The Streptococci and the Neisseria

Put the two enzyme lists side by side and the clinical difference writes itself.

S. aureus makes coagulase — it clots plasma and builds an abscess. S. pyogenes makes streptokinase, which lyses fibrin; hyaluronidase, which digests the ground substance holding tissue together; and streptodornase, which thins pus by degrading its DNA.

Every one of those dissolves a barrier. So streptococcal infection is not a boil — it is erysipelas and cellulitis, spreading through tissue with an advancing edge and thin, watery pus.

Streptokinase is, incidentally, the same molecule once given intravenously to dissolve coronary thrombi. And streptodornase is why dornase alfa — recombinant DNAase — thins the sputum in cystic fibrosis: pus is viscous because it is full of DNA from dead neutrophils.

The three-way division above is the answer to “how many ways can one bacterium make you ill?”

Pyogenic — the organism is present and invading. Sore throat, erysipelas.

Toxigenic — the pyrogenic exotoxin acts, giving the rash and strawberry tongue of scarlet fever.

Hypersensitive — and this is the one to understand rather than memorise. Weeks after the infection has resolved, the immune response turns on the patient. Unit 6 §5 gave both routes: M protein antigenically mimics heart tissue, so anti-M antibody cross-reacts and damages the heart — rheumatic fever; or immune complexes deposit in the glomeruli — acute glomerulonephritis.

That delay is why sore throats are treated with penicillin even though most resolve alone: not to shorten the pharyngitis, but to prevent the rheumatic fever that follows it.

The gonococcus's IgA1 protease. Secretory IgA is the antibody guarding every mucosal surface — the first line of defence at the urethra and the cervix. The gonococcus makes an enzyme that cleaves it. This is Unit 5's sixth virulence factor — mechanisms for escaping elimination by host defences — in its purest form, and it is shared by the meningococcus, H. influenzae and S. pneumoniae: all four are mucosal invaders.

Ophthalmia neonatorum. Infected from the mother during delivery is vertical transmission — the 2019 Section I term. A gonococcal conjunctivitis in the first days of life will destroy the cornea, which is why every newborn in most countries receives prophylactic eye drops. Note that the same route with Chlamydia trachomatis gives a later, milder conjunctivitis — a comparison the Chlamydia unit returns to.

And note that the meningococcus's main pathogenic factor is endotoxin — Unit 6 §3. The fulminant purpuric rash and shock of meningococcal septicaemia is lipid A acting on TLR4, not tissue invasion.