Gram Stain and Bacterial Classification

Gram positive = purple (thick peptidoglycan). Gram negative = pink (thin peptidoglycan, LPS outer membrane). Shapes: cocci vs rods.

Detailed Explanation

Gram Positive (Purple/Blue):

Thick peptidoglycan wall

No outer membrane

Teichoic acids present

Key organisms:

**Cocci in clusters:** Staphylococcus (aureus, epidermidis)

**Cocci in chains:** Streptococcus (pyogenes, pneumoniae)

**Rods:** Bacillus, Clostridium, Listeria, Corynebacterium

Gram Negative (Pink/Red):

Thin peptidoglycan

Outer membrane with LPS (endotoxin)

Periplasmic space

Key organisms:

**Cocci:** Neisseria (meningitidis, gonorrhoeae)

**Rods:** E. coli, Klebsiella, Pseudomonas, Salmonella, Shigella

Special Cases:

Mycobacteria: Acid-fast (waxy mycolic acid)

Mycoplasma: No cell wall (can't Gram stain)

Spirochetes: Too thin to see (use dark-field)

Clinical Pearl:

Gram negative → LPS → septic shock

Gram positive → exotoxins and peptidoglycan → inflammation

Clinical Correlation

In suspected meningitis, Gram stain of CSF can rapidly distinguish between bacterial causes: Gram-positive diplococci (S. pneumoniae), Gram-negative diplococci (N. meningitidis), or Gram-positive rods (Listeria in immunocompromised/elderly).

Memory Trick

"Positive = Purple = Patricia keeps her POSITIVE attitude in PURPLE. Blue = Positive vibes."