I am a second year medical student. I am weak at my microbiology subject. Help me to explain the topic pharyngitis. ... pharyngitis and take the content from Apoorva Shastri.
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I now have all the key content. Let me note that "Apoorva Shastri" is not a book in the library - this appears to be a reference to a popular Indian medical microbiology textbook/notes. The available standard microbiology textbooks (Medical Microbiology 9e by Murray et al.) cover the same content. Let me now compile a comprehensive, student-friendly explanation.
Note on Apoorva Shastri: The medical library does not have a book specifically authored by Apoorva Shastri. However, the content below is drawn from Medical Microbiology 9e (Murray et al.), which covers pharyngitis with the same depth as standard Indian medical microbiology curricula. The core microbiology facts are identical across sources.
Pharyngitis - A Complete Microbiology Guide for 2nd Year MBBS
What is Pharyngitis?
Pharyngitis means inflammation of the pharynx (the back of the throat). It is one of the most common infections seen in clinical practice. From a microbiology standpoint, you need to know the causative organisms, how they cause disease (pathogenesis), how to diagnose it, and how to treat it.
1. Etiology (Causative Organisms)
Pharyngitis can be caused by viruses, bacteria, or fungi.
Viral Causes (Most Common - ~70% of cases)
| Virus | Notes |
|---|---|
| Rhinovirus | Most common cold virus; often accompanies pharyngitis |
| Adenovirus | Causes pharyngoconjunctival fever (pharyngitis + conjunctivitis + fever) |
| Epstein-Barr Virus (EBV) | Infectious mononucleosis - "mono" or "glandular fever" |
| Influenza A & B | Flu-associated pharyngitis |
| Parainfluenza | Common in children |
| Herpes Simplex Virus | Vesicular lesions on pharynx |
| Coxsackievirus A | Herpangina and hand-foot-mouth disease |
| HIV | Acute retroviral syndrome (primary HIV infection) presents with pharyngitis |
| Cytomegalovirus (CMV) | Mononucleosis-like syndrome |
Bacterial Causes (Important for exams!)
| Organism | Notes |
|---|---|
| Group A beta-hemolytic Streptococcus (GABHS) = Streptococcus pyogenes | Most important bacterial cause - pharyngitis, tonsillitis, scarlet fever |
| Non-Group A beta-hemolytic Strep (groups B, C, G) | Less common bacterial pharyngitis |
| Corynebacterium diphtheriae | Diphtheria - grayish-white pseudomembrane (bleeds on removal) |
| Neisseria gonorrhoeae | Pharyngitis in sexually active individuals |
| Arcanobacterium haemolyticum | Pharyngitis + scarlatiniform rash |
| Mycoplasma pneumoniae | Atypical pathogen; also causes pneumonia |
| Chlamydia pneumoniae | Atypical pathogen |
| Mixed anaerobes (Fusobacterium, spirochetes) | Vincent's angina (ulcerative pharyngitis) |
| Yersinia enterocolitica | Pharyngitis + enterocolitis |
| Treponema pallidum | Secondary syphilis |
| Francisella tularensis | Oropharyngeal tularemia |
Fungal Causes
- Candida species - usually in immunocompromised patients (white patches/"thrush")
2. Focus on the Most Important Organism: Streptococcus pyogenes (GABHS)
This is the most tested organism for pharyngitis. Here's the complete breakdown:
Biology and Identification
- Gram-positive cocci arranged in chains
- Beta-hemolytic (complete hemolysis on blood agar - clear zone)
- Group A Lancefield classification
- Cell wall has:
- Group-specific carbohydrate - Group A antigen (C carbohydrate)
- Type-specific protein - M protein (most important virulence factor)
Virulence Factors
| Factor | Function |
|---|---|
| M protein | Antiphagocytic (resists opsonization); also helps adhesion |
| Capsule (hyaluronic acid) | Antiphagocytic; looks like "self" (molecular mimicry) |
| C5a peptidase | Inactivates complement, avoids phagocytosis |
| Lipoteichoic acid + F protein | Adhesion to host cells |
| Streptolysin O (SLO) | Oxygen-labile hemolysin; immunogenic → ASO test |
| Streptolysin S (SLS) | Oxygen-stable hemolysin; beta-hemolysis on plates |
| Streptokinase (Fibrinolysin) | Dissolves fibrin clots - helps spread |
| DNases (streptodornase) | Breaks down DNA in pus |
| Pyrogenic exotoxins (SPE A, B, C) | Superantigens - cause scarlet fever, toxic shock |
| Hyaluronidase | "Spreading factor" - breaks down connective tissue |
Epidemiology
- Pharyngitis is most common in children 5-15 years old
- Spread by respiratory droplets (person-to-person)
- Transient colonization occurs before protective antibodies develop
- Pharyngitis strains differ from pyoderma (skin infection) strains by M protein type
3. Clinical Features of Streptococcal Pharyngitis
Onset: abrupt, 2-4 days after exposure
Classic symptoms:
- Sore throat (sudden severe)
- Fever
- Malaise and headache
- Posterior pharynx is erythematous with exudate (pus)
- Cervical lymphadenopathy (swollen, tender neck nodes)
- Palatal petechiae
Important: Clinical features cannot reliably distinguish streptococcal from viral pharyngitis. You need lab tests!
Centor Criteria (used clinically to assess likelihood of GABHS)
- Tonsillar exudate
- Tender anterior cervical lymphadenopathy
- Fever (history or documented)
- Absence of cough (Each criterion = 1 point; 3-4 points = high probability)
4. Complications
Suppurative (Direct spread of infection)
- Otitis media (ear infection)
- Sinusitis
- Peritonsillar abscess (quinsy)
- Retropharyngeal abscess
- Mastoiditis
- Meningitis (rare)
Non-Suppurative / Post-Streptococcal (Immune-mediated - VERY IMPORTANT)
| Complication | Mechanism | Key Point |
|---|---|---|
| Acute Rheumatic Fever (ARF) | Autoimmune - antibodies against M protein cross-react with heart, joints | Follows pharyngitis only (not skin infection) |
| Acute Post-Streptococcal Glomerulonephritis (APSGN) | Immune complex deposition in kidney | Follows pharyngitis or skin infection |
Scarlet Fever (a complication of streptococcal pharyngitis)
- Caused when infecting strain carries a bacteriophage that codes for pyrogenic exotoxin (SPE)
- Appears 1-2 days after pharyngitis symptoms
- Features:
- Diffuse erythematous rash (starts upper chest → spreads to extremities)
- Circumoral pallor (area around mouth spared)
- Palms and soles spared
- "Strawberry tongue" - white coating shed to reveal red raw surface
- Rash blanches on pressure
- Pastia lines (rash prominent in skin folds)
- Rash fades in 5-7 days → desquamation
5. Laboratory Diagnosis
| Test | Details |
|---|---|
| Throat swab culture (blood agar) | Gold standard; shows beta-hemolysis; 24-48 hrs |
| Rapid Antigen Detection Test (RADT) | Detects Group A antigen directly; quick (minutes); ~70-90% sensitive |
| Gram stain | Not useful for pharyngitis diagnosis |
| Bacitracin sensitivity | S. pyogenes is sensitive (zone of inhibition); other streps are resistant |
| PYR test | S. pyogenes is PYR positive (L-pyrrolidonyl arylamidase) |
| ASO (Antistreptolysin O) titre | NOT useful for acute pharyngitis; used to confirm ARF or glomerulonephritis after pharyngitis |
| Anti-DNase B | Used to confirm glomerulonephritis after pharyngitis or skin infection |
Memory tip for identification: Catalase (-), PYR (+), Bacitracin sensitive, Group A antigen present = S. pyogenes
6. Treatment
| Situation | Drug of Choice |
|---|---|
| Streptococcal pharyngitis | Penicillin V (oral) or Amoxicillin |
| Penicillin-allergic patient | Oral cephalosporin or macrolide (azithromycin/clarithromycin) |
| Severe/systemic infections | IV Penicillin + Clindamycin |
Key points:
- Starting antibiotics within 10 days of pharyngitis prevents rheumatic fever
- No specific antibiotic treatment prevents post-streptococcal glomerulonephritis
- Prolonged asymptomatic carriage does NOT need treatment (antibiotics disturb normal flora)
- Patients with history of rheumatic fever need antibiotic prophylaxis before dental procedures (endocarditis prevention)
7. Quick Summary Table for Exams
| Feature | Details |
|---|---|
| Most common cause overall | Virus (rhinovirus) |
| Most common bacterial cause | S. pyogenes (GABHS) |
| Age group | Children 5-15 years |
| Spread | Respiratory droplets |
| Key virulence factor | M protein |
| Exotoxin (scarlet fever) | Pyrogenic exotoxin (SPE A, B, C) via bacteriophage |
| Diagnosis gold standard | Throat swab culture |
| Rapid test | RADT (Group A antigen) |
| Treatment | Penicillin V / Amoxicillin |
| Prevented by early treatment | Rheumatic fever (NOT glomerulonephritis) |
| Non-suppurative complication after skin OR throat | APSGN |
| Non-suppurative complication after throat ONLY | ARF |
A Note on "Apoorva Shastri"
The medical library does not contain a textbook authored by Apoorva Shastri. The content above is compiled from Medical Microbiology 9e (Murray, Rosenthal & Pfaller) and K.J. Lee's Essential Otolaryngology, which are standard reference textbooks. The facts align directly with the standard Indian MBBS microbiology syllabus (as taught by most professors and popular Indian textbooks).
- Medical Microbiology 9e, Murray et al. - Streptococcus pyogenes section
- K.J. Lee's Essential Otolaryngology - Acute Pharyngitis section
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