Micro lab exam 2

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What is the consequence of leaving a stain on the bacterial smear to long? (overstaining)

In a simple stain, probably the worst consequence is that the cell may appear larger than it really is.

What is the consequence of not leaving a stain on the smear long enough?

Understanding will result in the cells being difficult to see.

Consider a coccus and a rod of equal volume, which is more likely to survive in a dry environment?

Cocci, with their low surface to volume ratio are less efficient at exchange with the environment than rods, but are at an advantage in a dry environment where they lose water dehydrate more slowly than rods.

Consider a coccus and rod of equal volume, which is more likely to survive in a moist environment?

Organisms with high surface to volume ratio (rods,spirilla) often survive better in moist environments where their ability to exchange materials with their surroundings is an asset for nutrient acquisition of water loss is not a concern.

failure to add iodine in a gram stain will affect

probable decolorization of gram positives and no noticeable effects on gram negatives

failure to apply the decolorizor in a gram stain

all cells will appear purple since the crystal violet stains both gram positive and gram neg cells and the safranin is not likely to be seen over the darker crystal violet.

failure to apply the safranin in a gram stain

gram positives will be purple and the gram negatives will be colorless since they have been decolorised but not counterstained

Reversal of crystal violet and safranin stains

a mess. Safranin will probably be washed out during decolorization, so both gram positive and gram negatives will be colorized by the "counterstain" crystal violet. Even if the safranin os not completely washed away, the darker crystal violet will cover the lighter safranin.

Both crystal violet and safranin are basic stains and may be used to do simple stains on gram positive and gram negative cells. This being the case explain why they can be used in a differential staining technique

It is the ability of gram positive cells to retain the crystal violet when subjected to alcohol decolorization. This ability is enhanced with the use of the mordant iodine.

if you saw large eukaryotic cells in the preparation made from your gumline, they were most likely your own epithelial cells. Are you gram positive or gram negative?

Normal people are gram negative. It is the thick peptidoglycan of gram positive walls that makes them able to retain the crystal violet during decolorization. In fact, most cells, prokaryotic and eukaryotic are gram negative.

are acid fast negative cells stained by carbolfuchsin? if so can this be a differential stain?

Yes it can be a differential stain because the carbolfuchsin is easily removed from the walls of the acid fast negative cells, but is locked into the mycolic acid fast positive cells. This is a differential test because of the decolorization.

Why cant flagella be observes in action

They are too thin to be seen on a wet mount, and staining them to see them kills the cells.

flagella have a diameter of about 1nm. A hypothetical question: In order to resolve flagella with the maximum wavelength of the electromagnetic spectrum that would have to be used to create the image (given the numerical apertures of 1.25 for both the condenser and the oil lense) ?

2.5 nm

curved rods


short rods


flexible spirals



a variety of cell shapes may be seen in a given sample


a third division plane perpendicular to the other two produces a cube shape arrangement of eight cells called sarcina

tetrads and sarcina are seen only in


mycolic acid is

a waxy substance that gives acid fast cells a higher affinity for the primary stain and resistance to decolorization by an acid alcohol solution.


with flagella at both ends


turfs of flagella at the end of the cell


with flagella emerging from the entire cell surface

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