The host–parasite relationship

8 The host–parasite relationship

The normal flora

Advantages and disadvantages of the normal flora

Some of the species of the normal flora are positively beneficial to the host

The importance of these species for health is sometimes revealed quite dramatically under stringent antibiotic therapy. This can drastically reduce their numbers to a minimum, and the host may then be over-run by introduced pathogens or by overgrowth of organisms normally present in small numbers. After treatment with clindamycin, overgrowth by Clostridium difficile, which survives treatment, can give rise to antibiotic-associated diarrhea or, more seriously, pseudomembranous colitis.

Ways in which the normal flora prevents colonization by potential pathogens include the following:

Gut bacteria also release organic acids, which may have some metabolic value to the host; they also produce B vitamins and vitamin K in amounts that are large enough to be valuable if the diet is deficient. The antigenic stimulation provided by the intestinal flora helps to ensure the normal development of the immune system.

Symbiotic associations

All living animals are used as habitats by other organisms; none is exempt from such invasion – bacteria are invaded by viruses (bacteriophages) and protozoans have their own flora and fauna – for example, amoeba are natural hosts for Legionella pneumophila infection. As evolution has produced larger, more complex and better regulated bodies, it has increased the number and variety of habitats for other organisms to colonize. The most complex bodies, those of birds and mammals (including humans), provide the most diverse environments, and are the most heavily colonized. Relationships between two species – interspecies associations or symbiosis – are therefore a constant feature of all life.

As the normal flora demonstrates, disease is not the inevitable consequence of interspecies associations between humans and microbes. Many factors influence the outcome of a particular association, and organisms may be pathogenic in one situation but harmless in another. To understand the microbiologic basis of infectious disease, host–microbe associations that can be pathogenic need to be placed firmly in the context of other symbiotic relationships, such as commensalism or mutualism, where the outcome for the host does not normally involve any damage or disadvantage.

Commensalism, mutualism and parasitism are categories of symbiotic association

All associations in which one species lives in or on the body of another can be grouped under the general term ‘symbiosis’ (literally ‘living together’). Symbiosis has no overtones of benefit or harm and includes a wide diversity of relationships. Attempts have been made to categorize types of association very specifically, but these have failed because all associations form part of a continuum (Fig. 8.3). Three broad categories of symbiosis – commensalism, mutualism and parasitism – can be identified on the basis of the relative benefit obtained by each partner. None of these categories of association is restricted to any particular taxonomic group. Indeed, some organisms can be commensal, mutualist or parasitic depending upon the circumstances in which they live (Fig. 8.4).

Jul 9, 2017 | Posted by in MICROBIOLOGY | Comments Off on The host–parasite relationship

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