Overview of Major Pathogen Groups
-
Bacteria: Unicellular prokaryotes classified by cell wall structure (Gram-positive/negative, acid-fast), shape (cocci, bacilli, spirilla), and oxygen requirements (aerobic, anaerobic). Cause disease via toxins, enzymes, and immune evasion.
-
Viruses: Acellular agents containing DNA or RNA, encapsulated by a protein coat (capsid) and sometimes a lipid envelope. Require host cells for replication. Cause cell lysis, hijack cell machinery, and evade immune responses.
-
Fungi: Eukaryotic organisms, either unicellular (yeasts) or multicellular (moulds). Can be superficial, mucosal, or systemic pathogens, particularly in immunocompromised hosts.
-
Parasites: Eukaryotic; includes protozoa (unicellular) and helminths (multicellular worms). Transmission via vectors, ingestion, or direct contact. Pathogenicity due to invasion, immune modulation, and tissue damage.
Mechanisms of Pathogenesis
| Step | Description | Example |
|---|---|---|
| Infection Initiation | Entry through skin, respiratory, GI, urogenital tracts, or vectors. | Inhaled TB bacilli |
| Host-Cell Interaction | Adhesion (pili, adhesins), invasion, toxin delivery, or viral entry/fusion. | S. aureus adhesins |
| Host Damage | Direct cytopathic effect, immune-mediated injury, or toxin/enzyme release. | Cholera toxin, HIV |
Mnemonic: “I-H-D” (Infect, Hold, Damage)
Key Bacterial Pathogens
Gram-Positive Bacteria
| Pathogen | Features | Diseases | Virulence Factors |
|---|---|---|---|
| Staphylococcus aureus | Cocci in clusters, coagulase positive. | Skin infections, endocarditis, sepsis. | Coagulase, toxins, protein A. |
| Streptococcus pneumoniae | Lancet-shaped diplococci, encapsulated, alpha-haemolytic. | Pneumonia, meningitis, otitis media. | Capsule, pneumolysin, IgA protease. |
Gram-Negative Bacteria
| Pathogen | Features | Diseases | Virulence Factors |
|---|---|---|---|
| Escherichia coli | Bacilli, lactose fermenter, varied strains. | UTIs, sepsis, diarrhoeal illness. | Pili, shiga/enterotoxins. |
| Pseudomonas aeruginosa | Bacilli, non-lactose fermenter, pigment-former. | Pneumonia (CF), wound infections. | Biofilm, exotoxin A, efflux. |
Mycobacteria
-
Mycobacterium tuberculosis: Acid-fast bacilli, slow-growing. Causes tuberculosis via airborne spread, granuloma formation, and chronic inflammation. Cell wall rich in mycolic acids—resistant to many antibiotics.
Anaerobes
-
Bacteroides fragilis: Anaerobic Gram-negative rod. Part of gut flora, causes intra-abdominal, pelvic, and wound infections. Produces β-lactamase, polysaccharide capsule (immune evasion).
Viral Pathogens
| Virus | Genome Type | Key Diseases | Features & Pathogenesis |
|---|---|---|---|
| Influenza | RNA, segmented | Flu, pneumonia | Antigenic drift/shift, seasonal. |
| HIV | RNA, retrovirus | AIDS | Infects CD4+ T-cells, immune failure. |
| Hepatitis (B/C) | DNA (B)/RNA (C) | Hepatitis, cirrhosis | Chronic infection, hepatocarcinoma. |
| Coronaviruses | RNA, enveloped | COVID-19, SARS, MERS | Spike protein, respiratory tropism. |
Mnemonic: “I-H-He-Co” (Influenza, HIV, Hepatitis, Coronaviruses)
Fungal Pathogens
| Pathogen | Morphology | Diseases | Risk Factors |
|---|---|---|---|
| Candida spp. | Yeast, pseudohyphae | Oral/vaginal thrush, candidemia | Immunosuppression, antibiotics |
| Aspergillus spp. | Filamentous mould, septate | Aspergillosis, sinusitis | Neutropenia, lung disease |
| Dermatophytes | Filamentous, keratinophilic | Tinea (ringworm), athlete’s foot | Warm, moist skin, contact |
Parasitic Infections
| Parasite | Type | Diseases | Transmission/Pathogenesis |
|---|---|---|---|
| Plasmodium spp. | Protozoa | Malaria | Mosquito vector, RBC invasion |
| Toxoplasma gondii | Protozoa | Toxoplasmosis | Cat faeces, undercooked meat |
| Helminths | Nematodes, etc. | Ascariasis, schistosomiasis, others | Ingestion/skin, tissue migration |
Mnemonic for Parasites: “MT-H” (Malaria, Toxoplasma, Helminths)
Visual Revision Aid: Pathogen Table
| Group | Example Pathogen | Key Feature | Common Disease |
|---|---|---|---|
| Bacteria | S. aureus (G+), E. coli (G-) | Toxins, resistance | Skin/soft tissue, UTI |
| Virus | Influenza, HIV | Intracellular replication | Flu, AIDS |
| Fungi | Candida, Aspergillus | Eukaryotic, opportunistic | Thrush, aspergillosis |
| Parasite | Plasmodium, helminths | Complex life cycle | Malaria, worm infection |
References
-
NHS England Clinical Guidelines, 2024
-
BMJ, 2023
-
University of Edinburgh, Infectious Diseases Monograph
-
Public Health England, Microbiology Standards, 2023
-
Cardiff University, Review of Fungal Pathogens, 2022
-
St George’s, University of London, Antimicrobial Resistance Review
