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Microbio Lab Midterm

Why is it essential that smears be air-dried? Why can't they be gently heated over a flame to speed up the drying process? Heating can denature or rupture the cell wall. Excess water may boil during fixation.
Why should you be careful not to overheat the smear during the heat fixing process? Overheating distorts the morphology of the cell through plasmolysis of the cell wall.
Why can't methylene blue be used in place of nigrosin for negative staining? Nigrosin is a negatively charged acidic stain whereas methylene blue is a positively charged basic stain.
What are the practical advantages of negative staining? The cell's natural size and shape can be seen because there is no heat fixation. It is also possible to see bacteria which are difficult to stain.
What are the advantages of differential staining procedures over the simple staining technique? Differential staining provides means to classify organisms as gram positive or gram negative.
Purpose of primary stain: To impart its color onto all cells.
Purpose of mordant: To increase the cells affinity for stain.
Purpose of decolorizing agent: To remove the excess dye and possibly the primary stain.
Purpose of the counterstain: Cells will assume color if primary stain is not held.
What is the function of water in spore staining? Water removes the excess primary stain and removes all color from vegetative cells.
Explain the medical significance of a capsule. Capsules are resistant to white blood cell phagocytosis and give the cell virulance properties.
Phenylethyl alcohol agar Isolation of gram + from gram - cells
Crystal violet agar Isolation of gram - from gram + cells
7.5% sodium chloride agar Selects for halophiles
Mannitol salt agar Selects for organisms which ferment mannitol (staphlococci)
MacConkey agar Isolation of gram - bacteria on ability to ferment lactose
Eosin-methylene blue agar Differentiation between lactose fermenters and E.coli more prevalent
Blood agar Selects for more fastidious organisms and those with hemolytic properties
Explain the mechanism by which buffers prevent radical shifts in pH. If the pH decreases, the weak base accepts protons to form a weak acid and vice versa.
Why are proteins and amino acids considered to be natural buffers? Amino acids and proteins are amphoteric and can either donate or accept protons.
Why are streptococcus species catalase negative but are capable of growth in the presence of oxygen? Streptococcus are facultative anaerobes.
Explain the chemical methods for measuring cell growth. Protein and DNA concentration, oxygen uptake
Why are microorganisms able to cause dairy products to curdle? Microorganisms can break down nutrients in the dairy, producing acid end products.
What is the purpose of the TSI test? Designed for the rapid speraration and identification of enteric organisms.
Explain the purpose of thiosulfate in the medium. Serves as a substrate for hydrogen sulfide production.
Explain why the test observations must be made within 18 to 24 hours after inoculation. Limited incubation prevents breakdown of proteins which may skew results.
Created by: goberoi



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