To this point, we have discussed how microbes have evolved to deal with the abiotic (nonliving) environment, oxygen concentration, temperature, pH, and the like. But microorganisms must also deal with one another, the so-called biotic environment. And though the number of organisms, microscopic and otherwise, is tremendous, there are only five types of relationships that typically exist.
Symbioses, are defined as close associations between organ isms that are advantageous to at least one of the members (each member is known as a symbiont). A few facts about symbiotic relationships include:
● Can be obligatory or nonobligatory
● Can involve animals, plants, and other microbes
● Can include complex multipartner interactions
Endosymbionts live within their partner, whereas exosymbionts are associated with the outside surfaces of their partner. Figure 1 provides an overview of the major categories of symbiosis with examples. Symbiotic relationships differ based on who receives benefits from the relationship.
Mutualism is a relationship in which all members share in the benefits produced by the partnership. Many mutualistic associations have developed over hundreds of millions, and even billions, of years of shared evolution, a process termed coevolution. Coevolving symbionts remain in very close contact and must evolve together to sustain themselves. A change in one partner exerts a selective pressure on the other partner to adapt to these changes, and vice versa. A classic example of mutualism occurs between termites and a protozoan endosymbiont, Trichonympha. While termites eat wood, it is the Trichonympha living within the termite’s gut that digests the cellulose in the wood fibers (figure 1b). Both organisms receive benefits from the relationship (the relationship is actually far more complex, as many species of protozoans live in the termite’s gut and help to digest cellulose, but that is a minor point in our discussion).
Fig1. Part 1—Symbiosis: A shared existence. These relationships vary in the degree of interaction and its outcome. (a1): Scott Bauer/USDA; (a2): Louisa Howard, Dartmouth Electron Microscope Facility; (b1): jeridu/Getty Images; (b2): Alamy Stock Photo; (c): Dr. Ralf Wagner; (d): Nick Hill/USDA; (e): McGraw Hill; (f1): Dr. CSBR Prasad, Vindhya Clinic and Diagnostic Lab, India/CDC-DPDx; (f2): Janice Carr/CDC; (g): Cynthia Goldsmith/CDC; (h1): CDC/James Gathany; (h2): Source: Blaine A. Mathison/CDC-DPDX
Fig1. Part 2—Additional microbial adaptations. (j): MedicalRF.com; (k): Paul Bertner/Flickr/Getty Images; (l1): Jorg Barke, Ryan F Seipke, Sabine Grüschow, Darren Heavens, Nizar Drou, Mervyn J Bibb, Rebecca JM Goss, Douglas W Yu, and Matthew I Hutchings; (l2): Microfield Scientific Ltd/Science Source
The nondependent forms of mutualism are sometimes referred to as cooperation, to emphasize the fact that the organisms gain mutual benefit from their association but can survive independently outside of the partnership. Many of these cooperative relationships are coevolving to greater dependency. One example is the protozoan Euplotes, which harbors endosymbiotic algae in its cells (figure 1c). Although both members can survive apart from this mutualistic interaction, they appear to receive significant survival benefits from being together. Other examples include fungi that grow within plant tissues and protect the plant from drought and insects while being harbored and fed (figure 1d).
When only a single member of a symbiotic partnership benefits, while the other is neither benefited nor harmed, the relationship is described as commensalism. The members of the association are called commensals. One form of commensalism observed in cultured microbes is termed the satellite phenomenon, in which one species releases various growth factors that are required by a nearby species to grow. The feeder microbe grows around its partner in tiny colonies, while the passive partner is neither harmed nor helped (figure 1e). Many of the microbes that occupy a niche on the human body are considered commensals. They make a living by feeding off dead skin and secretions, but do not usually cause harm (figure 1f1, 2).
The last type of symbiotic interrelationship is parasitism, in which one member benefits at the expense of the other. There are various degrees of parasitism. The most extreme form is obligate intracellular parasitism, which means the microbe spends all or most of its life cycle inside the host cell, from which it derives essential nutrients and other types of support. Prime examples are viruses that are incapable of surviving without a host (figure 1g). Other examples include intracellular bacteria like Rickettsia and Chlamydia, which poach energy from their (bacterial) hosts, and protozoans like Plasmodium (the cause of malaria) that parasitize red blood cells (figure 1h1, 2). The kinds of harm that parasites do to their hosts varies from superficial damage (ringworm on the skin from a fungal infection) to significant damage (peptic ulcers) to rapid death (rabies virus). It’s also interesting to note that the most successful parasites are often the ones that have evolved to cause the least damage to their host, sometimes even evolving to a commensal or mutualistic existence. After all, a dead host is no host at all.
The final two types of relationships don’t require a particularly close association between microbes. One of these, syntrophy, or crossfeeding, is communal feeding between organisms sharing a habitat. In essence, products given off by one organism are usable by another (figure 7.13 i j). The organisms involved do not require this relationship for survival, but they generally benefit from it. Many syntrophic associations occur in aquatic habitats and soils within biofilms and are related to nutrient and bioelement recycling. A simple example involves a pair of free-living soil bacteria that share their metabolic products cyclically (figure 1i). Cellulomonas uses its enzymes to digest plant cellulose into glucose, but it cannot fix nitrogen. Azotobacter fixes nitrogen gas from the air and releases ammonium, but it does not digest cellulose. The glucose is a source of carbon for Azotobacter, and the ammonium supplies Cellulomonas with usable nitrogen. Even more complex interactions occur in the cycling of elements such as sulfur, phosphorus, and nitrogen.
Lastly, amensalism describes an action of one microbe that causes harm to another microbe. It usually involves antagonism or competition and occurs in a community where microbes are sharing space and nutrient sources. Some microbes effectively compete by using up a vital nutrient to grow faster and dominate the habitat. Other microbes may release specific inhibitory chemicals (antibiotics) into the surrounding environment that inhibit or kill microbes. Many fungi and bacteria are adapted to this survival strategy. An intriguing example can be found in the complex symbiosis of certain ants that actively cultivate specialized gardens of fungi as a source of food. To protect the fungi from other microbes, the ants also cultivate species of filamentous bacteria called actinomycetes that produce antibiotics. The ants spread the actinomycetes through their gardens to protect the fungi from invasion by parasites. Figures 1k and l feature some aspects of this relationship.
Biofilms—A Microbial Conversation
Among the most common and prolific associations of microbes are biofilms, first described chapter 4. Biofilms result when organisms attach to a substrate by some form of extracellular matrix that binds them together in complex organized layers. Biofilms are so prevalent that they dominate the structure of most natural environments on earth, and microbes within the biofilm participate in such an intimate association that they begin to resemble a single “superorganism,” influencing such microbial activities as adaptation to a particular habitat, content of soil and water, nutrient cycling, and even the course of infections.
It is generally accepted that the individual cells in the bodies of multicellular organisms such as animals and plants have the capacity to produce, receive, and react to chemical signals such as hormones made by other cells. Microbes show a well developed capacity to communicate and cooperate in the formation and function of biofilms. This is especially true of bacteria, although fungi and other microorganisms can participate in these activities.
One idea to explain the development and behavior of biofilms is termed quorum sensing. This process occurs in several stages, including self-monitoring of cell density, secretion of chemical signals, and genetic activation (process figure 2). Early in biofilm formation, free-floating or swimming microbes—often described as planktonic—are attracted to a surface and come to rest or settle down. Settling stimulates the cells to secrete a slimy or adhesive matrix, usually made of polysaccharide, that binds them to the substrate. Once attached, cells begin to release inducer molecules that accumulate as the cell population grows. By this means, they can monitor the size of their own population. In time, a critical number of cells, termed a quorum, accumulates, and this ensures that there will be sufficient quantities of inducer molecules. These inducer molecules enter biofilm cells and stimulate specific genes on their chromosomes to begin expression.
Fig2. Process Figure 2 Stages in biofilm formation, quorum sensing, induction, and expression.
The nature of this expression varies, but generally it allows the biofilm to react as a unit. For example, by coordinating the expression of genes that code for specific proteins, the biofilm can simultaneously produce large quantities of a digestive enzyme or toxin. This regulation of expression accounts for several observations made about microbial activities. For instance, it explains how saprobic microbes in soil and water rapidly break down complex substrates and how some pathogens release their toxins into the tissues simultaneously.
The effect of quorum sensing has also given greater insight into how pathogens invade their hosts and produce large quantities of substances that damage host defenses. It has been well studied in a number of pathogenic bacteria. Infections by Pseudomonas aeruginosa give rise to tenacious lung biofilms in cystic fibrosis patients and some types of pneumonia. Staphylococcus aureus commonly forms biofilms on inanimate medical devices and in wounds. Streptococcus species are the initial colonists that create dental plaque on tooth surfaces.
Although the best-studied biofilms involve just a single type of microorganism, most biofilms observed in nature are polymicrobial. In fact, many symbiotic and cooperative relationships are based on complex communication patterns among coexisting organisms. As each organism in the biofilm carries out its specific niche, signaling among the members sustains the overall partnership. Biofilms are known to be a rich ground for genetic transfers among neighboring cells. As our knowledge of biofilm patterns grows, it will likely lead to greater understanding of their involvement in infections and their contributions to disinfectant and drug resistance.
Microbiota—Our Human Ecology
Microbial interrelationships affect humans in many ways, but those having the most impact are microbes that naturally live on or in the body and parasites that attack the body. The normal residents, called microbiota, are so abundant that in most people, each hu man cell has a microbial counterpart. Obviously, a community of microbes this large is bound to have enormous effects on our physiology, immunity, and genetics.
Resident microbes come from nearly all groups and include viruses, archaea, bacteria, fungi, and protozoa that have adapted to specialized niches throughout our bodies. Some are commensals that make a living on the body without harm or benefit, but a greater number are mutualistic, providing some form of nutrition or protection. For example, lactobacilli growing in the vagina maintain an acidic pH that can protect the female reproductive tract from infections (figure3), and other species are involved in digestive processes and the function of the intestine. The mutualist Bacteroides thetaiotaomicron processes complex food molecules that provide nutrients for other organisms in the gut, promoting a stable gastro intestinal microbiota. The stabilizing effects of bacteria on the intestinal environment are the basis for probiotic supplements and foods. Results of hundreds of new studies are rapidly accumulating information on the complex interrelationships we have with our microbiota.
Fig3. Stained vaginal smear reveals a large pink epithelial cell harboring gram-positive rods of Lactobacillus, an important member of the normal microbiota that helps protect against infection. Dr. Mike Miller/CDC
Most of the time, our 40 trillion microbial passengers do more good than harm, but if certain types are displaced from their niche or are allowed to enter the sterile tissues, they become opportunistic pathogens and can cause infections. Microbes living in biofilms can create mixed infections, meaning that several microbes acting together invade the tissues and cause damage. This is the pattern in dental caries, periodontal disease, and gas gangrene.
