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