Microorganisms and Host

57

Microorganisms and Host

Based on their relationship to their host they can be divided into saprophytes and parasites.
A. Saprophytes
B. Parasites

A. Saprophytes
Saprophytes (from Greek sapros decayed, and phyton plant) are free-living microbes that live on dead or
decaying organic matter. They are found in soil and water and play an important role in the degradation of organic materials in nature. They are of little relevance in infectious disease because they are generally incapable of multiplying on living tissues. However, saprophytes like Bacillus subtilis may cause infection sometimes when host resistance is lowered.

B. Parasites
Parasites are microbes that can establish themselves and multiply in the hosts. There are many parasitic agents or organisms among the viruses, bacteria, fungi, plants, and animals. By convention, when the word parasite is used without quantification, it refers specifically to a protozoan or helminthic (nematode, trematode, cestode) organisms. Parasite microbes may be either pathogens or commensals:

Pathogens
Pathogens (from the Greek pathos, disease, and gen, to produce) are the microorganisms or agents, which are capable of producing disease in the host. Its ability to cause disease is called pathogenicity.
Types of Pathogens
They are two types: Primary and opportunist pathogens.
a. Primary (frank) pathogens
Primary (frank) pathogens are the organisms, which are capable of producing disease in previously healthy individuals with intact immunological defences. However, these bacteria may more readily cause disease in individuals with impaired defences.
b. Opportunist pathogens
Opportunist pathogens rarely cause disease in individuals with intact immunological and anatomical
defences. These bacteria are able to cause disease only when such defences are impaired or compromised, as a result of congenital or acquired disease or by the use of immunosuppressive therapy or surgical techniques. Many opportunistic pathogens are part of the normal human flora, e.g. coagulase-negative staphylococci and Escherichia coli.

Commensals
Commensals (organisms of normal flora) are the microorganisms that live in complete harmony with the host without causing any damage to it. Skin and mucous membranes are sterile at birth. The normal bacterial flora of the body consists largely of commensals. Many commensals behave as facultative pathogens in that they can produce disease when the host resistance is lowered