Competition: Mechanisms
What are the mechanisms that enable microbes to survive and flourish in this competitive environment?
1. Rapid Response
If two cells are in close proximity, the release of digestive enzymes into the surrounding solution will result in breakdown of the organic matter (e.g. cellulose), and release of simple organic compounds (e.g. glucose). The simple compounds will be in solution surrounding the source of those compounds. The cell that absorbs the simple organic compounds will benefit from the energy. Thus a microbe that detects the presence of and identifies a potential food, releases the enzymes that degrade the food, and then absorbs the simple compounds, will be better able to compete with slower microbes.
Note that all microbes in the vicinity of digestion will be able to absorb any simple compounds (amino acids, sugars, vitamins, fatty acids etc). Further, microbes close to digestion, will be bathed by a greater the concentration of available nutrients. Thus some cells will benefit without expressing enzymes, provided that they are in the locality of extracellular digestion by another microbe.
Cells may react rapidly when they detect food. They may also reproduce rapidly once food has been absorbed. Further, cells able to shut down metabolic processes when food is depleted may also survive more readily. A rapid response in all aspects of growth and development are important in competitive interactions.
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animation illustrating the rapid response mechanism (5 Kb).
2. Inhibit Competitors
Cells that are bathed in the digestion process of another cell benefit without the cost of detecting the potential food and expressing appropriate enzymes. However, if the microbe that is active also expresses molecules that inhibit competitors, then more of the digestion will pass to the active microbe.
Various inhibitors are known. Some of these compounds are toxins (e.g. bactericidal compound), that is, the compounds kill cells. Other inhibitors cause the competitor to go into stasis (e.g. fungistatic compound). The mechanisms of action are highly varied, and specific. Thus we are aware of compounds that kill plants, animals, bacteria and fungi, and others that are specific to one genotype of one species. Further compounds produced by fungi (or bacteria) act against other fungi, bacteria or plants. Humans use some inhibitors (e.g. penicillin). Other inhibitors are potent toxins that cause considerable damage when present in foods (e.g. aflotoxins). While we do not understand the function of some inhibitors, it is clear that many have the potential to play an important role in the competitive interactions between microbes.
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animation of inhibition (10 Kb).
3. Overcome Inhibitors
Microbes have evolved together over a long period of time. Mechanisms to detect inhibitors and neutralise their impact have also evolved. In bacteria, the plasmid often carries genes that enable cells to resist, bypass, neutralise, or excrete inhibitors. As plasmids can be transferred between cells, use of inhibitory compounds (especially by humans) will result in the survival of many different cells carrying plasmids that provide the host with resistance.
Transfer of plasmids under the selection pressure of antibiotics results in the widespread acquisition of resistance to antibiotics. Continued misuse of antibiotics (for treatment of viral infection, for instance) will inevitably result in most commonly used antibiotics being useless, especially in hospitals where they play very important role. Multidrug resistance is one of the greatest current threats to human health. Penicillin has been available since 1945. With continued spread of resistance to multiple drugs in hospitals, the penicillin story will become an odd, brief and unusual part of human history.
Reference: George, A. (2003) March of the Superbugs. New Scientist, Inside Science 162, 19 July, pp.1-4.
4. Special Niche
Cooperation (1):
Many inhibitors have little or no impact on most microbes. Thus, movement
of plasmids between cells may also result in the movement of genes that
enable the host cell to overcome the inhibitor. Inhibition and overcoming
inhibition is widespread within microbial communities, and is thought
to have resulted in a situation where different microbes tend to live
together, rather than in opposition. That is, each microbe has a specific
suite of foods, and processes, and the action of digestion results in
sharing of resources. As complex resources are shared by all partners,
the outcome is survival of all components of the community.
The concept underlying this belief is that each microbe has a specific array of foods that it can access. It is possible to add complex and unusual molecules to natural environments and the molecules will eventually disappear. These complex materials degrade, albeit slowly in many cases. Each microbe contributes its specific enzymatic capacity. The resource is shared. Complex microbial communities are supported because they act on the resource synergistically.
The capacity to degrade specific compounds has some unexpected consequences. For instance, if petroleum products become contaminated by the fungus Amorphotheca resinae, the fungus will grow quite happily, much to the concern of the oil industry. However, that same group of fungi can also be used to aide the removal of oil products from contaminated soil, such as following oil spills.
References:
Casterton, J. W. et al (1995) Bacterial Biofilms. Annual Review
Microbiology 79: 711-745.
Fry, C. (2003) Iron Rations. New Scientist, Vol 179, No 2405, 26 July,
pp.36-37.
Cooperation (2):
Microbes associate with a wide diversity of organisms. Perhaps the most common group of microbes is associated with the roots of plants. The mycorrhizal fungi (the association is called a mycorrhiza) gain access the products of photosynthesis directly from the plant. Interestingly, the fungi benefit the plant by increasing plant access to minerals in soil. In many plant species, mycorrhizal fungi are essential for access to and uptake of organic and inorganic phosphate and to a lesser extent organic nitrogen. Further, the fungi influence water relations of their host, improving plant response to relief of drought. They also reduce the impact of some pathogens, by modifying hormonal and response reactions of the host plant.
Other widespread symbioses include lichens and corals. In most cases, one partner supplies organic energy from photosynthesis. Other benefits are seen. Microbes are essential for herbivores because they degrade complex carbohydrate in the digestive system, and provide complex fatty acids, vitamins and amino acids. Some microbes also supply a protective function for their host. In general terms, microbes have biochemical and functional diversity, larger organisms have morphological diversity.
Microbes play an essential role in all ecosystems. Their capacity to interact determines many of the functional attributes of those ecosystems. Alteration of microbial components has profound consequences, many of which we see when we attempt to rehabilitate disturbed ecosystems. We need to better understand the attributes supplied by microbes, to complement our understanding of the larger organisms.
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