General Microbiology - Schlegel, H. 1987

Microorganisms and the Environment
Microorganisms as Symbiotic Partners

Various types of interactions exist among different microorganisms. Over the course of evolution, dependency relationships—either mutual or unilateral—have developed that extend far beyond interactions driven strictly by food chains. Once prokaryotes and numerous eukaryotic microorganisms reached their current level of evolutionary development, higher life forms emerged, creating new potential habitats for lower ones. Animals and plants evolved in an environment where nearly all metabolic pathways characteristic of prokaryotes were already established. Naturally, this led to The formation of numerous partnerships between microorganisms on the one hand, and animals and plants on the other. The close cohabitation of two different organisms is known as Symbiosis.

Regarding the relative benefits derived by partners from symbiosis, several scenarios can be distinguished: 1) the cohabitation creates favorable conditions for both partners (mutually beneficial symbiosis, or mutualism); 2) one partner experiences harmful effects from the other (in which case it is termed parasitism); 3) in many instances, the partners may have no discernible impact on one another (neutralism).

Spatial relationships between partners can also vary widely. If one partner resides outside the Cells of the other, it is referred to as ectosymbiosis, whereas residence within cells is called endosymbiosis. The larger of the partners is typically designated as the host.

Functional types of symbiosis can be distinguished based on The Nature of the benefits that one or both partners derive from the cohabitation. Sometimes a close association improves Nutrition, for instance, because one partner fixes molecular nitrogen, breaks down Cellulose, or supplies essential nutrients, Vitamins, and the like. A symbiont may also serve a signaling function, as seen in the association between luminous Bacteria and fish. Furthermore, the symbiont can play a protective role. A host Organism often simply provides shelter for ecto- and endosymbiotic microorganisms; occasionally, however, the latter protect the host against other microbes—whether parasitic or pathogenic—for example, within the intestinal tract or on the body surface.

Since various symbiotic associations have already been addressed in the sections dedicated to metabolic types, we will provide here only a Brief Overview of some important forms of symbiosis.

17.2.1 Mutualistic Symbiosis

Associations between microorganisms. The microbial world offers numerous Examples of syntrophy—the mutual supply of essential substances. Syntrophic associations are formed, for instance, by Desulfuromonas acetoxidans with Chlorobium, or Desulfovibrio with Chromatium. In both cases, the first-named partner provides the second with a hydrogen donor, while the second supplies the first with a hydrogen acceptor (Section 9.3). Syntrophy may also involve the provision of vitamins or their precursors. The fungus Mucor ramannianus and the Yeast Rhodotorula rubra both require vitamin B₁ (thiamine). The former species can synthesize the pyrimidine component but is incapable of synthesizing thiazole; the latter produces thiazole but lacks The ability to synthesize pyrimidine. When both organisms are grown in a mixed culture, the first species secretes the pyrimidine component and the second releases the thiazole component, thereby satisfying the Nutritional Requirements of both species.

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Fig. 17.4. Cross-section through a lichen thallus. B — alga (phycobiont); Г — fungal hyphae (mycobiont); ВК and НК — upper and lower cortical layers.

Lichens serve as a prime example of highly developed ectosymbiosis among microorganisms. Within the thallus of a lichen, the fungus and alga (or cyanobacterium) are so intimately connected that they form a unified plant body (Fig. 17.4). Both partners benefit from this symbiosis. As a rule, the fungal component of the lichen—the mycobiont—acts as the morphogenetic partner. The fungus receives organic substances (the products of CO₂ fixation) from the algal cells, while in turn supplying the alga with mineral salts and protecting it from adverse environmental factors, particularly desiccation. The phycobionts in lichens can be green Algae or cyanobacteria. The partners can be separated and cultivated independently, and artificial lichens can also be synthesized. These combined organisms successfully colonize extreme habitats where neither partner could survive in isolation.

If only one partner benefits from the symbiosis without causing harm to the other, it is called commensalism. To illustrate this with an example, The production of Harz cheese involves a single fungus and a single bacterium. Only the bacterium benefits from this association; the fungus oxidizes the acids present in the curd, thereby creating favorable environmental conditions for the bacterium. Another example is the association between aerobic and anaerobic bacteria, in which the aerobes rapidly consume O₂, rendering the habitat suitable for anaerobes. In other cases, certain microorganisms may secrete depolymerases that break down Polysaccharides, Proteins, or Nucleic Acids, thereby making these nutrients accessible to other species as well.

Microorganisms and plants. In the rhizosphere (the zone surrounding the roots) of many plants, bacteria are frequently far more abundant than in the rest of the soil. This bacterial proliferation is evidently supported by nutrients exuded by the roots. The precise benefit of this rather loose association for the plants is difficult to establish. Nevertheless, because many soil bacteria perform Functions critical to plants—such as fixing nitrogen or converting sparingly soluble nutrient salts into readily available forms—one may reasonably infer a mutualistic relationship. In recent years, certain bacteria consistently associated with specific grass species have been recognized as nitrogen fixers (e.g., Azotobacter paspali in the rhizosphere of Paspalum notatum; Azospirillum lipoferum in the ROOT zone of Digitaria and maize).

Many plants form a close root-fungus association known as mycorrhiza. Numerous soil Fungi, including agarics, can penetrate plant roots and invade their cells, simultaneously stimulating root growth by secreting Auxins. Experienced mushroom hunters know that certain edible fungi grow exclusively near specific tree species (such as spruce, larch, pine, and oak), which act as host plants in the mycorrhizal association. The fungus invading the root cortex forms specialized branching structures known as vesicles and arbuscles (vesicular-arbuscular mycorrhiza). The benefit of this association for the fungus is that it obtains assimilation products from the plant, whereas the plant benefits from a more efficient uptake of soil minerals (such as phosphate and fixed nitrogen).

We have previously examined plant associations with nitrogen-fixing endo- or ectosymbiotic bacteria (Section 13.1). The symbiosis between Rhizobium species and legume root cells represents one of the most highly differentiated symbiotic interactions. It serves as a striking example of the evolution of an intimate association between an intracellular symbiont and a host Cell—a fact that strongly Supports the hypothesis regarding the endosymbiotic origin of certain cellular Organelles (p. 26).

Microorganisms and animals. An immense variety of symbiotic relationships exists between microorganisms and animals. Only in a few instances is the function of the partners as transparent as it is in the rumen of ruminants (Section 14.1). Investigating symbiotic relationships, particularly in lower animals including Protozoa, presents numerous complex challenges. A central question is frequently whether the intestinal tract harbors a specific microbial community that confers protection against pathogens or performs a specialized digestive function. Out of a multitude of examples, we will examine only a few.

Protozoa—whether free-living or inhabiting the digestive tracts of ruminants, termites, and cockroaches (such as Ciliates, flagellates, and amoebae)—frequently coexist with ecto- and endosymbiotic bacteria.

Early in The history of microbial genetics, the "killer" phenomenon in the Ciliate Paramecium aurelia became widely known. As Sonneborn discovered, strains of this species can be divided into two groups: certain strains ("killers") secrete a toxic substance to which they are themselves resistant, whereas other strains are sensitive to this toxin and perish upon exposure. It turned out that the capacity to produce the toxic substance and eliminate other cells is governed by the Cytoplasm rather than nuclear genes. The "killer" trait is transmitted during conjugation, with the sensitive recipient similarly transformed into a "killer." This trait was found to be associated with the presence of so-called "kappa particles," which ultimately proved to be endosymbiotic bacteria. "Killer" strains can be "cured" of kappa particles, for instance, through antibiotic Treatment. Upon microscopic examination, kappa particles are readily identified by the presence of highly refractile inclusions (R-bodies); such a Structure consists of a tightly wound protein ribbon. This body is presumably responsible for the toxic substance. The situation grew even more complex when phage heads were discovered within the kappa particles, suggesting that the bacterial symbionts harbor temperate phages. Unfortunately, these undeniably endosymbiotic bacteria have still not been successfully cultured outside their host. Research on kappa particles entered a new, active phase following the recent isolation of soil bacteria containing R-bodies.

Many protozoa serve as hosts for green, brown, or yellow unicellular algae known as zoochlorellae and zooxanthellae; other protozoa, such as the flagellate Cyanophora, harbor cyanobacteria termed cyanelles. The utility of these photosynthetic partners for the protozoa is self-evident: they provide the host with a complete photosynthetic apparatus for carbohydrate synthesis, much like the phycobiont supplying the mycobiont with assimilation products in lichens.

In many insects, ciliates, Yeasts, and bacteria inhabit the gut or specialized gut appendages as symbionts. They may exist as extracellular symbionts or reside within the cells of specialized Tissues. The function of these microorganisms is easy to deduce, especially when their hosts feed on recalcitrant substrates (such as wood-eating termites) or consume unbalanced diets (like many plant-sucking aphids and bugs). The symbionts either perform a digestive function or supply the host with essential supplementary nutrients (such as Steroids, vitamins, and Amino Acids). While morphological and anatomical studies of insect symbiosis have advanced considerably (largely thanks to the work of P. Buchner), PHYSIOLOGICAL AND BIOCHEMICAL investigations have thus far yielded limited results, primarily because the symbiotic microorganisms have resisted cultivation outside their host.

In many insects, the endosymbiont-containing Organs (mycetomes) develop from appendages of the hindgut. By contrast, the previously mentioned rumen of ruminants represents an organ of the foregut. However, hindgut appendages also occur in mammals: in many herbivores, microbial Digestion takes place in enlargements of the hindgut. When symbionts are localized near the terminal end of the intestinal tract, the animal cannot extract the full nutritional benefit from the symbiosis; this explains why certain animals practice coprophagy (ingesting their own feces), thereby utilizing all nutrients more thoroughly.

The COMPOSITION OF THE human intestinal flora was addressed in Section 8.2. The mutualistic Nature of the relationship between our organism and its bacterial flora becomes glaringly apparent when this balance is disrupted and bacterial populations are decimated by Antibiotics AND CHEMOTHERAPEUTIC agents. The function of the intestinal flora is demonstrated most clearly in germ-free animals. Under a proper diet, these animals develop quite normally, but their Susceptibility to infectious diseases is drastically increased. Thus, the normal bacterial flora plays a critical role in defense against pathogenic and other foreign microorganisms.

Similar relationships exist between The Human Body and its cutaneous flora. Our Skin harbors a characteristic bacterial flora composed primarily of mycobacteria, streptococci, staphylococci, and propionibacteria, which consume nutrients present in sweat. Under normal conditions, this skin flora causes no adverse effects, save for the production of odorous compounds. The beneficial Functions of the skin flora become obvious when topical bacteriostatic antibiotics or systemic overmedication suppresses these cutaneous bacteria. In such instances, yeasts (Candida albicans) and other pathogenic fungi begin to proliferate unchecked.

Numerous marine fish possess specialized luminous organs. Morphologically, these are highly differentiated structures that harbor growing populations of luminescent bacteria (Photobacterium fischeri).

17.2.2 Antagonistic Symbiosis

Symbiotic relationships that inflict more or less pronounced harm upon the host occur both among microorganisms themselves and between microbes on the one hand, and animals and plants on the other. We have already touched upon Various Forms of parasitism when discussing, for example, the bacterium Bdellovibrio bacteriovorus, which parasitizes other bacteria (Section 3.14); rickettsiae and chlamydiae as Obligate Intracellular Parasites (Section 3.18); and pathogens affecting plants (Erwinia, Corynebacterium, Pseudomonas, Uredo, Ustilago, Puccinia, Claviceps) and animals. The Theoretical Aspects of parasitism and the intervention in the equilibrium between parasitic microorganisms and their hosts are investigated by veterinary and medical microbiology, as well as plant pathology.