A pathogen is a disease causing agent that disrupts the normal physiology of the infected organism
- Pathogens can be cellular (e.g. parasites, protozoa, bacteria) or acellular (viruses and prions)
Types of Pathogens
Viruses
- Viruses are metabolically inert and incapable of reproducing independently of a host cell (hence are non-living)
- They typically consist of an inner core of nucleic acid surrounded by a protein coat (capsid)
- Simpler viruses may lack a capsid (viroids), whilst more complex viruses may possess an external lipid envelope
- Viruses can be either DNA-based (adenoviruses) or RNA-based (retroviruses)
Prions
- A prion is an infectious protein that has folded abnormally into a structure capable of causing disease
- Prions can cause normally folded proteins to refold into the abnormal form and hence propagate within a host body
- Prion proteins aggregate together to form amyloid fibres that cause holes to form in the brain (spongiform encephalopathy)
- Infectious prion proteins have a higher beta-sheet content, making them more resistant to denaturation and difficult to treat
Bacteria
- Bacteria are unicellular prokaryotic cells that can reproduce quickly and compete with host cells for space and nutrition
- Most bacteria are relatively harmless and some may even form mutualistic relationships with hosts (e.g. normal gut flora)
- Bacteria may cause disease by producing toxic compounds (exotoxins) or releasing the substances when destroyed (endotoxins)
- As the toxins retain their destructive capacity beyond bacterial death, they are often the cause of food poisoning
Fungi
- Disease-causing fungi usually attack the body surfaces, including the skin and mucous membranes
- They can be categorised according to whether they are unicellular (yeasts) or multicellular (moulds) – most fungi are moulds
- Moulds consist of branching filaments called hyphae, which may form a mass of invading threads called mycelium
- Examples of fungal infections include thrush (yeast infection) and athlete’s foot (mould infection)
Parasites
- A parasite is an organism that grows and feeds on an organism to the detriment of the host’s survival
- Parasites can be classed as either ectoparasites (living on the surface of the host) or endoparasites (living within the host)
- Endoparasites can include microparasites (e.g. single-cell protozoa) or macroparasites (multicellular helmithes)