Biology Questions

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Qu. 1. In a sample of bacteria which stain positively with the Gram stain, not all organisms will appear stained with the same intensity. This can be because uptake of the stain was not taken up evenly by all cells, or that excessive washing out of the stain has occurred in some cells (History, 2004). It can also be due to a prolonged decolorization step in the procedure, which is the critical step for differentiating Gram-positive and Gram-negative organisms. During the Gram staining procedure, if washing or decolorizing is continued too long, all the stain may be removed from some cells, making them appear Gram-negative when they are actually Gram-positive. Most cocci are Gram-positive, and so in this case, there was probably some leaching of the stain during the washing step. Since this sample was from a pure culture, in a correctly applied Gram-stain, all cocci should have stained more or less equally.

Qu. 2. Selective media are media to which compounds have been added which will selectively inhibit the growth of certain microorganisms but not others (Growth, 2004). Differential media are media to which an indicator such as a dye has been added which allows the observer to differentiate between different organisms by their chemical reactions during growth because some of the organisms can use this ingredient but not others.

Eosin-methylene-blue (EMB) agar is both a selective and a differential medium which is often used for the isolation of Gram-negative enteric

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agar. Hydrogen sulfide production by bacteria can be identified by observing growth in a medium containing ferric iron. If sulfide is produced, ferric iron complexes with H2S and forms a black precipitate of iron sulfide. The utilization of citric acid by bacteria causes a pH rise and the incorporation of a dye in the medium causes a color change to occur which identifies the presence of these bacteria. Xylose lysine deoxycholate agar is a differential medium for the isolation of gram-negative enteric pathogens from fecal and other clinical specimens (Biol, 2004). MacConkey agar can be used to separate lactose-fermenting coliforms from non-lactose fermenting enteric pathogens. Qu. 3. Gram-negative bacteria appear to be more resistant to biocides than Gram-positive bacteria because of the impermeability of their outer membranes or cell walls (Russell, 2002, 599). Efflux systems may also be responsible for removing toxic drugs and molecules of biocide from the cells. Biocides are used mainly for external purpose at concentrations much higher than the minimum bactericidal concentrations of antibiotics, and such concentrations are unlikely to be effluxed from cells. However, ôlow residual concentrations could act could act a
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Some common words found in the essay are:
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Approximate Word count = 1222
Approximate Pages = 5 (250 words per page)

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