Medical Microbiology and Communicable Diseases
Domain 8 tests the causative agents, transmission and laboratory diagnosis of communicable diseases, plus general pathogenesis. Expect matching of pathogens to diseases and questions on specimen collection and antibiotics.
Core rules and concepts
- Pathogenicity is the ability to cause disease; virulence factors include capsules, toxins, adhesins and enzymes. Exotoxins are proteins, often heat-labile and specific; endotoxin is LPS from Gram-negative outer membranes.
- Bacterial examples: Mycobacterium tuberculosis (tuberculosis, airborne), Vibrio cholerae (cholera, faecal-oral, toxin-driven watery diarrhoea), Salmonella Typhi (typhoid, faecal-oral), Corynebacterium diphtheriae (diphtheria, toxoid-preventable), Clostridium tetani (tetanus, wound contamination).
- Viral examples: hepatitis B (blood/sexual), HIV (blood/sexual/vertical), influenza (respiratory). Not all infections are communicable; carriers can transmit without obvious illness.
- Diagnosis combines clinical features, microscopy, culture, serology and molecular tests. Antibiotics are ineffective against viruses.
Worked example
A patient with suspected pulmonary tuberculosis provides a sputum sample. Reasoning: the laboratory performs Ziehl-Neelsen staining for acid-fast bacilli and cultures on Lowenstein-Jensen medium; molecular testing may also be used. A routine Gram stain is not the primary test because mycobacteria do not stain reliably by Gram's method. The result must be interpreted with the clinical picture, not by Gram stain alone.
Mark-losing traps
- Do not confuse cholera with typhoid; both are faecal-oral but have different pathogens and presentations.
- Endotoxin is not an exotoxin; it is released from Gram-negative cell walls.
- A positive culture does not always mean disease; colonisation and contamination are possible.
- Antibiotics do not cure viral hepatitis or influenza.