Why must cultures between 18 24 hours old be used for the Gram stain?

Why must cultures between 18 24 hours old be used for the Gram stain?

Gram stain is reliable only on cells from cultures that are in the exponential phase of growth. Older cultures contain more ruptured and dead cells. Cells from old cultures may stain Gram negative even if the bacteria are Gram positive.

How old should a culture be for the best gram reaction?

18-48 hours old
Some bacterial species tend towards gram variable, and will show both colors although most often gram +. Over decolorizing the smear, too long a time. Using old cultures (preferably, the cultures should be 18-48 hours old).

Why will an old gram-positive culture more than 48 hours old stain gram negative instead?

Why will gram-positive cells more than 24 hours old stain gram-negative? The wall begins to degenerate after 24 hours. No because the iodine allows the crystal violet stain to bind to the peptidoglycan in cell walls.

Why is it important to perform Gram staining on cultures younger than 24 hrs?

Why is it recommended that Gram staining be performed on young bacterial cultures (not older than 24 hours)? Over time, gram-positive cells lose their ability to retain the crystal violet stain after decolorization.

How does the age of a bacterial culture affect the interpretation of a Gram stain reaction?

Gram-positive bacteria have a thicker layer of peptidoglycan that retains the crystal violet-iodine complex. How does the age of a culture affect the gram stain reaction? Old cultures can convert to gram-variable or gram-negative giving erroneous results.

How is gram staining used to identify bacteria?

Gram staining differentiates bacteria by the chemical and physical properties of their cell walls. Gram-positive cells have a thick layer of peptidoglycan in the cell wall that retains the primary stain, crystal violet. They are stained pink or red by the counterstain, commonly safranin or fuchsine.

What is the general rule in Gram staining?

Bacteria with thick cell walls keep the first (purple) stain and are called Gram positive. Thin walled bacteria cannot keep the first stain (purple) so when the second stain (red) is placed on the organisms they become red or Gram negative. For a video of the Gram stain procedure click here .

In what way does the age of a culture affect the Gram stain reaction?

Old cultures tend to lose the peptidoglycan cell walls, which predisposes gram-positive cells to be gram-negative or gram variable. Gram stain is not useful for organisms without a cell wall like Mycoplasma species, and for smaller bacteria like Chlamydia and Rickettsia species.

What determines if a bacterial cell is Gram positive or Gram negative quizlet?

Because of the differences in the structure of their cell wall. Because of the thickness of peptidoglycan in their cell walls. Those with thick peptidoglycan in their wall are stained purple and are known as Gram positive. Those with thin peptidoglycan and an outer membrane stain pink and are known as Gram negative.

What is responsible for the differences in the Gram stain reactions of the above microorganisms?

The differences in cell wall composition of Gram-positive and Gram-negative bacteria account for the Gram staining differences. Gram-positive cell wall contains a thick layer of peptidoglycan with numerous teichoic acid cross-linking which resists the decolorization.

How does culture age affect the results of a Gram stain?

Home › Science › How Does Culture Age Affect the Results of a Gram Stain? How Does Culture Age Affect the Results of a Gram Stain? Culture specimens that are over 24 hours old may stain incorrectly, losing their ability to hold on to the crystal violet stain.

How to perform Gram staining on an unknown organism?

One should be a known Gram positive organism like Staphylococcus aureus . The other should be a known Gram negative organism like Escherichia coli . In the middle, make a smear of your unknown organism. Then perform the Gram stain as usual.

What are the conditions when Gram positive bacteria can appear Gram negative?

The Conditions When Gram Positive Bacteria Can Appear Gram Negative? When over-decolourized by either prolonged exposure to decolourizer or using acetone alone. When cell wall gets damaged by exposure to lysozyme or cell wall acting antibiotics such as Penicillin. Old cultures, where cell wall is weakened or action of autolytic enzymes.

What is the morphology of the unknown bacteria on this slide?

For this project you will only be classifying the unknown bacteria into 2 possible morphologies; cocci and bacilli. The bacteria on this slide are two different Gram reactions and two different species. What is the morphology of the bacteria at the end of each arrow: