Microbiology
Respiratory Tract Infections
Respiratory infections for MBBS: pharyngitis, diphtheria, pneumonia, tuberculosis and respiratory viruses with sputum and AFB diagnosis, mapped to NMC codes MI6.1 to MI6.3.
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Respiratory Tract Infections
Respiratory infections for MBBS: pharyngitis, diphtheria, pneumonia, tuberculosis and respiratory viruses with sputum and AFB diagnosis, mapped to NMC codes MI6.1 to MI6.3.
This chapter covers infections of the upper and lower respiratory tract. It deals with pharyngitis, diphtheria, otitis media and sinusitis, community and hospital-acquired pneumonia, tuberculosis and its national programme, and the viral and fungal causes of respiratory disease, with their laboratory diagnosis.
High-yield: Respiratory Tract Infections
- Streptococcus pyogenes is the chief bacterial cause of pharyngitis and its untreated infection can lead to rheumatic fever.
- Corynebacterium diphtheriae produces a grey pseudomembrane in the throat and an exotoxin causing myocarditis and neuropathy.
- Streptococcus pneumoniae is the commonest cause of community-acquired pneumonia and shows Gram-positive lancet-shaped diplococci.
- Klebsiella pneumoniae causes a severe pneumonia with red-currant-jelly sputum, especially in alcoholics and diabetics.
- Mycoplasma pneumoniae causes atypical walking pneumonia and is associated with cold agglutinins.
- Legionella pneumophila causes pneumonia linked to contaminated water and air-conditioning systems.
- Mycobacterium tuberculosis is an acid-fast bacillus diagnosed by Ziehl-Neelsen stain and cartridge-based nucleic acid tests.
- The GeneXpert MTB/RIF assay rapidly detects Mycobacterium tuberculosis and rifampicin resistance.
- Hospital-acquired and ventilator-associated pneumonia often involve Pseudomonas aeruginosa and other Gram-negative bacilli.
- Respiratory syncytial virus is the leading cause of bronchiolitis in infants.
- Influenza virus causes seasonal epidemics and antigenic shift can trigger pandemics.
- Bordetella pertussis causes whooping cough with a paroxysmal cough and is prevented by the DPT vaccine.
- Aspergillus can cause invasive pulmonary infection in the immunosuppressed and colonises pre-existing lung cavities as a fungus ball.
- Sputum Gram stain and culture guide the diagnosis of bacterial lower respiratory infection.
- The tuberculin skin test and interferon-gamma release assays detect exposure to Mycobacterium tuberculosis but not active disease.
Pneumonia by setting
- **Community-acquired, typical:** Streptococcus pneumoniae, Haemophilus influenzae, Klebsiella pneumoniae.
- **Community-acquired, atypical:** Mycoplasma pneumoniae, Legionella, Chlamydophila pneumoniae.
- **Hospital-acquired:** Pseudomonas aeruginosa, Klebsiella, Staphylococcus aureus.
- **Immunosuppressed:** Pneumocystis jirovecii, Aspergillus, cytomegalovirus.
NMC competencies in this chapter
- **MI6.1:** Aetiology and pathogenesis of upper and lower respiratory tract infections
- **MI6.2:** Tuberculosis, pneumonia and laboratory diagnosis of respiratory infections
- **MI6.3:** Prevention, immunisation and national tuberculosis programme
Frequently Asked Questions
Which organism most commonly causes community-acquired pneumonia?
Streptococcus pneumoniae is the leading cause of community-acquired pneumonia and appears as Gram-positive lancet-shaped diplococci that are optochin sensitive.
How is tuberculosis diagnosed rapidly?
The Ziehl-Neelsen stain shows acid-fast bacilli in sputum, while the cartridge-based GeneXpert MTB/RIF assay rapidly detects both the organism and rifampicin resistance.
What is atypical pneumonia?
Atypical pneumonia is caused by organisms such as Mycoplasma pneumoniae, Legionella and Chlamydophila, which do not respond to standard beta-lactams and often present with prominent constitutional symptoms and a walking illness.
Why is diphtheria dangerous?
Corynebacterium diphtheriae forms a throat pseudomembrane that can obstruct the airway and releases an exotoxin that damages the heart and nerves, which is why immunisation with the DPT vaccine is essential.
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