Types of Microorganisms
Microorganisms may be of the following types:
a) Saprophytes: These are free-living microorganisms that liveon dead or decaying organic matter. They are usually present in soil and water. They are generally unable to invade the living body.
b) Parasites: These are microorganisms that live on a livinghost and derive nutrition from the host without any benefit to the host and causes harm to the infected host.
c)Commensals: These are microorganisms that live on a liv-ing host without causing any injury to the host. Most human microbes are commensals.
d) Pathogens: A microorganism capable of causing disease,especially if it causes disease in immunocompetent people, is called as apathogen. These pathogens, however, represent a very small proportion of the microbial species.
e) Opportunistic pathogens: A microbe that is capable ofcausing disease only in immunocompromised people is known asopportunistic pathogen. These organisms can cause disease only if one or more of the usual defense mechanisms of humans are reduced or altered by accident, by intent (e.g., surgery), or by an underlying metabolic disorder or an infectious disease (e.g., AIDS).
There are two major differences between primary pathogens and opportunistic ones:
a) The first and foremost being that the primary pathogens regularly cause overt disease, whereas opportunistic ones take the opportunity offered by reduced host defenses to cause disease.
b) Long-term survival of a microbe is another difference.
Long-term survival in a primary pathogen is absolutely dependent on its ability to replicate and to be transmit-ted in a particular host. However, this is not necessarily the case for a number of the opportunistic pathogens that infect humans.