Gram Positive Versus Gram Negative Bacteria

Feature	Gram Positive	Gram Negative
Peptidoglycan thickness	Thick (multiple layers)	Thin (single or few layers)
Outer membrane	Absent	Present
Lipopolysaccharide	Absent	Present (outer leaflet of outer membrane)
Teichoic acids	Present	Absent
Periplasmic space	Minimal/absent	Present, between inner membrane and outer membrane
Porins	Not required	Present in outer membrane, control permeability
Lipid content	Lower overall	Higher overall (outer membrane plus inner membrane)
Gram stain appearance	Purple (retains crystal violet-iodine complex)	Pink/red (counterstained with safranin)
Susceptibility patterns	Generally more susceptible to beta-lactams targeting peptidoglycan directly; vulnerable to agents disrupting the thick wall	Intrinsically more resistant to many antibiotics and disinfectants due to the outer membrane permeability barrier

Crystal violet stains all bacteria purple initially.
Iodine forms a crystal violet-iodine complex within the cell.
Alcohol (or acetone) decolourises: in Gram positive cells, the thick, multilayered peptidoglycan traps the complex, so the cell remains purple. In Gram negative cells, the thin peptidoglycan and the lipid-rich outer membrane are disrupted by the alcohol, allowing the complex to wash out.
Safranin counterstains decolourised (Gram negative) cells pink/red; Gram positive cells, already purple, are unaffected by the counterstain.

Thick peptidoglycan (up to 40 layers), providing the primary structural barrier and antigenic surface.
Teichoic acids and lipoteichoic acids embedded within/anchored to the wall, contributing to immune activation via TLR2.
No outer membrane, so no LPS, and no periplasmic space in the classic Gram negative sense.
Stains purple.
Because there is no outer membrane barrier, Gram positive organisms are generally more directly accessible to antibiotics that act on the cell wall or that must cross the membrane, though intrinsic and acquired resistance mechanisms still occur (e.g. altered penicillin-binding proteins in MRSA).

Thin peptidoglycan, located within the periplasmic space.
Outer membrane present, asymmetric bilayer with LPS on the outer leaflet.
Periplasm contains hydrolytic enzymes, including beta-lactamases, positioned to intercept antibiotics before they reach their target.
Stains pink or red.
The outer membrane, controlled by porins, is a major determinant of intrinsic antibiotic resistance: many large or hydrophobic drugs cannot cross it at all, and porin loss/mutation further reduces entry of drugs that otherwise could.

LPS (endotoxin), released from the outer membrane on bacterial lysis or during active growth, is recognised by host TLR4, triggering release of TNF-alpha, IL-1, and IL-6. Clinically this produces fever, hypotension, disseminated intravascular coagulation, and septic shock in severe Gram negative infection (full mechanistic detail in Microbial Toxins).
The outer membrane permeability barrier explains several recurring clinical patterns: Gram negative organisms are intrinsically resistant to vancomycin (too large to cross the outer membrane) and to many disinfectants that work well against Gram positive organisms; narrow-spectrum penicillins (e.g. benzylpenicillin) are largely ineffective against Gram negatives for the same reason, while agents specifically developed or modified to cross the outer membrane (aminopenicillins, later-generation cephalosporins, fluoroquinolones) extend coverage.
Gram positive organisms, lacking this barrier, are generally susceptible to a broader range of cell-wall-active agents from first principles, though clinically significant resistance (MRSA, VRE) still arises through other mechanisms covered in Antimicrobial Agents and Resistance.

Comparison Table