Describe Corynebacterium diphtheriae with its morphology, toxin, clinical features, laboratory diagnosis and prevention.

Morphology and Cultural Characteristics

• Gram-positive, non-motile, non-sporing, non-capsulated bacilli that are club-shaped (swollen at one end) and markedly pleomorphic
• Arranged at angles in "chinese letter" or cuneiform patterns, because the cells snap after division rather than separating cleanly
• Metachromatic granules (volutin, Babes-Ernst granules) of polymetaphosphate are demonstrated by ALBERT stain, appearing bluish-black against a green bacillus, and by Neisser and Ponder stains

Diphtheria Toxin

The tox gene is carried by a bacteriophage, not by the bacterium — lysogenic conversion by beta-phage → Toxin production requires a low iron concentration in the medium and in tissue → The toxin has two fragments — B binds the cell surface receptor and A is the active enzyme → Fragment A catalyses ADP-ribosylation of elongation factor 2 (EF-2) → protein synthesis stops in the affected cell → A single molecule can inactivate all the EF-2 in a cell, so the toxin is extraordinarily potent → Local necrosis produces the pseudomembrane; absorbed toxin damages heart, nerves and kidney

Clinical Features

• Faucial diphtheria is commonest — a tough, adherent, greyish-white pseudomembrane over the tonsils and pharynx, which bleeds when removed (unlike a candidal or streptococcal exudate). It consists of fibrin, necrotic epithelium, leucocytes and organisms
• "bull neck" from marked cervical lymphadenopathy and oedema, with severe toxaemia disproportionate to the modest fever
• Laryngeal diphtheria — hoarseness, stridor and respiratory obstruction from membrane extension, which may require tracheostomy; a particular danger in young children
• Other sites — nasal (often mild and a source of carriage), cutaneous (common in tropical countries and an important reservoir), conjunctival, genital
• Complications from absorbed toxin — myocarditis in the second week (arrhythmia, heart block, cardiac failure — the commonest cause of death); palatal palsy with nasal regurgitation and a nasal voice around the third week; and later polyneuritis including ocular and diaphragmatic paralysis; acute tubular necrosis

Laboratory Diagnosis and Tests for Toxigenicity

• Swabs from the edge of and beneath the membrane, and from the nose
• Direct smear — Albert stain for granules and Gram stain; suggestive but not confirmatory, since diphtheroids are normal throat flora
• Culture — Loeffler slope (rapid), tellurite (selective) and blood agar, examined at 6 to 8 hours and again at 24 to 48 hours
• ELEK test — an in-vitro immunoprecipitation test: a filter paper strip soaked in antitoxin is laid on the agar and the strain streaked at right angles; arrowhead lines of precipitation where toxin meets antitoxin indicate toxigenicity
• PCR for the tox gene — rapid, though it detects the gene rather than its expression
• In-vivo tests (guinea-pig inoculation, intradermal test) are of historical interest only
• SCHICK test — a historical intradermal test of susceptibility, measuring circulating antitoxin; now replaced by serology and no longer performed

Other Corynebacteria and Differential Diagnosis

Treatment, Prevention and Applied Aspects

• Antitoxin is given immediately on clinical suspicion, without waiting for laboratory confirmation, because it neutralises only circulating toxin — once the toxin has bound to tissue the damage cannot be reversed. Delay of even a day increases mortality substantially. A test dose is given, since it is horse serum
• Antibiotics (penicillin or erythromycin) eliminate the organism and stop further toxin production and transmission, but they do not substitute for antitoxin
• Supportive care — strict bed rest for weeks because of myocarditis, cardiac monitoring, airway management with tracheostomy if needed, and isolation until two negative cultures
• Prevention by toxoid — formalin-treated toxin that is immunogenic but non-toxic, given as DPT or pentavalent vaccine in India's Universal Immunisation Programme, with boosters. Immunity wanes, so adult boosters matter — the resurgence of diphtheria in the former Soviet Union followed a fall in adult immunity
• Contacts — culture, antibiotic prophylaxis, and a booster dose
• Diphtheria persists in parts of India in older children and adults with waning immunity and in under-vaccinated pockets, so it must remain in the differential of membranous pharyngitis; the diagnosis is clinical and treatment cannot wait
• Do not attempt to remove the membrane; it bleeds and increases toxin absorption, and may obstruct the airway
• Nurse the patient with strict bed rest for 2 to 3 weeks and monitor the electrocardiogram; myocarditis appears in the second week, when the throat is improving and the patient feels well enough to move about
• Cutaneous diphtheria is an important and neglected reservoir in tropical countries, maintaining transmission silently in communities where faucial disease is rare
• Immunisation does not prevent carriage, only disease — so a vaccinated population can still transmit, which is why outbreak control requires contact tracing and prophylaxis rather than vaccination alone
• Send swabs before starting antibiotics, but never delay antitoxin for the result; the laboratory confirms the diagnosis, it does not make it
• Antitoxin is horse serum, so a test dose is given and adrenaline kept ready; serum sickness may follow a week later, and neither risk justifies withholding it in suspected diphtheria
• Notify the case and investigate the contacts; diphtheria is a notifiable disease and a single case indicates a gap in immunisation coverage in that community
• Check and complete immunisation in every child presenting with any illness; opportunistic vaccination catches those missed by the routine schedule, and diphtheria is a disease of the unimmunised
• Isolate the patient until two consecutive negative cultures taken at least 24 hours apart and after antibiotics have finished
• Give a booster to close contacts as well as prophylaxis, since their immunity is likely to have waned in the same way as the index case's
• Toxoid is one of the oldest and most effective vaccines in use, and the near-disappearance of diphtheria from well-vaccinated populations is among the clearest demonstrations of what immunisation achieves
• Low iron promotes toxin production, which is a curious detail with a practical corollary: the organism is most dangerous in exactly the iron-poor conditions found at a mucosal surface
• Diphtheria antitoxin supply is limited worldwide, and delays in obtaining it contribute to mortality; hospitals in endemic areas should know where their nearest supply is held before they need it
• The Elek test remains the standard toxigenicity test in many laboratories, and its result determines whether contacts need prophylaxis and whether the case is notifiable as diphtheria
• Consider diphtheria in an unimmunised adult with a sore throat and disproportionate toxaemia, particularly with cervical swelling; the low fever is deceptive and delays the diagnosis
• Laryngeal obstruction may develop rapidly in a small child, so the airway must be assessed repeatedly and tracheostomy facilities kept available