The host–parasite relationship

8 The host–parasite relationship

The normal flora

In the gut the density of microorganisms increases from the stomach to the large intestine

The stomach normally harbours only transient organisms, its acidic pH providing an effective barrier. However, the gastric mucosa may be colonized by acid-tolerant lactobacilli and streptococci. Helicobacter pylori, which can cause gastric ulcers (see Ch. 22), is carried without symptoms by large numbers of people, the bacterium being in mucus and neutralizing the local acidic environment. The upper intestine is only lightly colonized (104 organisms/g), but populations increase markedly in the ileum, where streptococci, lactobacilli, enterobacteriaceae and Bacteroides may all be present. Bacterial numbers are very high (estimated at 1011/g) in the large bowel, and many species can be found (Fig. 8.2). The vast majority (95–99%) are anaerobes, Bacteroides being especially common and a major component of faecal material; E. coli is also carried by most individuals. Bacteroides and E. coli are among the species capable of causing severe disease when transferred into other sites in the body. Harmless protozoans can also occur in the intestine (e.g. Entamoeba coli) and these can be considered as part of the normal flora, despite being animals.

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.

What happens when the normal flora is absent?

Germ-free animals tend to live longer, presumably because of the complete absence of pathogens, and develop no caries (see Ch. 18). However, their immune system is less well developed and they are vulnerable to introduced microbial pathogens. At the time of birth, humans are germ free, but acquire the normal flora during and immediately after birth, with the accompaniment of intense immunologic activity.

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).

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

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