Microbiology and Infectious Diseases
Items test pattern recognition from specimen and staining plus the choice of antimicrobial - increasingly framed around stewardship and infection control.
Core rules
- Gram stain divides bacteria by cell wall; culture and sensitivity guide therapy.
- Tuberculosis: sputum Xpert MTB/RIF is first line; AFB microscopy is cheap but less sensitive; extrapulmonary disease often needs biopsy or fluid testing.
- Malaria: thick film for detection, thin film for species; falciparum causes severe disease with high parasitaemia, hypoglycaemia and cerebral features.
- Dengue: NS1 antigen in the first five days; IgM later; warning signs are abdominal pain, vomiting, bleeding and fluid accumulation.
- Typhoid: blood culture is most useful in the first week.
- Sterilisation: autoclave at 121°C, 15 psi, for 15 minutes; hand hygiene remains the single most effective control measure.
Worked example
A child has fever and neck stiffness; cerebrospinal fluid shows 500 white cells with 90% neutrophils, glucose 30 mg/dL against a blood glucose of 90 mg/dL, and protein 120 mg/dL. This is bacterial meningitis - start empirical intravenous ceftriaxone after blood culture, and do not wait for the CSF culture.
Traps
- Sending dengue NS1 after day 7, when IgM is the right test.
- Using CSF lymphocyte predominance to diagnose bacterial meningitis.
- Prescribing antibiotics for every undifferentiated fever.
- Forgetting that TB meningitis has low glucose and raised protein with lymphocytes.