Mannitol Agar Lab Report

Decent Essays
4-3 Mannitol Salt Agar is used to differentiate Staphylococcus areus from other microorganisms. The agar contains mannitol, salt, and a red pH indicator. Most organisms cannot grow in such a salty environment, but organisms of the genus Staphylococcus are uninhibited, making the agar selective for Staphylococcus. Therefore, if an organism is able to grow well on the agar, it is likely that it is Staphylococcus. The agar is differential because of the mannitol and the indicator. Only Staphylococcus aureus is able to ferment mannitol, which produces an acid end product. This acid accumulation will lower the pH of the agar plate, causing it to change colors to yellow. I was able to observe 3 organisms on a mannitol plate; one S. aureus that…show more content…
This agar contains phenylethyl alcohol, which damages their membrane permeability layer and ultimately disrupts their DNA synthesis. The thick cell walls of gram positive organisms allow them to resist this effect and grow uninhibited on the agar. I was able to observe a gram positive S. aureus grow abundantly on PEA and a gram negative E. coli grow very poorly on…show more content…
Some organisms produce an enzyme called catalase, which breaks down the toxic hydrogen peroxide (that is produced as a result of their electron transport chain) into water and oxygen gas. This test determines the presence of catalase in an organism by applying hydrogen peroxide directly to the organisms. If the tube immediately bubbles, it means that catalase was present and it turned hydrogen peroxide into water and oxygen gas. If it does not bubble, it means that catalase is absent and did not convert hydrogen peroxide into water and oxygen gas. I observed S. aureus bubble with the addition of hydrogen peroxide, meaning catalase was present. I also observed S. pyrogenes not bubble with the addition of hydrogen peroxide, meaning the organism does not produce
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