Biofilm

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Many unicellular microorganisms, prokaryotes in particular, form dense films called biofilms. Cells lay down gel-like polysaccharide matrix when contacting solid surface. This matrix traps other bacteria, forming a biofilm. Thus, biofilms are structured groups of one or more microbial species encased in an extracellular polysaccharide (EPS) matrix and attached to a solid surface.

Mineral oxidation occurs both at the point at which the microorganisms are attached to the mineral (biofilm growth) or planktonic growth [1]. Mineral oxidation has been exploited in bioleaching [2].

Contents

Biofilm formation on minerals

Con-focal microscopy images of F. acidarmanus biofilm development on the surface of pyrite after 3 (A), 10 (B), 21 (C), and 38 days (D) . The pyrite is red and the cells are stained green (the greater the signal the brighter the image). The signal from planktonic cells (visible after 3 days) is too weak to be observed as the biofilm fluorescence increases.
Con-focal microscopy images of F. acidarmanus biofilm development on the surface of pyrite after 3 (A), 10 (B), 21 (C), and 38 days (D) [3]. The pyrite is red and the cells are stained green (the greater the signal the brighter the image). The signal from planktonic cells (visible after 3 days) is too weak to be observed as the biofilm fluorescence increases.

Surface attachment, the pre-cursor to biofilm formation, is very important for mineral oxidation. Acidophilic species that have been shown to bind and form biofilms on the surface of sulfide minerals include Acidithiobacillus ferrooxidans, At. caldus and Ferroplasma acidarmanus [4] [5] [6]. Surface attachment is not random, and occurs at dislocation sites such as cracks and the boundaries of mineral grains [7]. Physical attachment to the mineral surface is aided by EPS and in At. ferrooxidans the exopolymer is complexed with Fe3+ and attaches to the mineral by electrostatic interactions [8][9].

Numerous structural and genetically encoded regulatory determinants of biofilm development have been revealed. In some bacteria biofilm development is mediated by the quorum sensing molecules, N-acyl homoserine lactones (HSLs) that allow cell-cell communication [10] and At. ferrooxidans HSL production has been characterized [11].

Possible function of biofilms (in the perspective of a microbe)

  • increased availability of nutrients for growth
  • increased binding of water molecules, which reduces the possibility of desiccation,
  • some protection against UV radiation, perhaps also physical protection. Biofilms protect microorganisms from antimicrobial agents such as metals, thereby conferring an advantage during metal leaching as well as concentrating Fe3+ that oxidizes the metal sulfide bond
  • the establishment of complex consortia, which allows for the recycling of substances
  • easier genetic exchange due to the proximity to progeny and other bacteria

Questions to answer

  • When do biofilms cause problems for humanity? Describe how the microenvironment created by a biofilm enhances corrosion.
  • When are biofilms helpful? Especially, describe biofilms relevant for bioleaching, metal removal in wastewater treatment plants or other examples from the Biohydrometallurgy area.

Citations about biofilms

"Biological strategies: biofilms and microbial mats Biofilms (a word coined by John William Costerton in 1978) develop when microorganisms attach to surfaces in aquatic environments and produce exopolysaccharides, which help cells to adhere to submerged surfaces. Biofilms can consist of either a single species or a community consisting of many microbial species, most of which are prokaryotes. Microbial mats can be considered complex biofilms. A major aspect of their species composition is the presence or absence of representatives from the three basic functional groups: primary producers (i.e. autotrophs), consumers, and decomposers. Biofilms without primary producers depend on exogenous sources of organic matter. Single-species biofilms are particularly dependent on their environment not only to provide a source of oxygen and nutrients but also to transport wastes. Microbial mats, with their rich diversity of organisms, are sites of complex elemental transformations [50]. At a higher level of organization, microorganisms in microbial mats or in biofilms make up coordinated functional communities much more efficient than mixed populations of floating planktonic organisms. In fact, biofilms or mats resemble the tissue formed by eukaryotic cells in their physiological cooperativity and in the extent to which they are protected from variations in bulk-phase conditions by a kind of primitive homeostasis provided by the matrix of exopolysaccharides [7]. The analogy with eukaryotic organisms can be extended even to dissemination strategies, in which well-protected communities of cells are the most successful and their genomes are thus the most competitive, whereas planktonic cells are produced to disseminate and to colonize new localizations. The controlled shedding of planktonic cells from biofilms is a major strategy in the bacterial struggle for survival and predominance in aquatic ecosystems. We can consider the survival value of this strategy in the milieu of the early Earth. The advantages that this kind of growth confers to sessile bacteria are: (1) increased availability of nutrients for growth, (2) increased binding of water molecules, which reduces the possibility of desiccation, (3) some protection against UV radiation, (4) the establishment of complex consortia, which allows for the recycling of substances, and (5) easier genetic exchange due to the proximity to progeny and other bacteria [23]. The ability to remain in an optimal or even permissive local environment was one of the most valuable contributions of sessile growth to bacterial survival. The biofilm ‘‘phenotype’’ demonstrates that adhesion and 180 biofilm development were selected for early in the evolution of bacteria. The persistence of this positive selection for the biofilm phenotype today is evident by the predominance of this sessile mode of growth in most ecosystems [8]. [12]

References

  1. Rawlings (2002) Ann Rev Microbiol 56, 65-91.
  2. Rohwerder et al. (2003) Appl Microbiol Biotechnol 63, 239-248.
  3. Dopson et al. (2006) Un-published.
  4. Fu et al. (2004) Trans Nonferrous Metals Soc China 14, 383-387.
  5. Edwards et al. (2001) FEMS Microbiol Ecol 34, 197-206.
  6. Edwards et al. (2000) Environ Microbiol 2, 324-332.
  7. Gehrke et al. (1998) Appl Environ Microbiol 64, 2743-2747.
  8. Gehrke et al. (1998) Appl Environ Microbiol 64, 2743-2747.
  9. Kinzler et al. (2003) Hydrometallurgy 71, 83- 88.
  10. Stanley & Lazazzera (2004) Mol Microbiol 52, 917-924.
  11. Farah et al. (2005) Appl Environ Microbiol 71, 7033-7040.
  12. Guerrero, R., Piqueras, M., Berlanga, M., Microbial mats and the search for minimal ecosystems. Int Microbiol (2002) 5:177-188, DOI 10.1007/s10123-002-0094-8

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