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It is a common misimpression that microbes exist as free-living, single-celled organisms. This idea underlies the famous postulates of the German biologist Robert Koch, the first to isolate the cause of anthrax. In his view, to prove unambiguously that a particular organism was the cause of a particular disease it was necessary to show that the presumptive pathogen
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For many pathogens, and for a true appreciation of microbial diversity, this view is likely to be seriously misleading. The vast majority, over 99.9% of all microbes, have yet to be grown in monoculture. Even if it were possible, studying the properties of an organism in monoculture could be misleading. |
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Most microorganisms live within complex communities composed of a number of different types of organisms. They are often critically dependent upon one another for nutrients, more to the point their behavior will be altered by their environment. These types of communities are known as biofilms. The properties of a biofilm are not necessarily a simple function of its inhabitants. A biofilm is more than the sum of its parts.
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| The importance of biofilms: Biofilms form on essentially any surface. Their properties influence how their component organisms respond to antibiotics and how they interact with their host. The organisms within the biofilm secrete glue-like molecules that hold the film together (link to bacterial super-glue) Biofilms mature over time. They have a natural progression, like a forest. |
| There is colonization, maturation and climax, each characterized by specific intrabiofilm and biofilm-host interactions. Not all of the organisms within a biofilm are necessarily "friendly" towards one another, and many antibiotics are the result of interfilm competition. |
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We are in close association with a number of biofilms. Their properties and dynamics, a topic that is beyond our scope here, play a critical role in health and disease. |
Making metazoans: Biofilms are communities or microecologies composed of distinct organisms leaving in various states of interdependence and competition. |
An early step on the path to true organismic multicellularity is the development of different cell types of the same organism. One example of multiple cell types is found in the bacteria Caulobacter crescentus. It can produce stalk cells , that are attached to a substrate and swimming cells. |
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A next level of integration occurs in the eukaryotic slime molds, in particular the the cellular slime molds, such as Dictyostelium discoideum, which form temporary multicellular organisms. Cellular slime molds live in soil and eat bacteria - they are predators. Most of the time they are small, amoeba-like haploid cells. Upon starvation, however, they undergo a dramatic aggregation process. |
Aggregation is triggered by the release from isolated cells of pulses of cyclic adenosine monophosphate (cAMP) . This leads to the streaming aggregation of cells towards the cAMP source. Between 10,000 to 50,000 amoebae aggregate but remain as distinct cells. They adhere to one another to form a slug. The slug can migrate and eventually will differentiate, forming a fruiting body that rises above the soil. |
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Some of the cells of the slug differentiate to form the stalk. These cells are terminally differentiated and die with reproducing themselves. They sacrifice themselves for the good of the aggregate. Others form the fruiting body, which differentiates to form spores. When released, the spores can be widely dispersed and can form single celled amoebae. The slime molds are temporary metazoans. |
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The steps to true metazoans and metaphyta (multicellular animals and plants) remain to be completely defined, but important events were the development of permanent adhesions and distinct cell types. There are a number of colonial eukaryotes that contain multiple cell types; specifically cells that possess flagella and those that do not. The flagellum is a motile organelle used to move the organism through space, or to move fluid over the organism. |
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In some cases, the cells are held together by a gelatinous matrix and form loose colonial groups. |
Alternatively, the cells can be regularly organized, as in a Volvox. Volvox can reproduce asexually, by forming daughter colonies that develop within the mother. There is also a sexual cycle, mediated by the formation and fusion of gametes. These colonial organisms appear closely related to higher plants or metaphyta. |
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The animals or metazoa appear to have been derived from a different lineage. Their sister group (closest unicellular relatives) appears to be the choanoflagellates. |
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Choanoflagellates have cells that are characterized by a single flagellum surrounded by a distinctive collar. Choanoflagellates exist in both unicellular and simple colonial forms. The collar cells are located on the periphery of cell aggregate, while amoeboid cells are located in the interior. |
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Sponges (porifera) are among the simplest of the metazoans. They contain only a few different types of cells. These include
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As fluid is pumped through the surface pores, particles are food are captured. Aside from these fluid movements, sponges are sessile, unmoving. Fossils of extinct sponges, such as the Archaeocyathids, are found in Cambrian rock that is over 500 million years old. Earlier sponge-like organisms, for example Paleophragmodictya have been found in Precambrian rock. PBS : SHAPE OF LIFE - THE FIRST ANIMALS |
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The next level of metazoan complexity is represented by hydra and related organisms, the hydrozoa, which include the jellyfish. Some of these organisms alternate between between a sessile and benthic or floating lifestyle. They can swim and they can move in subtle ways. |
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The hydrozoa contain more distinct cell types than the porifera. The most dramatic difference is their ability to produce coordinated organismic movement. While sponges are like sieves, the hydrozoa have a single distinct mouth and an internal stomach-like cavity, specialized to digest prey. Their mouth also serves an their anus, where wastes are released. |
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Hydrozoan movement is coordinated by a network of cells, known as a nerve net, that acts to regulate contractile muscle cells. Together the nerve net and muscles cells generate coordinated movements. There is no central brain, which in its simplest form is just a dense mass of nerve cells. Nevertheless, a hydra can displays movement complicated enough to capture and engulf small fish! |
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Stinging cells, nematocysts, are located in the "arms". Triggered by touch, they explode outward, embedding themselves in prey and delivering a paralyzing poison. Hydrozoans are complex enough to be true predators. |
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