Introduction to Microbiology
Microbiology: the study of microorganisms: bacteria, viruses, fungi, protozoa, helminths, ectoparasites, and prions.
- Medical microbiology: the subset concerned with organisms that cause human disease, their diagnosis, and their control.
| Group | Cell Type | Key Feature |
|---|---|---|
| Bacteria | Prokaryotic | Peptidoglycan cell wall (most species); 70S ribosomes |
| Viruses | Acellular | Obligate intracellular parasites; DNA or RNA genome, no independent metabolism |
| Fungi | Eukaryotic | Chitin cell wall; ergosterol membrane; yeasts, moulds, dimorphic forms |
| Protozoa | Eukaryotic | Unicellular; trophozoite and cyst stages |
| Helminths | Eukaryotic, multicellular | Nematodes, cestodes, trematodes |
| Ectoparasites | Eukaryotic, multicellular | Lice, mites, fleas; live on host surface |
| Prions | Acellular, non-nucleic-acid | Misfolded protein; no genome, no classic replication |
- Pathogen: an organism capable of causing disease in a host.
- Nonpathogen: an organism that does not cause disease under normal circumstances.
- This binary is an oversimplification in practice. Pathogenicity is context-dependent (host immune status, site of the organism, inoculum size, presence of other organisms), which is why the categories below (commensal, opportunistic) exist.
- Organisms that colonise the host without causing harm, typically deriving benefit (nutrients, habitat) while the host is unaffected.
- Form the bulk of the normal microbiota (covered in full in a dedicated later subsection).
- Not inert. Commensals actively contribute to colonisation resistance, competing with potential pathogens for the same niche.
- Organisms of low intrinsic virulence that cause disease only when host defences are impaired or normal anatomical barriers are breached.
- Examples of the underlying mechanism: neutropenia (loss of phagocytic defence), breach of skin/mucosa (surgery, catheters, burns), disruption of normal microbiota (antibiotic therapy), immunosuppression (HIV, transplant medication, chemotherapy).
- The same organism can be commensal in one host context and pathogenic in another. This context-dependence is a recurring theme throughout clinical microbiology.
- Organisms whose natural habitat is soil, water, or other non-host environments, encountered by humans incidentally.
- Most are nonpathogenic; some cause disease only via a specific exposure route (inhalation, wound contamination, ingestion), for example Legionella (water systems), Clostridium tetani (soil, via wound inoculation), Aspergillus (airborne spores, inhaled).
- Microbiology describes the organism.
- Immunology describes the host response to that organism.
- Infectious disease is the clinical discipline where organism and host response are integrated into diagnosis and management.
- Outcome of any given exposure is determined by the interaction of these two sides, organism virulence versus host defence, not by the organism alone. This host-pathogen interaction is developed in full in a dedicated later subsection.
