Definitions

Symbiosis, the living together of dissimilar organisms as originally defined by Anton de Bary in 1879, encompasses: (1) commensalism - two organisms living together, with little benefit or detriment to themselves or each other; (2) mutualism - two organisms benefiting each other; and (3) parasitism (sometimes referred to as pathogenesis) - one organism benefiting at the expense of another (Figure 1). Many textbooks now define commensalism as a situation where one member benefits and the other is unaffected. Commensals belong to the Archaea, Eubacteria, or

Parasitism

Figure 1 Cartoon depicting the differences between commensalism, mutualism, and parasitism based on the classical definitions of de Bary. These definitions are flexible because the behaviors intergrade (double-headed arrows). Illustration, courtesy of M. Kowalczyk.

Parasitism

Figure 1 Cartoon depicting the differences between commensalism, mutualism, and parasitism based on the classical definitions of de Bary. These definitions are flexible because the behaviors intergrade (double-headed arrows). Illustration, courtesy of M. Kowalczyk.

Eukaryota. This article concentrates on the Eubacteria because they are the most numerous and the most well known of the commensals. It also focuses on the resident or indigenous microbes rather than on the transient ones. Nevertheless, Eukaryota such as yeasts and other fungi as well as various protists are well-known inhabitants of both plant and animal surfaces. A few examples are presented.

Commensalism is much less studied than either mutualism or parasitism, most likely because very little seems to happen in contrast to mutualism and parasitism. However, this is an erroneous impression as we become more aware of the vast numbers and types of prokaryotic commensals that live in association with their eukaroytic hosts. Also, changes in the conditions of a commensal's habitat or a move to a new niche may cause a formerly neutral microbe to become parasitic or mutualistic. For example, Helicobacter pylori, the causative agent of ulcers, exists as a commensal or a pathogen depending on its host. Pseudomonas aeruginosa, found in the anterior portion of the nose, is a serious pathogen in the lungs of cystic fibrosis patients and can cause infections in other tissues if it becomes internalized. Other commensals protect their symbiotic partner from parasites (thus acting as mutualists), either by their very presence, that is, occupying sites where parasites could attach, or through their metabolism, synthesizing vitamins, catabolizing complex polymers, or mediating developmental processes.

Both plants and animals establish interactions with commensal bacteria. Often, the interactions with plant and animal surfaces involve the formation of biofilms -a community of bacterial cells, frequently of different species, adherent to a surface and to each other, and enclosed in a self-produced polymeric matrix. Overall, more is known about interactions between commensals and animals, mainly because of the relationship to human health, but new and exciting discoveries have been uncovered for plant-commensal interactions.

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