Clinical Microbiology (Bacteriology, Virology, Mycology, Parasitology)
Clinical microbiology spans bacteriology, virology, mycology and parasitology. The exam tests specimen selection, staining, culture media, identification schemes, and how susceptibility results are reported and interpreted.
Bacteriology essentials
- Gram stain sequence: crystal violet, iodine, alcohol decolouriser, safranin. Report Gram reaction, shape and arrangement.
- MacConkey agar: lactose fermenters such as Escherichia coli form pink colonies; non-lactose fermenters such as Salmonella and Shigella are pale; bile salts inhibit Gram-positive organisms.
- Blood agar reveals haemolysis; chocolate agar supplies the X and V factors needed by Haemophilus influenzae.
- Core identification tests include catalase, oxidase, coagulase, bile esculin, urease and triple sugar iron.
Virology, mycology, parasitology
- PCR is the preferred test for acute viral infection; IgM suggests recent infection but can persist for months, IgG indicates past infection or vaccination.
- KOH mount for fungal elements; Sabouraud dextrose agar for culture; germ tube test for Candida albicans.
- Stool ova and parasite examination uses saline and iodine wet mounts plus formol-ether concentration; Giemsa-stained thick and thin films for malaria.
Worked example
CSF from a 19-year-old with fever and neck stiffness shows Gram-negative diplococci within neutrophils, low glucose and high protein. Treat as Neisseria meningitidis. Culture requires chocolate agar in 5% CO2; the organism is oxidase positive, catalase positive, and ferments glucose and maltose but not lactose. Telephone the Gram stain result immediately, because empirical therapy must not wait for culture.
Traps
Calling coagulase-negative staphylococci from one blood culture bottle a pathogen; accepting a sputum sample that microscopy shows to be saliva; reporting susceptibility without a standardised inoculum and incubation conditions.