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USMLE Step 1MicrobiologyEP 03 of 14

Endotoxin vs exotoxin: lipid A, A-B toxins and toxoids

Endotoxin is a piece of the gram-negative outer membrane. An exotoxin is a protein that living bacteria secrete. Both can cause shock, but they differ in source, chemistry, mechanism and effect.

  • 5:46 video
  • 3 min read
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  • Published 15 Nov 2026
Video lesson · 5:46 · Watch on YouTubeSeries: Microbiology · @medservatory
Chapters (12)
By the end of this lesson

Contrast endotoxin (the lipid A of lipopolysaccharide) with exotoxins by source, chemistry, mechanism and clinical effect.

The short version

  1. Endotoxin is the lipopolysaccharide (LPS) of the gram-negative outer membrane, and its toxic part is lipid A.
  2. The O side chain varies between strains and is what antibodies see; lipid A is relatively conserved.
  3. Endotoxin acts through LBP, CD14 and the TLR4–MD-2 complex, which switches on NF-κB and releases TNF-α, IL-1β and IL-6.
  4. Exotoxins are secreted proteins with specific targets, often A-B toxins, mostly heat labile, and they can be made into toxoid vaccines.
  5. Endotoxin is a general alarm; an exotoxin is a precise tool.

Endotoxin vs exotoxin: two toxins, one question

Endotoxin vs exotoxin is a classic Step 1 contrast. One toxin is a protein, made and secreted by living bacteria. The other is part of the bacterial coat itself. Both can drive a patient into shock, so the task is to tell them apart and to explain why the coat behaves so differently.

The contrast runs along four lines: source, chemistry, mechanism and clinical effect.[1][3]

Endotoxin is LPS, and lipid A is the toxic part

Start from the gram-negative envelope. The outer membrane carries lipopolysaccharide, made of lipid A, a core polysaccharide and an O side chain. Endotoxin is that lipopolysaccharide, and only gram-negative bacteria have it.[1][2][3]

The O side chain differs between strains. Lipid A is relatively conserved across gram-negative genera, so its effects are broadly similar whichever organism makes it.[1][2]

Because endotoxin sits in the wall, it is released when the bacteria die and break up, and also when they divide.[1][6] It is also heat stable: it survives temperatures that denature most proteins.[1][3]

How endotoxin hurts: LBP, CD14, TLR4 and NF-κB

Lipopolysaccharide binding protein (LBP) in the blood picks up the LPS and hands it to CD14. CD14 presents it to the TLR4–MD-2 complex on macrophages.[4]

  1. LBPLipopolysaccharide binding protein in the blood picks up the LPS.
  2. CD14LBP hands the LPS to CD14.
  3. TLR4–MD-2CD14 presents it to the TLR4–MD-2 complex on macrophages.
  4. NF-κBSignalling switches on NF-κB.[5]
  5. CytokinesTumour necrosis factor α (TNF-α), interleukin 1β (IL-1β) and interleukin 6 (IL-6) are released.[5]

The result is fever, vasodilation, a fall in blood pressure, and activation of coagulation, which can end in septic shock and disseminated intravascular coagulation (DIC).[1][3][5]

Endotoxin is therefore a general alarm: much the same inflammatory response, whatever the organism.

Exotoxins: secreted proteins and A-B toxins

Exotoxins differ from endotoxin in almost every way that matters. They are proteins, secreted by living bacteria, both gram-positive and gram-negative.[1][3]

Many are A-B toxins: the B part binds a receptor, and the A part does the damage inside the cell.[1][7] Diphtheria toxin is the classic case. Its A subunit adds ADP-ribose to elongation factor 2, and protein synthesis stops.[1][7]

Because they are proteins, most exotoxins are heat labile. They are also potent: botulinum toxin is far more lethal than endotoxin.[1]

Toxoids: why exotoxins make vaccines

Proteins make good antigens. Treat an exotoxin with formaldehyde and you get a toxoid, which keeps its antigenicity but has lost its toxicity. That is the basis of the diphtheria and tetanus vaccines.[3][8][9]

Membrane-disrupting toxins and superantigens

Exotoxins also include membrane-disrupting toxins, which punch holes in host cells.[1]

A third class, after the intracellular-targeting toxins (which include the A-B toxins) and the membrane-disrupting toxins, is the superantigens. They bridge MHC class II on antigen-presenting cells to the T-cell receptor. Many T cells fire at once and release a flood of cytokines, which can cause shock.[1][3]

Endotoxin vs exotoxin side by side

Endotoxin compared with exotoxins
FeatureEndotoxinExotoxin
SourceGram-negative outer membrane onlySecreted by living bacteria, gram-positive and gram-negative
ChemistryLipopolysaccharide; lipid A is the toxic partProtein
HeatHeat stableMostly heat labile
MechanismLBP, CD14, TLR4–MD-2, NF-κB, cytokinesSpecific target, often an A-B toxin
EffectFever, low blood pressure, septic shock, DICDepends on the target (diphtheria toxin stops protein synthesis)
  1. Try a pairing: gram-negative rods in the bloodLipid A, TLR4, cytokines, fever, low blood pressure.
  2. Try a pairing: diphtheriaA secreted A-B toxin, blocked elongation factor 2, and a toxoid vaccine.

Practice question

Original · USMLE style
Your turnQB-MS-S1-MICRO-002-01

A 62-year-old woman is admitted with fever, flank pain and confusion. Temperature is 39.4 °C, blood pressure is 84/50 mmHg and heart rate is 118/min. Blood cultures grow gram-negative rods.

Which bacterial component most directly activates TLR4 on her macrophages?
Pick one option. Your answers stay on this device.

Common questions

What is the difference between endotoxin and exotoxin?

Endotoxin is the lipopolysaccharide of the gram-negative outer membrane, and its toxic part is lipid A. An exotoxin is a protein secreted by living bacteria, with a specific target.

Which part of lipopolysaccharide is toxic?

Lipid A is the toxic part. The O side chain is the variable part that antibodies recognise.

Why can exotoxins be made into vaccines?

They are proteins, so they make good antigens. Treating an exotoxin with formaldehyde gives a toxoid that keeps its antigenicity but has lost its toxicity, as in the diphtheria and tetanus vaccines.

What is an A-B toxin?

An A-B toxin has a B part that binds a receptor and an A part that does the damage inside the cell. Diphtheria toxin is the classic example: its A subunit blocks elongation factor 2.

Which receptor complex recognises endotoxin?

The TLR4–MD-2 complex on macrophages, after LBP and CD14 deliver the lipopolysaccharide. This switches on NF-κB and releases TNF-α, IL-1β and IL-6.

Transcript

Read the full narration, by chapter688 words

0:00Two toxins, one question

One toxin is a protein, made and secreted by living bacteria. Another is part of the bacterial coat itself. Both can drive a patient into shock. How do we tell them apart, and why does the coat behave so differently? Today: contrast endotoxin with exotoxins by source, chemistry, mechanism and clinical effect.

0:23Endotoxin is LPS: lipid A is the toxic part

Recall the gram-negative envelope. The outer membrane carries lipopolysaccharide, made of lipid A, a core polysaccharide and an O side chain. Endotoxin is that lipopolysaccharide, and only gram-negative bacteria have it. The toxic part is lipid A, not the sugars. The O side chain varies between strains and is what antibodies see. Lipid A is relatively conserved across gram-negative genera, so its effects are broadly similar whichever organism makes it.

0:57Release and heat stability

It sits in the wall, so it is released when the bacteria die and break up, and also when they divide. It is heat stable: it survives temperatures that denature most proteins.

1:09How endotoxin hurts: LBP, CD14, TLR4, NF-kB

How does it hurt? Lipopolysaccharide binding protein in the blood picks it up, hands it to CD14, and CD14 presents it to the TLR4 and MD-2 complex on macrophages. That switches on NF-κB and releases cytokines: tumour necrosis factor-α, interleukin-1β and interleukin-6. The result is fever, vasodilation, a fall in blood pressure, and activation of coagulation, which can end in septic shock and disseminated intravascular coagulation. Endotoxin is a general alarm: much the same inflammatory response, whatever the organism.

1:51Exotoxins: secreted proteins and A-B toxins

Exotoxins differ in almost every way that matters. They are proteins, secreted by living bacteria, both gram-positive and gram-negative. Many are A-B toxins: the B part binds a receptor, and the A part does the damage inside the

2:09Diphtheria toxin and elongation factor 2

cell. Diphtheria toxin is the classic case. Its A subunit adds ADP-ribose to elongation factor 2, and protein synthesis stops.

2:20Heat lability, potency and toxoids

Because they are proteins, most exotoxins are heat labile. They are also potent: botulinum toxin is far more lethal than endotoxin. And proteins make good antigens. Treat an exotoxin with formaldehyde and you get a toxoid, which keeps its antigenicity but has lost its toxicity. That is the basis of the diphtheria and tetanus vaccines.

2:47Membrane-disrupting toxins and superantigens

Exotoxins also include membrane-disrupting toxins, which punch holes in host cells. A third kind, the superantigens, bridges MHC class II on antigen-presenting cells to the T-cell receptor, so many T cells fire at once and release a flood of cytokines, which can cause shock.

3:08Try a pairing

Try a pairing. Gram-negative rods in the blood: lipid A, TLR4, cytokines, fever, low blood pressure. Diphtheria: a secreted A-B toxin, blocked elongation factor 2, and a toxoid vaccine.

3:26Practice question

Your turn. A 62-year-old woman is admitted with fever, flank pain and confusion. Her temperature is 39.4 degrees Celsius, blood pressure 84 over 50, and heart rate 118. Blood cultures grow gram-negative rods. Which bacterial component most directly activates TLR4 on her macrophages? A, the O-specific side chain of lipopolysaccharide at the cell surface. B, the polysaccharide capsule that surrounds the outer membrane. C, a secreted A-B protein exotoxin that acts inside host cells. D, the core polysaccharide that links lipid A to the side chain. E, the lipid A anchor of lipopolysaccharide in the outer membrane. Pause here and decide.

4:22Answer and explanation

A is out: the O side chain is the variable, antigenic part that antibodies see, not the toxic part. B is out: a capsule is not lipopolysaccharide, and lipopolysaccharide is what the TLR4 complex recognises. C is out: an exotoxin is a secreted protein with a specific target, not a general trigger of TLR4. D is out: the core is part of lipopolysaccharide, but the toxic activity belongs to lipid A. So the answer is E. Lipid A, anchored in the outer membrane, binds the MD-2 and TLR4 complex and triggers the cytokine release behind fever and shock.

5:08Recap and next lesson

Three things to keep. One: endotoxin is the lipid A of lipopolysaccharide, from gram-negative bacteria, heat stable, and it causes a general inflammatory response through TLR4. Two: exotoxins are secreted proteins with specific targets, often A-B toxins, mostly heat labile, and they can be made into toxoid vaccines. Three: lipid A is the toxic part, and the O side chain is the antigen. Next, bacterial toxins that hijack second messengers.

Sources

Numbers in the text, e.g. [1], point here
  1. OpenStax Microbiology. 15.3 Virulence Factors of Bacterial and Viral Pathogens. openstax.org/books/microbiology/pages/15-3-virulence-factors-of-bacterial-and-viral-pathogens
  2. OpenStax Microbiology. 3.3 Unique Characteristics of Prokaryotic Cells. openstax.org/books/microbiology/pages/3-3-unique-characteristics-of-prokaryotic-cells
  3. Wikipedia: Endotoxin, Exotoxin, Superantigen, Toxoid and Gram stain. en.wikipedia.org/wiki/Endotoxin · en.wikipedia.org/wiki/Exotoxin · en.wikipedia.org/wiki/Superantigen · en.wikipedia.org/wiki/Toxoid · en.wikipedia.org/wiki/Gram_stain
  4. LPS recognition by LBP, CD14 and TLR4-MD-2 (Europe PMC PMC3880462). europepmc.org/article/PMC/PMC3880462
  5. NF-kB induction of IL-1 beta, TNF-alpha and IL-6 (Europe PMC PMC5748601; PMC7904555). europepmc.org/article/PMC/PMC5748601 · europepmc.org/article/PMC/PMC7904555
  6. Release of LPS after lysis, division and vesicle formation (Europe PMC PMC13480389). europepmc.org/article/PMC/PMC13480389
  7. AB toxins (Europe PMC PMC3153263); diphtheria toxin ADP-ribosylates EF-2 (PMC11054074). europepmc.org/article/PMC/PMC3153263 · europepmc.org/article/PMC/PMC11054074
  8. CDC Pink Book. Tetanus (formaldehyde-inactivated toxoid). cdc.gov/pinkbook/hcp/table-of-contents/chapter-21-tetanus.html
  9. Ramon's toxoid principle (Europe PMC PMC10819242). europepmc.org/article/PMC/PMC10819242

The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.

Image credits. Illustrations: NIAID NIH BioArt Source (bioart.niaid.nih.gov), public domain, adapted (recoloured, outlined): Gram Negative Bacteria (item 179), Macrophage (item 311), Toll-like Receptor (item 700), Blood Vessel (item 55). Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (cropped, recoloured): antibody. Receptor membrane illustration by Servier via Bioicons (bioicons.com), CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/), adapted (recoloured). Icons: Health Icons (healthicons.org), CC0; blood sample tube by Marcel Tisch (via bioicons.com), CC0. The lipopolysaccharide molecules, the TLR4/MD-2 schematic, the cascade, the toxins and all handwriting are drawn in code (original).

Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-MICRO-002.

For exam study only, not medical advice. Read the medical disclaimer.

In this series

Playlist on YouTube
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  1. EP 01Gram-positive vs gram-negative cell walls5:39
  2. EP 02Acid-fast stain and the mycobacterial cell wall5:01
  3. EP 03Endotoxin vs exotoxin: lipid A, A-B toxins and toxoids5:46
  4. EP 04Bacterial toxins that raise cAMP: cholera, pertussis, anthrax5:54
  5. EP 05Clostridial toxins compared: tetanus, botulinum, alpha, C. difficile5:22
  6. EP 06Staph vs strep: catalase, coagulase and haemolysis5:23
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  8. EP 08Bacterial genetics: how bacteria swap genes5:13
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