Questions and Answers
What is the primary characteristic used to distinguish Gram-positive bacteria from Gram-negative bacteria?
Which of the following bacterial shapes is characterized as spherical?
What is the role of iodine solution in the Gram staining process?
What mistake could occur if over-decolorization happens during Gram staining?
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Which of the following bacteria would be classified as Gram-negative?
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During the Gram staining procedure, what is the purpose of applying safranin after decolorization?
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What is the function of heat-fixing in the Gram staining protocol?
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What type of bacteria are classified as spirochetes?
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Study Notes
Bacteria Classification Overview
- Bacteria are classified based on shape, cell wall structure, metabolism, and genetic characteristics.
Shape of Bacteria
- Cocci: Spherical forms; examples include Streptococcus and Staphylococcus.
- Bacilli: Rod-shaped; notable examples are Escherichia coli and Bacillus.
- Spirilla: Spiral or corkscrew-shaped; represented by Spirillum.
- Vibrios: Comma-shaped; for instance, Vibrio cholerae.
- Spirochetes: Flexible, spiral shapes; an example is Treponema pallidum.
Gram Staining Classification
- Relies on cell wall composition to classify bacteria into two categories:
- Gram-Positive: Thick peptidoglycan walls; retain crystal violet stain; examples include Staphylococcus aureus and Streptococcus pneumoniae.
- Gram-Negative: Thin peptidoglycan walls with an outer membrane; do not retain crystal violet but absorb the counterstain (safranin); examples are Escherichia coli and Neisseria gonorrhoeae.
Protocol for Gram Staining
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Materials Required:
- Bacterial culture, glass microscope slides, inoculating loop, burner, crystal violet stain, iodine solution, 95% ethanol or acetone-alcohol, safranin or fuchsine, distilled water, staining rack, bibulous paper or towels.
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Preparation of Smear:
- Place water and aseptically transfer bacterial culture onto a slide.
- Spread thinly to create a smear, air dry, and heat-fix by passing through flame.
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Application of Crystal Violet:
- Cover smear with crystal violet for 1 minute, allowing it to stain.
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Application of Iodine Solution:
- Add iodine solution for 1 minute, forming a complex with crystal violet to help retain dye; rinse with distilled water.
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Decolorization Step:
- Angle slide and carefully apply decolorizer (95% ethanol/acetone) until runoff is clear (10-20 seconds).
- Excessive decolorization can remove stain from gram-positive bacteria; insufficient can fail to remove from gram-negative.
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Application of Safranin (Counterstain):
- Cover with safranin for 1 minute, staining decolorized gram-negative bacteria; rinse to remove excess.
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Drying the Slide:
- Conclude by allowing the stained slide to dry.
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Description
This quiz covers the classification of bacteria based on their shape, cell wall structure, metabolism, and genetic makeup. Explore the different types such as cocci, bacilli, spirilla, vibrios, and spirochetes. Test your knowledge of these classifications and their examples.
