non-pathogens is extremely important to the field of medical microbiology. In order to do this, selective and differential media are often used. Particularly when members of the normal flora can be opportunistic pathogens, it is important to have on hand easy ways of identification so that useful and appropriate treatment can be begun. The bacterial genus Staphylococcus contains three species of medical importance: S. aureus , S. epidermidis and S. saprophyticus . Of these three, only is typically considered to be pathogenic, except under special circumstances. All three, however, can be found residing on the skin and mucous membranes of healthy adults. Staphylococci are particularly well suited to this environment because they are facultative halophiles and can grow in media containing up to 10% NaCl. This capability can be used to isolate staphylococci from a mixed culture of organisms or from a patient sample It is important to note, however, that microscopic examination must confirm the presence of non-spore forming staphylococci before biochemical testing can be performed. Once a staphylococcal organism has been isolated, the species can be determined by performing a few simple biochemical tests, including mannitol fermentation. Because both salt tolerance and mannitol fermentation are useful criteria for identifying members of the staphylococci, a medium which examines both of these characteristics is commonly used, Mannitol Salt Agar (Figure 1). Table 1: Differential characteristics of the genus . Organism S. aureus S. epidermidis S. saprophyticus Coagulase positive negative negative Novobiocin Susceptibility sensitive sensitive resistant Mannitol Fermentation positive negative variable α -toxin Production positive negative negative Page 1 of 18
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Figure 1: The selective and differential medium, Mannitol Salt Agar. This medium is selective for salt- tolerant organisms, because it contains 7.5% NaCl and differential because the fermentation of mannitol in the medium results in a lowering of the pH and a change in the color of the pH indicator, phenol red, from reddish-pink to yellow. Both organisms are salt tolerant, but only the organism on the right is a mannitol fermenter. Streptococcus
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This note was uploaded on 12/23/2009 for the course BIO 205 taught by Professor Murphy during the Fall '09 term at Miramar College.

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11Tues_Staph Strep Enteric - Lab Exercise Staphylococcal...

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