Educational resource · Not a medical device · Infectious diseases review in progress · Does not replace clinical judgement

Diagram — Bacterial cell wall and Gram stain — SeptiMap

tell apart Gram-positive, Gram-negative, mycobacterial and Mycoplasma envelopes, and derive why some antibiotics cannot reach their target. Educational resource, not a medical device.

Diagrams: mechanisms and principles

17 sourced diagrams · for medical students and practice

Each diagram reads step by step: an objective, the steps, three key points, a classic pitfall, its sources and the related atlas pages. Revision mode hides the labels to test yourself.

Preclinical years

Bacterial cell wall and Gram stain

Objective: tell apart Gram-positive, Gram-negative, mycobacterial and Mycoplasma envelopes, and derive why some antibiotics cannot reach their target.

Step 6 / 6
Bacterial cell wall and Gram stain1. Gram-positive — Thick peptidoglycan, no outer membrane: the wall retains crystal violet. 2. Gram-negative — Thin peptidoglycan in the periplasmic space, beneath an outer membrane carrying LPS and porins. 3. Mycobacteria — Mycolic-acid-rich envelope: it resists acid decolorisation (acid-fast bacillus). 4. Mycoplasma — No cell wall: β-lactams, which target peptidoglycan synthesis, have nothing to act on. 5. β-lactams — They inhibit the penicillin-binding proteins (PBPs) that build peptidoglycan; in Gram-negatives they cross through porins. 6. Vancomycin — Glycopeptides act only on Gram-positives: the Gram-negative outer membrane is a permeability barrier.Gram +Thick peptidoglycanretains violetGram −Outer membrane, LPSporin →thin peptidoglycanMycobacteriaMycolic acidsacid-fastMycoplasmaMembrane only, no wallβ-lactams: no targetGram +Gram −β-lactamvancomycin
  • Pathogen
  • Structure
  • Toxin, damage
  • Host (cells, tissues)
  • Antibiotic

Key points

  • Gram-positive: thick peptidoglycan, no outer membrane.
  • Gram-negative: thin peptidoglycan and an outer membrane (LPS, porins), a permeability barrier.
  • No wall (Mycoplasma): no target for β-lactams.

Classic pitfall

Assuming vancomycin works on Gram-negatives because it targets the wall: the outer membrane stops it.

Related pathogen pages (6)

Sources

Educational content · Illustrative mechanisms · Not applicable to individual clinical decisions · MDR 2017/745

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