Gram-Positive Bacilli
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Direct answer
Spore formation splits the Gram-positive bacilli into two families. The spore formers are Bacillus and Clostridium — anthrax with its polypeptide capsule and painless black eschar, the food-poisoning Bacillus cereus, and the clostridia behind tetanus, botulism, gas gangrene and pseudomembranous colitis. The non-spore formers carry equally quotable facts: Corynebacterium diphtheriae with metachromatic granules, Chinese-letter arrangement and the Elek test; Listeria monocytogenes with tumbling motility at room temperature and neonatal meningitis; and Actinomyces versus Nocardia, the anaerobic versus partially acid-fast pair. Almost every examination question here resolves to a staining property or a toxin mechanism.
What you must remember
- Anthrax: Gram-positive rods in long chains with a D-glutamate polypeptide capsule shown by McFadyean's methylene blue (halo of unmethylated blue); Medusa-head colonies; the Ascoli thermoprecipitin test detects antigen in old animal material; malignant pustule is painless with a black central eschar; inhalation disease is wool-sorters' disease.
- Bacillus cereus: emetic syndrome from reheated fried rice (heat-stable toxin, vomiting within 1–6 hours) versus diarrhoeal syndrome (enterotoxin, 8–16 hours) — timing separates it from Staphylococcus aureus and Clostridium perfringens.
- Tetanus: drumstick terminal spore; tetanospasmin blocks glycine and GABA release in the cord — spastic paralysis, risus sardonicus, nuchal rigidity with a clear sensorium; treat with human tetanus immunoglobulin, wound debridement, metronidazole and toxoid.
- Botulism: toxin blocks acetylcholine release at the neuromuscular junction — descending flaccid paralysis with cranial nerve palsies, classically after home-canned food; no fever, no sensory loss.
- Clostridium perfringens: lecithinase (alpha toxin) demonstrated by the Nagler reaction, half-plate covered by antitoxin; stormy fermentation in litmus milk; gas gangrene with crepitus.
- Clostridium difficile: antibiotic-associated pseudomembranous colitis; diagnose by toxin (cytotoxin assay or ELISA) in stool, treat with oral vancomycin or fidaxomicin.
- Diphtheria: metachromatic volutin granules stained by Albert's or Neisser's stain, Chinese-letter arrangement, growth on Loeffler's serum slope and black colonies on potassium tellurite; toxin confirmed by the Elek immunodiffusion test; toxoid vaccine, antitoxin plus erythromycin for disease.
- Listeria monocytogenes: small Gram-positive coccobacillus with tumbling motility at 22–25°C, umbrella-shaped spread in semisolid agar, cold enrichment, beta-haemolytic colonies; neonatal sepsis or meningitis, infection in pregnancy from soft cheese and ready-to-eat foods; treat with ampicillin plus gentamicin.
A typical exam case
A five-year-old from a migrant family has a sore throat, low-grade fever and a grey membrane over the tonsils that bleeds on removal. The stem wants the sequence: swab before antibiotics, Albert-stained smear showing green-black granules in green bacilli arranged like Chinese letters, Loeffler's serum slope for early growth, potassium tellurite agar for black colonies, and the Elek test to prove toxigenicity. Management is diphtheria antitoxin without waiting for confirmation, plus erythromycin, isolation and immunising contacts — because the antitoxin neutralises only circulating, unbound toxin.
Contrast that with a farm worker who steps on a nail: the organism is tetani, the toxin ascends retrogradely and cleaves synaptobrevin in inhibitory neurons, and the answer changes completely — immunoglobulin, toxoid booster, metronidazole and debridement.
Where students slip
The painless anthrax eschar is mistaken for a painful lesion, and the B. cereus emetic syndrome is confused with staphylococcal food poisoning — the rice clue settles it. The Nagler and Elek tests are frequently interchanged: Nagler belongs to C. perfringens lecithinase, Elek to diphtheria toxin. Finally, Actinomyces israelii (anaerobic, sulphur granules, penicillin) and Nocardia (aerobic, partially acid-fast, sulphur granules, co-trimoxazole) keep swapping answer options — the acid-fast stain in a modified Ziehl–Neelsen is the discriminator.
Frequently asked questions
Which stain demonstrates the capsule of Bacillus anthracis?
McFadyean's methylene blue, which shows the capsule as an amorphous purple halo around the bacilli in blood or tissue smears.
What is the Elek test?
An immunoprecipitation test in which a filter-paper strip of diphtheria antitoxin on a serum agar plate produces a line of precipitation where the strain's toxin diffuses — proof of toxigenicity.
How does Clostridium perfringens cause gas gangrene?
Its alpha toxin, a lecithinase, destroys cell membranes; anaerobic sugar fermentation and tissue gas produce crepitus, with rapid myonecrosis after contaminated wound trauma.
Why does Listeria show tumbling motility at 25°C but not at 37°C?
Its peritrichous flagella are expressed optimally at room temperature, giving characteristic tumbling in hanging-drop preparations and umbrella growth in soft agar — a defining laboratory quirk.
Which Clostridium causes antibiotic-associated colitis?
Clostridium difficile, whose toxins A and B damage the colonic mucosa after gut flora disruption; oral vancomycin is standard therapy for significant disease.