Gram-positive rods !
- Bacteria
Gram-positive rods: spore-forming clostridia and bacilli, corynebacteria, Listeria and the branching Nocardia and Actinomyces.
Updated 2026-11-01
What they are
This group gathers very different Gram-positive rods. The spore formers are Clostridium and Clostridioides (anaerobes) and Bacillus (aerobic); many cause disease through powerful toxins. Non-spore formers include Corynebacterium and Listeria, plus the branching filaments Nocardia (aerobic, weakly acid-fast) and Actinomyces (anaerobic).
What they share
- Spores: Clostridium, Clostridioides and Bacillus
- Exotoxins drive the disease: tetanus, botulism, diphtheria, anthrax
- Listeria: intracellular, grows in the fridge
- Branching: Nocardia (aerobic) and Actinomyces (anaerobic)
Microorganisms in this group
- Clostridium tetaniA soil bacterium whose spores enter through wounds and make a toxin that causes severe muscle spasms; vaccination prevents it.Exam priority: high
- Clostridioides difficileA bacterium that causes diarrhoea and colitis, mostly after antibiotics; its spores resist alcohol and are removed by washing with soap and water.Exam priority: high
- Clostridium botulinumA soil bacterium whose toxin, the most potent known, causes a flaccid paralysis that can stop breathing.Exam priority: medium
- Clostridium perfringensA bacterium that causes gas gangrene in deep wounds and a common food poisoning from reheated meat and gravy.Exam priority: medium
- Corynebacterium diphtheriaeA bacterium whose toxin can form a grey membrane in the throat and damage the heart and nerves; vaccination prevents it.Exam priority: medium
- Listeria monocytogenesA bacterium of chilled foods that can cause miscarriage, newborn sepsis and meningitis in pregnant women, newborns, older and immunosuppressed people.Exam priority: medium
- Bacillus anthracisA spore-forming bacterium of soil and livestock that causes skin, lung or gut anthrax; it does not spread between people.For completeness
- Bacillus cereusA spore-forming bacterium that causes food poisoning from reheated rice (fast vomiting) or from other foods (diarrhoea).For completeness
- NocardiaA filamentous soil bacterium that causes lung infection and brain abscess in immunosuppressed people, and skin infection after wounds.For completeness
- Actinomyces israeliiA normal mouth bacterium that, after an injury, causes chronic abscesses with sinus tracts draining yellow grains, mostly in the jaw and neck.For completeness
Test yourself
Think of the answer, then tap the question to see it.
Which Gram-positive rods form spores?
Clostridium, Clostridioides (anaerobes) and Bacillus (aerobic or facultative).
How does the paralysis of tetanus differ from that of botulism?
Tetanus causes spastic paralysis (it blocks inhibitory neurons); botulism causes flaccid paralysis (it blocks acetylcholine at the neuromuscular junction).
Which Gram-positive rod grows at fridge temperature and crosses the placenta?
Listeria monocytogenes.
How are Nocardia and Actinomyces told apart?
Nocardia is aerobic and weakly acid-fast; Actinomyces is anaerobic, not acid-fast and forms sulfur granules.
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Tetanus (WHO, 2026); Botulism (WHO, 2023); Diphtheria (WHO, 2025); C. diff: Facts for Clinicians (CDC, 2024).
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Representative model, for educational purposes only.