Bio205 Gram Staining Lab
1. Why do some cells stain gram positive and some stain gram negative? Explain
The difference between Gram-negative and Gram-positive bacteria lies solely primarily in their cell walls. Bacteria which do not retain Gram staining, Gram-negative bacteria have a lipopolysaccharide (LPS) membrane as their outermost membrane.
2. Did your results agree with the published information?
Yes.
3. Suppose you are viewing a sample from a PURE culture and observe both red and purple cocci. What are the possible problems that would cause such an inconclusive result?
The culture may be getting too old. When bacteria die their cell walls degrade and some may not retain the primary stain. The bacteria that haven't completely degraded may retain the primary stain. Therefore you would get red and purple cocci. Gram stains should be done in less than 24 hours of culture or can be assumed that the sample is contaminated; a pure culture is expected to stain a single color. Gram negative bacteria stain red and gram positive bacteria stain purple. Therefore the samples may be contained and may be not considered pure.
4. Since you cannot identify bacteria based on a Gram stain alone, why might a physician perform a Gram stain on bacteria taken a patient’s infection?
To identify which type of antibiotics to use. There are antibiotics that are gram specific and there are broad spectrum antibiotics that are effective against both gram negative and gram positive.
5. What is the purpose of Gram’s Iodine? Alcohol?
Gram’s Iodine "fixes" the crystal violet to any cell walls that have absorbed it. The alcohol is a decolorized. It washes away the crystal violet that has hasn't been absorbed and fixed to cell walls by the iodine.
6. Why is it necessary to counterstain?
To give gram negative the color. After decolorization, gram negative will lose the color and gram positive will retain the purple color. Counterstain solution will color the gram negative pink.
The difference between Gram-negative and Gram-positive bacteria lies solely primarily in their cell walls. Bacteria which do not retain Gram staining, Gram-negative bacteria have a lipopolysaccharide (LPS) membrane as their outermost membrane.
2. Did your results agree with the published information?
Yes.
3. Suppose you are viewing a sample from a PURE culture and observe both red and purple cocci. What are the possible problems that would cause such an inconclusive result?
The culture may be getting too old. When bacteria die their cell walls degrade and some may not retain the primary stain. The bacteria that haven't completely degraded may retain the primary stain. Therefore you would get red and purple cocci. Gram stains should be done in less than 24 hours of culture or can be assumed that the sample is contaminated; a pure culture is expected to stain a single color. Gram negative bacteria stain red and gram positive bacteria stain purple. Therefore the samples may be contained and may be not considered pure.
4. Since you cannot identify bacteria based on a Gram stain alone, why might a physician perform a Gram stain on bacteria taken a patient’s infection?
To identify which type of antibiotics to use. There are antibiotics that are gram specific and there are broad spectrum antibiotics that are effective against both gram negative and gram positive.
5. What is the purpose of Gram’s Iodine? Alcohol?
Gram’s Iodine "fixes" the crystal violet to any cell walls that have absorbed it. The alcohol is a decolorized. It washes away the crystal violet that has hasn't been absorbed and fixed to cell walls by the iodine.
6. Why is it necessary to counterstain?
To give gram negative the color. After decolorization, gram negative will lose the color and gram positive will retain the purple color. Counterstain solution will color the gram negative pink.