Classification of virus bacteria and fungi
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Classification of Viruses, Bacteria, and Fungi
1. Viruses
Viruses are acellular infectious agents. They contain either DNA or RNA, enclosed in a protein coat, sometimes with a lipid envelope.
A. By nucleic acid
| Type | Examples |
|---|---|
| DNA viruses | Herpesvirus, Adenovirus, Papillomavirus, Poxvirus, Hepatitis B virus |
| RNA viruses | Influenza virus, Measles virus, Rabies virus, Poliovirus, HIV, Hepatitis C virus |
| Reverse-transcribing viruses | HIV (retrovirus), Hepatitis B virus |
B. By capsid symmetry
| Type | Examples |
|---|---|
| Icosahedral (cubical) | Adenovirus, Poliovirus, Herpesvirus |
| Helical | Influenza, Rabies, Measles |
| Complex | Poxvirus, bacteriophages |
C. By envelope
| Type | Examples |
|---|---|
| Enveloped viruses | HIV, Influenza, Herpesvirus, Hepatitis B |
| Non-enveloped viruses | Adenovirus, Poliovirus, Papillomavirus, Parvovirus |
D. Baltimore classification
Based on genome type and method of producing mRNA:
- Double-stranded DNA viruses
- Single-stranded DNA viruses
- Double-stranded RNA viruses
- Positive-sense single-stranded RNA viruses
- Negative-sense single-stranded RNA viruses
- Single-stranded RNA viruses with reverse transcriptase
- Double-stranded DNA viruses with reverse transcriptase
Viral taxonomy considers virion structure, genome, proteins, envelope, replication and biological properties. ICTV taxonomy overview
2. Bacteria
Bacteria are unicellular prokaryotes. They have no true nucleus or membrane-bound organelles.
A. By Gram stain
| Group | Main feature | Examples |
|---|---|---|
| Gram-positive bacteria | Thick peptidoglycan wall, stain purple | Staphylococcus, Streptococcus, Bacillus, Clostridium, Corynebacterium |
| Gram-negative bacteria | Thin peptidoglycan wall plus outer membrane, stain pink/red | Escherichia coli, Salmonella, Neisseria, Pseudomonas, Vibrio |
B. By shape
| Shape | Examples |
|---|---|
| Cocci - spherical | Staphylococcus, Streptococcus, Neisseria |
| Bacilli - rod-shaped | E. coli, Klebsiella, Bacillus, Clostridium |
| Coccobacilli - short rods | Haemophilus, Brucella |
| Vibrios - comma-shaped | Vibrio cholerae |
| Spirilla/spirochetes - spiral | Treponema, Borrelia, Leptospira |
C. By oxygen requirement
| Type | Examples |
|---|---|
| Obligate aerobes | Pseudomonas, Mycobacterium tuberculosis |
| Obligate anaerobes | Clostridium, Bacteroides |
| Facultative anaerobes | E. coli, Staphylococcus aureus |
| Microaerophiles | Campylobacter, Helicobacter pylori |
D. By spore formation
| Type | Examples |
|---|---|
| Spore-forming | Bacillus, Clostridium |
| Non-spore-forming | Most other bacteria |
E. Special groups
- Acid-fast bacilli: Mycobacterium species
- Cell-wall-deficient bacteria: Mycoplasma
- Obligate intracellular bacteria: Chlamydia, Rickettsia
- Branching filamentous bacteria: Actinomyces, Nocardia
Gram staining rapidly separates bacteria according to cell-wall staining characteristics, with Gram-positive organisms staining blue-purple and Gram-negative organisms staining red-pink. Schwartz's Principles of Surgery, “Bacteria” section.
3. Fungi
Fungi are eukaryotic organisms with chitin-containing cell walls. They may occur as yeasts, molds, or both.
A. By morphology
| Type | Features | Examples |
|---|---|---|
| Yeasts | Unicellular, budding forms | Candida, Cryptococcus |
| Molds | Multicellular filaments called hyphae | Aspergillus, Rhizopus, dermatophytes |
| Dimorphic fungi | Mold at about 25°C; yeast or tissue form at about 37°C | Histoplasma, Blastomyces, Sporothrix, Coccidioides |
B. By hyphae
| Type | Examples |
|---|---|
| Septate hyphae | Aspergillus, dermatophytes |
| Aseptate or pauciseptate hyphae | Mucor, Rhizopus |
C. Clinical classification of fungal infections
| Category | Main site / examples |
|---|---|
| Superficial mycoses | Outer keratin layer or hair shaft, e.g., pityriasis versicolor |
| Cutaneous mycoses | Skin, hair and nails, e.g., dermatophytosis or tinea |
| Subcutaneous mycoses | Skin and subcutaneous tissue after trauma, e.g., sporotrichosis |
| Systemic/endemic mycoses | Primarily acquired by inhalation, e.g., histoplasmosis, blastomycosis, coccidioidomycosis |
| Opportunistic mycoses | Mainly in immunocompromised hosts, e.g., candidiasis, aspergillosis, mucormycosis, cryptococcosis |
Traditional fungal classification uses observable features such as hyphal septation and sexual structures, though molecular methods are increasingly used. Sherris & Ryan’s Medical Microbiology, “Classification” section. Clinical mycoses are commonly grouped as superficial, cutaneous, subcutaneous, endemic/systemic, and opportunistic. Medical Microbiology 9e, “Classification of Human Mycoses” section.
Quick comparison
| Feature | Viruses | Bacteria | Fungi |
|---|---|---|---|
| Cellular nature | Acellular | Prokaryotic cells | Eukaryotic cells |
| Genetic material | DNA or RNA | Both DNA and RNA | Both DNA and RNA |
| Cell wall | Absent | Peptidoglycan | Chitin and glucans |
| Multiplication | Only inside living host cells | Binary fission | Budding, hyphal growth, spores |
| Examples | Influenza, HIV | E. coli, Staphylococcus | Candida, Aspergillus |
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