8 The host–parasite relationship
The normal flora
Why is it called the normal flora?
The organisms occur in those parts of the body that are exposed to, or communicate with, the external environment, namely the skin, nose and mouth and intestinal and urinogenital tracts. The main organisms found in these sites are shown in Figure 8.1. Internal organs and tissues are normally sterile.
The normal flora is acquired rapidly during and shortly after birth and changes continuously throughout life
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.
The urethra is lightly colonized in both sexes, but the vagina supports an extensive flora of bacteria and fungi
• Before puberty, the predominant organisms are staphylococci, streptococci, diphtheroids and E. coli.
• Subsequently, Lactobacillus aerophilus predominates, its fermentation of glycogen being responsible for the maintenance of an acid pH, which prevents overgrowth by other vaginal organisms.
Advantages and disadvantages of the normal flora
Some of the species of the normal flora are positively beneficial to the host
Ways in which the normal flora prevents colonization by potential pathogens include the following:
• Skin bacteria produce fatty acids, which discourage other species from invading.
• Gut bacteria release a number of factors with antibacterial activity (bacteriocins, colicins) as well as metabolic waste products that help prevent the establishment of other species.
• Vaginal lactobacilli maintain an acid environment, which suppresses growth of other organisms.
• The sheer number of bacteria present in the normal flora of the intestine means that almost all of the available ecologic niches become occupied; these species therefore out-compete others for living space.
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.
The disadvantages of the normal flora lie in the potential for spread into previously sterile parts of the body
• when the intestine is perforated or the skin is broken
• during extraction of teeth (when Streptococcus viridans may enter the bloodstream)
• when organisms from the perianal skin ascend the urethra and cause urinary tract infection.
• the local environment changes (e.g. increases in stomach or vaginal pH)
• the immune system becomes ineffective (e.g. AIDS, clinical immunosuppression).
Under these conditions, the potential pathogens take advantage of the opportunity to increase their population size or invade tissues, so becoming harmful to the host. An account of diseases associated with such opportunistic infections is given in Chapter 30.
Symbiotic associations
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).