1. Most microorganisms in complex communities have not been grown or characterized. This has limited our understanding of microorganism interactions and their roles in nature and disease. Molecular techniques are providing a better understanding of these uncultured organisms.

2. The term symbiosis, or "together-life," can be used to describe many of the interactions between microorganisms, and also microbial interactions with higher organisms, including plants and animals. These interactions may be positive or negative.

3. Microbial ecology is the study of microbial relationships with other organisms and also with their nonliving environments. These relationships, based on interactive uses of resources, have effects extending to the global scale.

4. Positive interactions include mutualism, protocooperation, and commensalism. Negative interactions include parasitism, predation, amensalism, and competition. These interactions are important in natural processes and in the occurrence of disease. The interactions can vary depending on the environment and changes in the interacting organisms.

5. Microorganisms, as they interact, can form complex physical assemblages that are often described as biofilms. These occur on living and inert surfaces and have major impacts on microbial survival and the occurrence of disease.

6. Microorganisms also interact by the use of chemical signal molecules, which allow the microbial population to respond to increased population density. Such responses include quorum sensing, which controls a wide variety of microorganism properties.

7. Energy, electrons, and nutrients must be available in a suitable physical environment for microorganisms to function. Microbes interact with their environment to obtain energy (from light or chemical sources), electrons, and nutrients, which leads to a process called biogeochemical cycling. Microorganisms change the physical state and mobility of many nutrients as they use them in their growth processes.

8. Microorganisms are an important part of ecosystems, or self-regulating biological communities and their physical environments. Microorganisms play an important role in succession, or the predictable changes that occur in ecosystems when they are disturbed.

9. Extreme environments restrict the range of microbial types able to survive and function. This can be due to physical factors such as temperature, pH, pressure, or salinity. Many microorganisms found in "extreme" environments are especially adapted not only to survive, but to function metabolically under these particular conditions.

10. Methods used to study microbial interactions and microbial ecology provide information on environmental characteristics; microbial biomass, numbers, types and activity; and community structure. Microscopic, chemical, enzymatic, and molecular techniques are used in these studies.

11. It is now possible to determine the nucleic acid sequences of specific microorganisms or organelles isolated from natural environments and to study the phylogeny of uncultured microorganisms. This should lead to important new advances in the study of microbial ecology.

In previous chapters microorganisms usually have been considered as isolated entities. The basic characteristics of microorganisms, including the structure and function of microbial cells, metabolism, growth and the control of growth, have been discussed. In addition, metabolism, genetics, and molecular aspects of microorganisms, including genomics, have been described. In this chapter, microbial interactions with both the physical environment and with other organisms will be considered.

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