Anaerobic Bacteria
Classification ★★
| Group | Members |
|---|---|
| Spore-forming anaerobes | Clostridium — Gram-positive |
| Non-spore-forming anaerobes | Gram-positive and Gram-negative; cocci and bacilli |
General characteristics of Clostridium
- Gram-positive, spore-forming bacilli
- Obligate anaerobes
- Motile — peritrichous flagella (exception: C. perfringens is non-motile)
- The sporangia are swollen
- Typical clinical symptoms
Four species matter: C. tetani, C. botulinum, C. perfringens and C. difficile — which is precisely the four the 2020 paper asks for.
- How are anaerobes classified? → Spore-forming (Clostridium, G+) and non-spore-forming (G+ and G−, cocci and bacilli)
- Give five general characteristics of Clostridium. → Gram-positive spore-forming bacilli · obligate anaerobes · motile with peritrichous flagella · swollen sporangia · typical clinical symptoms
- Which Clostridium is non-motile? → C. perfringens
Clostridium tetani ★★★
| Characteristics | Detail |
|---|---|
| Spores | Terminal spores — the classic “drumstick” |
| Motility | Peritrichous flagella |
| Toxin | Tetanospasmin |
Pathogenicity
| Detail | |
|---|---|
| Portal of entry | A wound |
| Conditions of infection | A regional anaerobic environment: a deep and narrow wound, contamination with soil or foreign bodies, necrotic tissue, contamination with aerobes or facultative anaerobes |
| Virulence factor | Tetanospasmin — a protein (neurotoxin), heat-labile (65 °C for 30 min) |
⭐ The mechanism of tetanospasmin
Toxin → inhibitory interneuron → blocks the release of neurotransmitters from the presynaptic membrane of inhibitory interneurons, e.g. glycine and γ-aminobutyric acid → loss of inhibition to the motor neuron → spastic paralysis (rigid paralysis).
The transmitters to keep straight: excitatory — acetylcholine; inhibitory — glycine and γ-aminobutyric acid (GABA).
| Disease — tetanus | Detail |
|---|---|
| Latent period | 4–5 days to several weeks |
| Typical symptoms | Lockjaw · sardonic smile (risus sardonicus) · opisthotonos — all rigid paralysis |
| Neonatal tetanus | A frequent cause of death in developing countries. Most commonly from cutting the umbilical cord with unsterilised instruments, or infection of the umbilical stump. Fatality rate around 50%; the common cause of death is respiratory failure |
Immunity and control
| Detail | |
|---|---|
| Immunity | Antitoxin immunity, but weak — because the exotoxin is potent and combines rapidly with target cells. Toxoid vaccine (made with formaldehyde) |
| Proper care of wounds | Surgical debridement |
| Active immunisation | Tetanus toxoid. For children, basic immunisation with DPT — diphtheria toxoid, pertussis vaccine, tetanus toxoid. For high-risk groups, a toxoid booster |
| Passive immunisation | Tetanus antitoxin — urgent prevention, along with toxoid, as soon as possible |
| Special treatment | Administration of antibiotics · supportive measures |
The slide says something that should stop you: antitoxin immunity is weak, because the exotoxin is potent and combines rapidly with target cells.
Read that carefully. Surviving tetanus does not protect you from tetanus. The amount of toxin needed to cause the disease is far below the amount needed to immunise, and what there is binds to nerve endings before the immune system sees it.
Three consequences, all examinable:
• Every patient who recovers must still be actively immunised — the illness itself confers nothing.
• Antitoxin must be given as early as possible, because it can only neutralise toxin that is still free in the circulation. Once the toxin is bound to nerve, no antibody will retrieve it — which is why treatment is largely supportive and recovery waits for new nerve terminals to grow.
• Toxoid and antitoxin are given together at the time of a dirty wound: the antitoxin protects now, the toxoid protects next time. That is Unit 6 §2's passive/active pair in clinical practice.
- Describe C. tetani. → Gram-positive bacillus with terminal spores and peritrichous flagella, producing tetanospasmin
- What conditions are needed for infection? → A regional anaerobic environment — a deep narrow wound, soil or foreign-body contamination, necrotic tissue, co-infection with aerobes or facultative anaerobes
- Give the mechanism of tetanospasmin. → It blocks release of the inhibitory transmitters glycine and GABA from inhibitory interneurons, removing inhibition of the motor neuron → spastic (rigid) paralysis
- Name the typical symptoms. → Lockjaw, sardonic smile, opisthotonos
- Describe neonatal tetanus. → From cutting the cord with unsterilised instruments or umbilical stump infection; fatality around 50%, usually from respiratory failure
- How is tetanus prevented and treated? → Wound debridement; active immunisation with toxoid (DPT in children, boosters for high-risk groups); passive immunisation with antitoxin for urgent prevention, with toxoid, as soon as possible; antibiotics and supportive measures
Clostridium perfringens ★★★
| Characteristics | Detail |
|---|---|
| Spore | Subterminal endospore |
| Other structures | Capsule · non-motile — the exception among the clostridia |
| Cultivation | Anaerobic; carbohydrate (lactose) fermentation — “stormy fermentation” |
| Classification | Five toxigenic types, A through E, defined by which of the α, β, ε and ι toxins they produce |
Virulence factors
| Factor | Detail |
|---|---|
| α toxin | The most potent toxin; produced by ALL strains. Acts as a lecithinase → destroys erythrocytes, leukocytes and platelets → haemolysis and tissue necrosis. Diagnosis: the Nagler reaction on egg-yolk agar |
| Enterotoxin | Produced by types A (most), C and D; heat-labile |
| Others | Collagenase · haemolysin · proteinase · DNase |
Diseases
| Disease | Detail |
|---|---|
| Gas gangrene | Transmission: trauma. 60–80% of cases are type A. Manifestations: sudden onset, emphysema, oedema, necrotic tissue, foul-smelling discharge, toxaemia, shock |
| Food poisoning | Transmission: gastrointestinal tract. Type A. Short incubation period (10 hours), diarrhoea, self-limiting |
⭐ How gas gangrene develops — the deck's own final slide
Development of an anaerobic environment (e.g. a deep wound) → spores become vegetative cells → tissue destruction and necrosis; carbohydrate fermentation with gas (H₂, CO₂) formation and accumulation in the tissue → restriction of blood supply → increased tissue necrosis.
That arrow diagram is the whole disease, and the point is that the last arrow feeds the first.
The organism needs an anaerobic environment. It gets one from the injury. It then produces gas that accumulates in the tissue and compresses the blood supply — which makes the tissue more anaerobic, which favours the organism further, which produces more gas.
Each turn of the loop is faster than the last, and that is why gas gangrene advances in hours and why the treatment is surgical: you cannot out-prescribe a positive feedback loop with an antibiotic that cannot reach tissue with no blood supply. Debridement, and hyperbaric oxygen where available, break the loop at the point where it closes.
The α toxin is the accelerant: as a lecithinase it dissolves cell membranes — which are made of lecithin — so it lyses red cells, white cells and platelets at once. The Nagler reaction is that same enzyme demonstrated on egg-yolk agar, where the toxin produces an opaque halo.
- Describe C. perfringens. → Gram-positive bacillus with a subterminal endospore and a capsule; NON-motile; anaerobic with stormy lactose fermentation; five toxigenic types A–E
- What is the α toxin, and what does it do? → The most potent toxin, produced by all strains; a lecithinase destroying erythrocytes, leukocytes and platelets, causing haemolysis and tissue necrosis
- How is it demonstrated? → The Nagler reaction on egg-yolk agar
- Name the two diseases. → Gas gangrene (trauma, 60–80% type A) and food poisoning (type A, 10-hour incubation, self-limiting diarrhoea)
- Describe the pathogenesis of gas gangrene. → Anaerobic environment → spores germinate → tissue necrosis plus carbohydrate fermentation with H₂ and CO₂ accumulating → blood supply restricted → more necrosis, a self-accelerating loop
Clostridium botulinum ★★★
| Characteristics | Detail |
|---|---|
| Morphology | Gram-positive rod with a subterminal endospore |
| Botulinum toxin | A neurotoxin; relatively heat-labile — 100 °C for 10 min, or 80 °C for 20 min. Types A, B, C, D, E, F, G. The most potent toxic material known — 10 000 times more toxic than potassium cyanide |
| Mechanism | Blocks the presynaptic release of the neurotransmitter acetylcholine → flaccid paralysis |
Disease — botulism
From the Latin botulus, “sausage”. The sequence is endospore → germinate → toxin → flaccid paralysis, and there are three forms.
| Form | Transmission | Manifestation |
|---|---|---|
| Food poisoning | Ingestion of toxin-contaminated food — sausages, seafood products, milk and canned vegetables | Flaccid paralysis: double vision, dysphagia, difficulty breathing and speaking. Gastrointestinal symptoms are rare. Cause of death: respiratory failure |
| Infant botulism | Ingestion of organism-contaminated food — honey | Constipation, poor feeding, difficulty sucking and swallowing, weak cry, loss of head control — the “floppy baby”. Prevention: keep free of honey |
| Wound botulism | Trauma | Rare |
The toxin also has a medical use — blepharospasm among others.
Both are clostridial neurotoxins. Both block neurotransmitter release. They produce opposite paralysis, and the reason is which transmitter they block.
Botulinum toxin blocks acetylcholine — the excitatory transmitter at the neuromuscular junction. Block the signal to contract and the muscle cannot contract: flaccid paralysis. Floppy baby, drooping eyelids, double vision.
Tetanospasmin blocks glycine and GABA — the inhibitory transmitters, released by interneurons in the spinal cord. Remove the brake and the motor neuron fires without restraint: spastic (rigid) paralysis. Lockjaw, sardonic smile, opisthotonos.
One blocks the accelerator; the other cuts the brake cable. Same class of action, opposite site, opposite patient.
And both kill the same way — respiratory failure, whether the respiratory muscles are too limp or too rigid to move air.
- Describe botulinum toxin. → A neurotoxin, relatively heat-labile (100 °C/10 min, 80 °C/20 min), types A–G, the most potent toxic material known — 10 000 times more toxic than KCN
- Give its mechanism. → Blocks presynaptic release of acetylcholine → flaccid paralysis
- Name the three forms of botulism. → Food poisoning, infant botulism, wound botulism
- Describe food-borne botulism. → Ingestion of toxin in sausages, seafood, milk, canned vegetables; flaccid paralysis with double vision, dysphagia, difficulty breathing and speaking; GI symptoms rare; death from respiratory failure
- Why must infants avoid honey? → It may carry spores, which germinate in the infant gut and produce toxin — the floppy baby
- Contrast the two toxins' effects on muscle. → Botulinum blocks excitatory acetylcholine → flaccid paralysis; tetanospasmin blocks inhibitory glycine and GABA → spastic paralysis
Clostridium difficile ★★★
| Detail | |
|---|---|
| Virulence factors | Exotoxin A — an enterotoxin · exotoxin B — a cytotoxin |
| Disease | Pseudomembranous colitis — from dysbacteriosis · antibiotic-associated diarrhoea |
| Treatment | Discontinuation of the causative antibiotic · administration of sensitive antibiotics |
| Prevention | No vaccine. Use antibiotics only when necessary |
Dysbacteriosis was defined in Unit 5 §4 — the state in which the proportion of bacterial species and the number of the normal flora colonising a certain site present large-scale alteration. C. difficile is that definition's clinical illustration, and Unit 5 §3 gave the mechanism: broad-spectrum antibiotics remove the antagonism of the normal flora, and a resistant organism expands unopposed.
- Name C. difficile's two toxins. → Exotoxin A (enterotoxin) and exotoxin B (cytotoxin)
- Which diseases? → Pseudomembranous colitis and antibiotic-associated diarrhoea
- How is it treated and prevented? → Stop the causative antibiotic and give a sensitive one; there is no vaccine, so use antibiotics only when necessary
- Which earlier term explains it? → Dysbacteriosis — large-scale alteration of the normal flora
Non-spore-forming anaerobes ★★★
These include both Gram-positive and Gram-negative bacilli and cocci. They are members of the normal flora, and they cause endogenous infection.
| Group | Main genera | Habitat |
|---|---|---|
| Gram-negative bacilli | Bacteroides Fusobacterium | Colon Mouth, colon |
| Gram-positive bacilli | Bifidobacterium Propionibacterium Actinomyces Lactobacillus Eubacterium | Mouth, colon Skin Mouth Vagina Mouth, colon |
| Gram-positive cocci | Peptostreptococcus | Colon |
| Gram-negative cocci | Veillonella | Mouth, colon |
⭐ The four conditions causing disease
- Change of habitat
- Decrease of host defence
- Dysbacteriosis
- Local anaerobic environment formation
⭐ Characteristics of the infections
- Endogenous infection throughout the body, most chronic
- Non-specific manifestations, most pyogenic
- Foul-smelling discharge, sometimes with gas formation
- Direct smear positive, aerobic culture negative
- No response to some antibiotics, such as the aminoglycosides
Diseases: septicaemia · infections of the central nervous system · dental sepsis · pulmonary infections · intra-abdominal infections · infections of the female genital tract.
Of the five characteristics, that fourth one is the most useful at the bedside — and it is a favourite examination line.
Pus arrives at the laboratory. The Gram film shows organisms, often several morphologies at once. The routine aerobic plates grow nothing. That combination — you can see them but they will not grow — is close to diagnostic of an anaerobic infection, because routine culture is aerobic and simply cannot recover them.
Two more clues sit alongside it. Foul-smelling discharge, from the short-chain fatty acids of anaerobic metabolism — a distinctive smell that experienced clinicians diagnose from the doorway. And no response to aminoglycosides, which is not resistance in the usual sense: aminoglycoside uptake requires oxygen-dependent active transport, so the drug never enters an anaerobe at all.
- What are non-spore-forming anaerobes, and what kind of infection do they cause? → G+ and G− bacilli and cocci that are normal flora; they cause endogenous infection
- Name the four conditions causing disease. → Change of habitat · decreased host defence · dysbacteriosis · local anaerobic environment formation
- Name the five characteristics of the infections. → Endogenous and mostly chronic · non-specific, mostly pyogenic · foul-smelling discharge sometimes with gas · direct smear positive but aerobic culture negative · no response to some antibiotics such as aminoglycosides
- Which genus lives in the colon and is a G− bacillus? → Bacteroides
- Why do aminoglycosides fail? → Their uptake needs oxygen-dependent transport, which anaerobes lack
⭐ The deck's own five questions
C. perfringens — α toxin (lecithinase), enterotoxin, collagenase, haemolysin, proteinase, DNase → gas gangrene (60–80% type A) and food poisoning.
C. botulinum — botulinum toxin (blocks acetylcholine release) → botulism: food-borne, infant and wound.
C. difficile — exotoxin A (enterotoxin) and exotoxin B (cytotoxin) → pseudomembranous colitis and antibiotic-associated diarrhoea.
Four species, each with its named toxin and its named disease. That is the seven marks.
Treatment: antitoxin as soon as possible, antibiotics, and supportive measures. Explain why early antitoxin matters: it neutralises only free toxin, not toxin already bound to nerve.
Botulinum toxin → flaccid paralysis, by blocking presynaptic release of the excitatory transmitter acetylcholine at the neuromuscular junction.
Both kill by respiratory failure.
“A neurotoxin that causes flaccid paralysis is…” — b. botulinum toxin. (Tetanospasmin causes spastic paralysis; α toxin is a lecithinase, not a neurotoxin; enterotoxin acts on the gut.)
Revision
⭐ The four Clostridium species on one line each
| Species | Spore | Toxin | Disease |
|---|---|---|---|
| C. tetani | Terminal | Tetanospasmin — blocks glycine/GABA | Tetanus — spastic paralysis |
| C. perfringens | Subterminal; non-motile | α toxin = lecithinase | Gas gangrene · food poisoning |
| C. botulinum | Subterminal | Botulinum toxin — blocks acetylcholine | Botulism — flaccid paralysis |
| C. difficile | — | Exotoxin A (entero-) and B (cyto-) | Pseudomembranous colitis · antibiotic-associated diarrhoea |
- Answer the 2020 question. → C. tetani/tetanospasmin/tetanus · C. perfringens/α toxin/gas gangrene and food poisoning · C. botulinum/botulinum toxin/botulism · C. difficile/toxins A and B/pseudomembranous colitis
- Spastic or flaccid, and why? → Tetanospasmin blocks inhibition (glycine, GABA) → spastic; botulinum blocks excitation (acetylcholine) → flaccid
- What is the α toxin and how is it shown? → A lecithinase, the most potent C. perfringens toxin, produced by all strains; the Nagler reaction on egg-yolk agar
- Name the four conditions for non-spore-forming anaerobic disease. → Change of habitat · decreased host defence · dysbacteriosis · local anaerobic environment
- Which laboratory finding suggests an anaerobe? → Direct smear positive, aerobic culture negative — with foul-smelling discharge