Microbial associations with animals

Termites are insects belonging to the order Isoptera that are found particularly in tropical regions. Their famous ability to destroy trees and wooden structures such as buildings and furniture is due to a resident population of flagellated protozoans in their hindgut, which are able to break down cellulose. Termites lack the enzymes necessary to do

Symbiosis is sometimes taken to mean a relationship between different organisms from which both participants derive benefit. We use the term in its broader sense, as described in the text.

Table 15.1 Types of symbiotic association

Association

Species A

Species B

Mutualism

+

+

Protocooperation

+

+

Commensalism

-

+

Parasitism

x

+

Participants in symbiosis may derive benefit, harm or neither from the association. + denotes benefit, x denotes harm and - denotes neither.

Participants in symbiosis may derive benefit, harm or neither from the association. + denotes benefit, x denotes harm and - denotes neither.

this, and would thus starve to death if the protozoans were not present. In return, they are able to provide the anaerobic conditions required by the protozoans to ferment the cellulose to acetate, carbon dioxide and hydrogen. The acetate is then utilised as a carbon source by the termites themselves.

In addition to the protozoans, anaerobic bacteria resident in the hindgut also play an important role in the metabolism of the termites. Acetogenic and methanogenic species compete for the carbon dioxide and hydrogen produced by the protozoans. The former contribute more acetate for the termite to use, whilst the latter produce significant amounts of methane. Some methanogens exist as endosymbionts within the protozoans.

In other types of termite, no resident population of cellulose digesters is present. Instead, the termite ingests a fungus, which provides the necessary cellulolytic enzymes.

Another example of a host's staple diet being indigestible without the assistance of resident microorganisms is provided by the brightly coloured African bird the honey guide. The honey guide eats beeswax, and relies on a two-stage digestion process by bacteria (Micrococcus cerolyticus) and yeast (Candida albicans) to render it in a usable form.

At the bottom of the deepest oceans, around geothermal vents, live enormous (two metres or more) tube worms belonging to the genus Riftia. These lack any sort of digestive system, but instead contain in their body cavity a tissue known as the trophosome. This comprises vascular tissue plus cells packed with endosymbiotic bacteria. These are able to generate ATP and NADPH by the oxidation of hydrogen sulphide generated by

The total global amount of methane production by termites is comparable to that generated by ruminants.

Table 15.2 Microorganism-animal associations

Microorganism

Animal

Type of relationship

Anaerobic bacteria Flagellated protozoans Sulphur-oxidising bacteria Luminescent bacteria Bacteria, yeasts Fungus

Resident bacteria of skin, large intestine, etc.

Ruminants Termites

Riftia (marine tube worm) Fish, molluscs Honey guide (bird) Leaf cutter ants Humans

Mutualism

Mutualism

Mutualism

Mutualism

Mutualism

Protocooperation

Commensalism

Table 15.3 Microorganism-plant associations

Microorganism

Plant

Type of relationship

N2-fixing bacteria

Legumes

Mutualism

Mycorrhizal fungi

Various

Mutualism

Agrobacterium

Various

Parasitism (Crown gall

tumefaciens

disease)

Acremonium

Grass

Mutualism

(fungus)

volcanic activity and fix carbon dioxide via the Calvin cycle, providing the worm with a supply of organic nutrients. Hydrogen sulphide is transported to the trophosome from the worm's gill plume by a form of haemoglobin present in its blood (Figure 15.1).

Warm-blooded animals such as humans play host in their lower intestinal tract to vast populations of bacteria. Although some of these are capable of producing useful metabolites such as vitamin K, most live as commensals, neither benefiting nor harming their host. It could be argued, however, that the very presence of the resident intestinal microflora acts as an important defence against colonisation by pathogens, thus making the association more one of mutualism.

A number of bacteria, viruses, fungi, protozoans and even algae act as pathogens in animals, and cause millions of human deaths every year. A detailed description of these falls outside of the scope of this introductory text, however examples of diseases caused by each group are described in Chapters 7 to 10.

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