cell wall

The cell wall is essential for bacterial survival, protecting against osmotic lysis and determining cell shape. It is composed of peptidoglycan (murein), a polymer of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) sugars cross-linked by peptide bridges (via transpeptidase/penicillin-binding proteins). Gram-positive bacteria have a thick peptidoglycan layer (retains crystal violet stain) with embedded teichoic acids that stimulate cytokine release. Gram-negative bacteria have a thin peptidoglycan layer sandwiched between an inner (cytoplasmic) membrane and an outer membrane containing lipopolysaccharide (LPS/endotoxin), which triggers strong innate immune responses via TLR4. Clinically, the cell wall is a major antibiotic target: beta-lactams (penicillins, cephalosporins, carbapenems) inhibit transpeptidase cross-linking; vancomycin blocks peptidoglycan polymer elongation by binding D-Ala-D-Ala; bacitracin inhibits dephosphorylation of the lipid carrier needed to move peptidoglycan precursors across the membrane. Mycobacteria have an atypical cell wall with mycolic acids (acid-fast), conferring resistance to standard Gram staining and many antibiotics. Cell wall-deficient organisms (e.g., Mycoplasma) lack peptidoglycan entirely and are resistant to beta-lactams. Understanding cell wall structure is fundamental for predicting antibiotic mechanism, resistance patterns, and Gram stain results—all high-yield for microbiology and pharmacology sections of Step 1.