Microbes as parasites

1 Microbes as parasites

The varieties of microbes

Prokaryotes and eukaryotes

A number of important and distinctive biologic characteristics must be taken into account when considering any organism in relation to infectious disease. One of these is the way in which the organism is constructed, particularly the way in which genetic material and other cellular components are organized.

Microparasites and macroparasites

Living inside or outside cells

The basis of all host–pathogen relationships is the exploitation by one organism (the pathogen) of the environment provided by another (the host). The nature and degree of exploitation varies from relationship to relationship, but the pathogen’s primary requirement is a supply of metabolic materials from the host, whether provided in the form of nutrients or (as in the case of viruses) in the form of nuclear synthetic machinery. The reliance of viruses upon host synthetic machinery requires an obligatory intracellular habit: viruses must live within host cells. Some other groups of pathogens (Chlamydia, Rickettsia) also live only within cells. In the remaining groups of pathogens, different species have adopted either the intracellular or the extracellular habit, or, in a few cases, both. Intracellular microparasites other than viruses take their metabolic requirements directly from the pool of nutrients available in the cell itself, whereas extracellular organisms take theirs from the nutrients present in tissue fluids, or, occasionally, by feeding directly on host cells (e.g. Entamoeba histolytica, the organism associated with amoebic dysentery). Macroparasites are almost always extracellular (though Trichinella is intracellular), and many feed by ingesting and digesting host cells; others can take up nutrients directly from tissue fluids or intestinal contents.

Pathogens within cells are protected from many of the host’s defence mechanisms

As will be discussed in greater detail in Chapter 13, the intracellular pathogens pose problems for the host that are quite different from those posed by extracellular organisms. Pathogens that live within cells are largely protected against many of the host’s defence mechanisms while they remain there, particularly against the action of specific antibodies. Control of these infections depends therefore on the activities of intracellular killing mechanisms, short-range mediators or cytotoxic agents, although the latter may destroy both the pathogen and the host cell, leading to tissue damage. This problem, of targeting activity against the pathogen when it lives within a vulnerable cell, also arises when using drugs or antibiotics, as it is difficult to achieve selective action against the pathogen while leaving the host cell intact. Even more problematic is the fact that many intracellular pathogens live inside the very cells responsible for the host’s immune and inflammatory mechanisms and therefore depress the host’s defensive abilities. For example, a variety of viral, bacterial and protozoal pathogens live inside macrophages, and several viruses (including HIV) are specific for lymphocytes.

Intracellular life has many advantages for the pathogen. It provides access to the host’s nutrient supply and its genetic machinery and allows escape from host surveillance and antimicrobial defences. However, no organism can be wholly intracellular at all times: if it is to replicate successfully, transmission must occur between the host’s cells, and this inevitably involves some exposure to the extracellular environment. As far as the host is concerned, this extracellular phase in the development of the pathogen provides an opportunity to control infection through defence mechanisms such as phagocytosis, antibody and complement. However, transmission between cells can involve destruction of the initially infected cell and so contribute to tissue damage and general host pathology.

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Jul 9, 2017 | Posted by in MICROBIOLOGY | Comments Off on Microbes as parasites

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