Life in a Biofilm. Most microbes live attached to solid surfaces (biotic or abiotic) within highly organized and functionally interactive communities called biofilms. These biofilms can be composed of populations that developed from a single species or a community derived from multiple species. All exhibit collective and interdependent behavior, with different genes rapidly brought into play as conditions dictate (see figure). Among the many advantages of biofilm living are nutrient availability with metabolic cooperation, acquisition of new genetic traits, and protection from the environment. Researchers are only beginning to realize the prevalence and significance of biofilms. These communities probably play major roles in such complex natural processes as the cycling of nitrogen and sulfur and the degradation of environmental pollutants and organic matter, activities that require a range of metabolic capabilities. Recent metagenomic and metaproteomic studies focused on biofilm members in an acid mine drainage environment. [Reference: M. E. Davey and G. A. O’Toole, “Microbial Biofilms: From Ecology to Molecular Genetics,” Microbiol. Mol. Biol. 64(4), 847–67 (2000).] Strength in Numbers. Microbes in biofilms live an interdependent, community-based existence (see accompanying text above). In this overhead view of an idealized biofilm, four microcolonies in the center of the figure represent organisms that both generate and consume hydrogen. Two participate in syntrophism, in which hydrogen producers use organic acids generated by fermenting organisms that gain their carbon and energy by using various sugars. In addition to potential metabolic interactions, signaling molecules may aid in inter- and intraspecies communication. These genetic factors and environmental influences contribute to the biofilms’ spatial organization. [Figure and caption adapted from Davey and O’Toole, 2000.] Quorum Sensing. Microbes communicate with each other by sending and detecting a wide variety of chemical signals (autoinducers). These molecules trigger group behaviors, including the formation and persistence of biofilms, symbiosis, and other processes. Many of these activities are density dependent, that is, when a threshold concentration of chemicals is detected (reflecting a certain number of cells), microbes respond with a change in gene expression. This process, called quorum sensing, facilitates coordination of gene expression by the entire community, in essence enabling it to behave like a multicellular organism. Quorum sensing allows microbial communities to adapt rapidly to environmental changes and reap benefits that would be unattainable as individuals. Quorum sensing first was described in the bioluminescent marine bacterium Vibrio fischeri. This microbe lives in symbiotic association with several marine animal hosts, who use the light it produces to attract prey, avoid predators, or find a mate. In exchange, V. fischeri obtains a nutrient-rich home environment. V. fischeri emits light only inside a specialized light organ of the host, where the concentration of these organisms becomes dense; it does not give off light when free living in the ocean. Light production depends on producing, accumulating, and responding to a minimum-threshold concentration of an autoinducer (acylated homoserine lactone). Only under the nutrient-rich conditions of the light organ can V. fischeri grow to such high populations. Also, trapping the diffusible autoinducer molecule in the light organ with the bacterial cells allows it to accumulate to a concentration sufficient for V. fisheri to detect it. Recent studies have revealed diverse chemical languages that enable bacterial communication both within and between species (the latter called cross talk). The extracellular matrix surrounding mature biofilms (composed of glycans and other components) plays a crucial role in transmitting these chemical signals into and between cells. Biotechnological researchers are developing molecules structurally related to autoinducers to exploit quorum-sensing capabilities and possibly improve industrial production of natural products.
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