Gram-Negative Cocci & Gram-Positive Bacilli: Key Bacteria

Gram-negative cocci are almost entirely the genus Neisseria: kidney-bean-shaped diplococci that cause gonorrhoea and meningococcal meningitis. Gram-positive bacilli are a mixed group of rods including Corynebacterium diphtheriae (diphtheria), Listeria monocytogenes (foodborne meningitis), Bacillus anthracis (anthrax) and Bacillus cereus (food poisoning). Both groups contain organisms that vaccines have nearly defeated and that return wherever vaccination lapses.

This matters because gonorrhoea is one of the commonest sexually transmitted infections, meningococcal meningitis can kill a healthy teenager within hours, diphtheria still appears in under-vaccinated pockets, and listeriosis threatens pregnant women through contaminated food.

Key Concepts

• Neisseria gonorrhoeae: intracellular gram-negative diplococci inside pus cells; causes urethritis, cervicitis and neonatal eye infection; grown on selective Thayer-Martin medium; treated with single-dose ceftriaxone.
• Neisseria meningitidis: capsular polysaccharide defines serogroups (A, B, C, Y, W135); causes epidemic meningitis with a petechial rash and Waterhouse-Friderichsen adrenal bleeding; oxidase-positive; close contacts need chemoprophylaxis.
• Corynebacterium diphtheriae: club-shaped rods with metachromatic granules; its toxin causes a grey adherent pseudomembrane in the throat and bull neck; Elek's test detects toxin; treatment is diphtheria antitoxin plus erythromycin.
• Listeria monocytogenes: short rods with tumbling motility at room temperature; grows in the cold (cold enrichment); crosses the placenta and the blood-brain barrier; danger from unpasteurised milk and soft cheese.
• Bacillus anthracis: large non-motile rods with McFadyean's reaction (polychrome methylene blue shows red bacilli with purple capsules); causes malignant pustule, woolsorter's disease and mediastinal widening; Bacillus cereus causes the fried-rice vomiting syndrome.

How It Works

1. Diphtheria toxin's B subunit binds the throat cell surface and the whole toxin is engulfed.
2. The A subunit escapes into the cytoplasm and ADP-ribosylates elongation factor 2 (EF-2).
3. With EF-2 disabled, the ribosome cannot build proteins and the cell dies.
4. Dead cells, fibrin and bacteria pile up into the leathery pseudomembrane that can choke the airway.
5. Toxin absorbed into blood attacks the heart (myocarditis) and nerves (palatal and diaphragmatic palsy weeks later), which is why antitoxin must be given before it binds.

Worked Example

A college student dies of meningococcal meningitis, and 40 hostel contacts need chemoprophylaxis with rifampicin. Step 1: adult dose = 600 mg twice daily for 2 days; total per adult = 600 times 2 times 2 = 2,400 mg. Step 2: a 20 kg child gets 10 mg per kg twice daily = 200 mg per dose, for 2 days; total per child = 200 times 2 times 2 = 800 mg. Step 3: with 30 adults and 10 children, total rifampicin needed = (30 times 2,400) plus (10 times 800) = 72,000 plus 8,000 = 80,000 mg, which is 80 grams. Contacts are also offered the quadrivalent meningococcal vaccine for longer-term protection.

Common Misconceptions

• "Gonorrhoea always causes symptoms." Most infected women and many men have no symptoms at all, yet they transmit the infection and risk infertility, which is why partner treatment is mandatory.
• "Diphtheria is a disease of the past." It persists wherever DPT coverage drops
• "Listeria comes only from meat." Outbreaks trace to soft cheese, unpasteurised milk, deli salads and even ice cream; pregnant women are told to avoid these foods entirely.

Real-World Applications

• STI clinics: nucleic-acid tests for gonorrhoea plus single-dose ceftriaxone 500 mg and partner therapy.
• Meningitis outbreaks: mass chemoprophylaxis and quadrivalent ACYW135 vaccination.
• DPT immunisation: diphtheria toxoid in the childhood schedule, with boosters every 10 years.
• Food safety: pasteurisation and cold-chain control to prevent listeriosis.

Neisseria are the gram-negative diplococci of gonorrhoea and meningitis, while the gram-positive rods span diphtheria's toxin, Listeria's cold-loving invasion, and anthrax's spores. Shape, oxidase, granules and motility identify them; antitoxin, ceftriaxone and chemoprophylaxis defeat them.