Gram-positive

**Gram stain mechanism:** Steps are crystal violet (primary stain), Gram's iodine (mordant, forms a crystal violet–iodine complex), alcohol or acetone decolorizer, then safranin (counterstain). Gram-positive cells have a thick, multilayered peptidoglycan wall that dehydrates and traps the complex during decolorization, so they stay purple. Gram-negative cells have a thin peptidoglycan layer and an outer membrane. The decolorizer dissolves the outer membrane and washes out the complex, so they take up safranin and appear pink/red.

**Cell envelope features (Gram-positive):** - Thick peptidoglycan layer (about 20–80 nm). - No outer membrane, so no LPS/endotoxin. Lipid A–mediated septic shock is therefore a Gram-negative feature. - Teichoic acids and lipoteichoic acids (LTA) are embedded in the wall. LTA anchors in the cytoplasmic membrane and can trigger TLR-2 and cytokine release. - A narrow periplasmic space. - Exotoxins are the key virulence factors (for example, diphtheria toxin, tetanus toxin, botulinum toxin, TSST-1, and alpha toxin).

**Clinical significance:** - The cell wall is the target of beta-lactams, vancomycin, and bacitracin. Gram-positive organisms are generally more susceptible to penicillin and vancomycin because there is no outer membrane barrier. Vancomycin does not cover Gram-negatives because it is too large to cross the outer membrane. - Lysozyme degrades peptidoglycan and is especially effective against Gram-positives.

**Classification (high-yield algorithm):** - **Cocci** - Clusters, catalase +: *Staphylococcus*. Coagulase + is *S. aureus*. Coagulase − is *S. epidermidis* (novobiocin sensitive) or *S. saprophyticus* (novobiocin resistant). - Chains/pairs, catalase −: - *Streptococcus*, which is alpha-hemolytic (*S. pneumoniae*: optochin sensitive, bile soluble; viridans: optochin resistant), beta-hemolytic (Group A *S. pyogenes*: bacitracin sensitive; Group B *S. agalactiae*: bacitracin resistant, CAMP +, hippurate +), or gamma-hemolytic (nonhemolytic). - *Enterococcus* (bile-esculin +, grows in 6.5% NaCl). *Enterococcus* and group D *Streptococcus bovis/gallolyticus* are both group D. *S. gallolyticus* is associated with colon cancer. - **Rods** - Spore-forming: *Bacillus* (aerobic: *anthracis*, *cereus*) and *Clostridium* (anaerobic: *tetani*, *botulinum*, *perfringens*, *difficile*). - Non-spore-forming: *Corynebacterium diphtheriae* (Chinese-letter/club-shaped rods, metachromatic granules; Elek test; toxin via β-prophage), *Listeria monocytogenes* (tumbling motility, facultative intracellular, cold growth, unpasteurized dairy; neonatal meningitis). - Filamentous/branching: *Actinomyces* (anaerobic, non-acid-fast, sulfur granules) and *Nocardia* (aerobic, weakly acid-fast).

**Caveats and exceptions:** - Some organisms stain poorly or are not visualized by Gram stain: *Mycobacteria* (acid-fast; have a waxy mycolic acid layer), *Mycoplasma* (no cell wall), *Chlamydia* (obligate intracellular), *Rickettsia*, *Treponema*, *Legionella* (stain poorly; use silver stain), and *Borrelia*. Only some of these are Gram-negative. - Older cultures of Gram-positive bacteria (especially *Clostridium*, *Bacillus*) may stain Gram-variable or Gram-negative as the wall degrades. - Over-decolorizing can make Gram-positives appear Gram-negative.

**Exam pearls:** - Exotoxin-mediated Gram-positive diseases: toxic shock syndrome, scalded skin syndrome, diphtheria, tetanus, botulism, anthrax, and pseudomembranous colitis. - Gram-positive cocci in clusters from a blood culture suggest *Staph* (look for IV drug use, endocarditis, or osteomyelitis). Treat empirically with vancomycin if MRSA is a concern (MRSA has altered PBP2a, encoded by *mecA*). - Teichoic acid and protein A (*S. aureus*) binds the Fc region of IgG and inhibits opsonization. - M protein (*S. pyogenes*) is antiphagocytic, and molecular mimicry underlies rheumatic fever.