Microbiological definitions
Colonisation refers to the presence and growth of microorganisms on body surfaces (such as skin or mucosa) without causing disease. It is usually a normal and non-pathological state.
Infection is the process by which a microorganism invades host tissues and causes disease or an inflammatory response.
A pathogen is a microorganism capable of causing disease in a host. Pathogenicity is the ability of a microorganism to cause disease. Virulence refers to the degree of pathogenicity of a microorganism, or its ability to cause severe disease in a host.
Opportunistic pathogens are microorganisms that typically do not cause disease in healthy individuals but can do so when host defences are compromised. Examples include Pneumocystis jirovecii in patients with HIV or Clostridium difficile in individuals whose normal gut flora has been disrupted by antibiotics.
A conditional pathogen is a subset of opportunistic pathogens that cause infection only under specific conditions, such as Staphylococcus aureus entering through a wound.
An obligate pathogen is a microorganism that always causes disease when it infects a host. It requires a living host to complete its life cycle and typically cannot survive or reproduce outside of a host
Obligate anaerobes are microorganisms that can only live and grow in the absence of molecular oxygen; exposure to oxygen is harmful or lethal to them.
Facultative anaerobes are capable of surviving and multiplying in both the presence and absence of molecular oxygen, adapting their metabolism accordingly.
Infection is the process by which a microorganism invades host tissues and causes disease or an inflammatory response.
A pathogen is a microorganism capable of causing disease in a host. Pathogenicity is the ability of a microorganism to cause disease. Virulence refers to the degree of pathogenicity of a microorganism, or its ability to cause severe disease in a host.
Opportunistic pathogens are microorganisms that typically do not cause disease in healthy individuals but can do so when host defences are compromised. Examples include Pneumocystis jirovecii in patients with HIV or Clostridium difficile in individuals whose normal gut flora has been disrupted by antibiotics.
A conditional pathogen is a subset of opportunistic pathogens that cause infection only under specific conditions, such as Staphylococcus aureus entering through a wound.
An obligate pathogen is a microorganism that always causes disease when it infects a host. It requires a living host to complete its life cycle and typically cannot survive or reproduce outside of a host
Obligate anaerobes are microorganisms that can only live and grow in the absence of molecular oxygen; exposure to oxygen is harmful or lethal to them.
Facultative anaerobes are capable of surviving and multiplying in both the presence and absence of molecular oxygen, adapting their metabolism accordingly.
| Pathogen Type | Examples |
|---|---|
| Obligate | HIV, Mycobacterium tuberculosis, Plasmodium species, Neisseria gonorrhoeae |
| Conditional | Escherichia coli (UPEC strain), Neisseria meningitidis, Staphylococcus aureus |
| Opportunistic | Clostridioides difficile, Pneumocystis jirovecii, Pseudomonas aeruginosa |