What is the difference between Gram-positive and Gram-negative bacteria?

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I remember our professor once brought this up during a lab session, and it really helped me understand how Gram-positive and Gram-negative bacteria differ—not just in color after staining, but in their structure, resistance to antibiotics, and overall behavior. This answer covers everything clearly, from peptidoglycan layers to LPS and clinical relevance.

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Anttilampinen 2025-06-01T19:04:06+00:00 1 Answer 3 views
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  1. fleming
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    2025-06-01T19:07:41+00:00
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    Gram Staining Procedure

    The Gram stain procedure involves treating bacterial cells with crystal violet dye, followed by an iodine solution (which forms a complex with the dye), decolorization with alcohol or acetone, and counterstaining with safranin.

    Gram-positive bacteria retain the crystal violet-iodine complex, appearing purple under microscopic examination, while Gram-negative bacteria lose this complex during decolorization and take up the counterstain, appearing pink or red.

    Structural Differences

    Gram-positive Bacteria

    • Possess a thick cell wall composed primarily of peptidoglycan (20–80 nm thick), constituting 50–90% of the wall.
    • Contains teichoic acids, which contribute to rigidity, ion exchange, and antigenicity.
    • Thick peptidoglycan layer traps the dye complex, resulting in a purple color during staining.

    Gram-negative Bacteria

    • Have a thin peptidoglycan layer (2–7 nm thick), forming only 5–20% of the wall.
    • Contain an outer membrane with phospholipids (inner leaflet) and lipopolysaccharide (LPS) in the outer leaflet.
    • The periplasmic space between membranes contains enzymes and transport proteins.
    • During staining, alcohol disrupts membranes, allowing dye complex to wash out, leading to pink/red appearance.

    Other Differences

    Antibiotic Susceptibility

    • Gram-negative outer membrane blocks many antibiotics, especially large or hydrophobic ones.
    • They possess beta-lactamases in the periplasm, degrading antibiotics.
    • Gram-positive bacteria are generally more susceptible to antibiotics like penicillin.
    • Polymyxins target Gram-negative bacteria by disrupting their outer membrane.

    Pathogenicity Mechanisms

    • Gram-positive bacteria secrete exotoxins and surface proteins for host adhesion.
    • Gram-negative bacteria use LPS (endotoxin) and secretion systems (Type III/IV) to affect host cells.

    Susceptibility to Physical and Chemical Agents

    • Gram-positive bacteria are more resistant to physical stress and desiccation.
    • Gram-negative outer membranes protect against detergents, bile salts, and enzymes.

    Nutritional and Metabolic Differences

    • Gram-negative bacteria utilize a broader range of carbon sources.

    Taxonomic Distribution

    Gram-positive Phyla Examples
    Firmicutes Bacillus, Staphylococcus, Streptococcus, Clostridium
    Actinobacteria Mycobacterium, Streptomyces
    Gram-negative Phyla Examples
    Proteobacteria Escherichia, Salmonella, Pseudomonas
    Others Bacteroidetes, Spirochaetes, Cyanobacteria

    Some bacteria, such as Mycobacteria, have features of both and are considered “Gram-variable” or “acid-fast”.

    Importance

    Understanding the differences between Gram-positive and Gram-negative bacteria is fundamental in clinical microbiology. It informs decisions about antibiotic treatment, infection control, and diagnostics. It also provides insight into bacterial evolution, physiology, and ecology.

    Source: Annual Review of Microbiology; Microbiology and Molecular Biology Reviews

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