Question | Answer |
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. |