Medical Microbiology: Bacterial, Viral, Fungal and Parasitic Pathogens
Introduction
This domain applies microbiology to medically important bacteria, viruses, fungi, and parasites. The exam tests specimen-to-pathogen reasoning, staining and culture clues, and the limits of direct microscopy and serology.
Core rules
- Bacterial meningitis in a neonate or child often shows neutrophils, low glucose, and high protein; Gram stain and culture of CSF are urgent.
- Mycobacterium tuberculosis is detected by acid-fast staining, culture, and nucleic acid amplification; smear sensitivity is limited.
- Viral diagnosis uses nucleic acid tests, antigen tests, and serology; IgM suggests recent infection but timing matters.
- Fungal diagnosis uses KOH mount, culture, and antigen tests; Candida isolated from a non-sterile site may be colonisation.
- Parasites are diagnosed by stool microscopy, concentration techniques, blood films, and antigen tests depending on the organism.
Worked example
CSF from a child shows 1200 white cells/µL with 85% neutrophils, glucose 20 mg/dL, protein 150 mg/dL, and Gram-negative diplococci. The likely diagnosis is meningococcal meningitis. The laboratory should notify the clinician immediately, subculture on chocolate agar, and perform susceptibility testing, because delay in treatment worsens outcome.
Traps
A lymphocytic CSF does not always mean viral meningitis; tuberculous and fungal meningitis can be lymphocytic. A negative acid-fast smear does not exclude tuberculosis. Growth of Candida from sputum or stool does not by itself prove invasive infection.