Microbiology: Pathogens, Diagnostics and Antimicrobials
Microbiology: what is tested
Microbiology tests bacterial structure and staining, culture and identification, virulence and toxins, sterilisation, antimicrobial mechanisms and resistance, virology and basic immunology. Questions often start with a specimen and a clinical syndrome.
Core rules
- Gram-positive bacteria retain crystal violet because of thick peptidoglycan; Gram-negative bacteria have an outer membrane containing lipopolysaccharide. Ziehl-Neelsen staining identifies acid-fast mycobacteria.
- Exotoxins are heat-labile proteins (tetanus, diphtheria, cholera); endotoxin is heat-stable lipopolysaccharide and causes fever and shock. Sterilisation kills all organisms including spores; disinfection reduces pathogens but may not kill spores.
- Antibiotic targets: cell wall (beta-lactams, vancomycin), protein synthesis (aminoglycosides, macrolides), nucleic acid (fluoroquinolones), folate synthesis (co-trimoxazole). Resistance arises by enzymes, efflux, target change or reduced permeability.
- Viral replication requires host cells; HIV uses reverse transcriptase; hepatitis B is a DNA virus; influenza undergoes antigenic shift and drift.
Worked example. A 2-year-old has fever, neck stiffness and a petechial rash. CSF shows neutrophils, protein 150 mg/dL, glucose 30 mg/dL with serum glucose 100 mg/dL, and Gram-negative diplococci. Reasoning: bacterial meningitis caused by Neisseria meningitidis. Low CSF glucose and high protein fit bacterial infection. Household contacts need chemoprophylaxis, for example rifampicin for 2 days, ciprofloxacin as a single dose, or ceftriaxone as a single intramuscular dose.
Traps that cost marks
- Calling viral meningitis when CSF glucose is low.
- Forgetting Listeria coverage in neonates and older adults.
- Confusing sterilisation with disinfection.
- Treating meningococcal contacts with vaccination alone instead of chemoprophylaxis.