Microbiology Cheat Sheet
Upper Airway: Toxins & Swabs
Upper airway toxins and swabs for NEET-PG: GAS pharyngitis, diphtheria toxin and Elek test, pertussis PCR, and the otitis media SHM organisms.
MedNext Academy | 3 min read
Upper Airway: Toxins & Swabs
Upper airway toxins and swabs for NEET-PG: GAS pharyngitis, diphtheria toxin and Elek test, pertussis PCR, and the otitis media SHM organisms.
This topic covers the etiopathogenesis, laboratory diagnosis, and prevention of upper respiratory tract infections, with emphasis on toxin-mediated diseases and swab-based identification.
High-yield lines
- Streptococcus pyogenes is the most important bacterial cause of acute pharyngitis, and diagnosis prevents acute rheumatic fever.
- Throat swab culture on blood agar is the gold standard for GAS pharyngitis, but a routine throat Gram stain cannot diagnose it due to mixed oral flora.
- Corynebacterium diphtheriae shows club-shaped bacilli in Chinese-letter arrangement with metachromatic granules demonstrated by Albert stain.
- Diphtheria is a toxin-mediated disease where the tox gene is carried by a lysogenic beta-corynebacteriophage, the classic example of lysogenic conversion.
- Diphtheria toxin fragment A is an ADP-ribosyltransferase that inactivates elongation factor 2, arresting protein synthesis, and toxin production is maximal at low iron.
- The Elek gel diffusion test is the gold standard for diphtheria toxigenicity testing, and treatment with antitoxin must not await confirmation.
- Diphtheria culture uses Loeffler serum slope for enrichment and potassium tellurite blood agar giving grey-black colonies.
- Bordetella pertussis is a fastidious Gram-negative coccobacillus whose pertussis toxin ADP-ribosylates Gi protein, causing lymphocytosis.
- Tracheal cytotoxin destroys ciliated epithelium in pertussis, and the paroxysmal stage features whooping cough with post-tussive vomiting.
- Pertussis nasopharyngeal PCR is the most sensitive test and is preferred in the paroxysmal stage when culture sensitivity declines.
- The three predominant organisms of acute otitis media and sinusitis are Streptococcus pneumoniae, non-typeable Haemophilus influenzae, and Moraxella catarrhalis.
- Diphtheria myocarditis is the most common cause of death, and palatal palsy is the earliest neuropathic feature.
- Diphtheria toxoid and pertussis vaccines are given in India's UIP as part of the pentavalent vaccine.
- The Centor criteria (absent cough, tonsillar exudates, tender anterior cervical nodes, fever) estimate the probability of GAS pharyngitis.
Mapped competency codes
- MI6.1
- MI6.2
Continue into the full chapter
This summary maps to MI6-respiratory-tract-infections.
Frequently Asked Questions
Why can a throat Gram stain not diagnose streptococcal pharyngitis?
The oropharynx harbours viridans streptococci that are morphologically indistinguishable from GAS on smear, so culture or antigen testing is required.
What is the mechanism and gold-standard test for diphtheria toxin?
Fragment A ADP-ribosylates elongation factor 2 to halt protein synthesis, and the Elek gel diffusion test is the gold standard for toxigenicity.
Which test is preferred for pertussis in the paroxysmal stage?
Nasopharyngeal PCR, which is the most sensitive method as culture sensitivity falls sharply after the catarrhal stage.
What are the three main bacterial causes of acute otitis media?
Streptococcus pneumoniae, non-typeable Haemophilus influenzae, and Moraxella catarrhalis.
Continue reading
Cheat SheetsAll Microbiology cheat sheets
Browse the complete authored set for this subject.
Open the complete revision pathway
Explore clinician-written learning resources, structured revision and practice across the MedNext platform.
Open in the MedNext appSee plans