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Bacteria frequently secrete chemicals into their environment in order to modify it favorably. The secretions are often proteins and may act as enzymes that digest some form of food in the environment.


A few bacteria have chemical systems that generate light. This bioluminescence often occurs in bacteria that live in association with fish, and the light probably serves to attract fish or other large animals.<ref>Dusenbery, David B. (1996). Life at Small Scale. Scientific American Library. ISBN 0-7167-5060-0.</ref>


Bacteria often function as multicellular aggregates known as biofilms, exchanging a variety of molecular signals for inter-cell communication, and engaging in coordinated multicellular behavior.<ref name=shapiro1>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref name=costerton1>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

The communal benefits of multicellular cooperation include a cellular division of labor, accessing resources that cannot effectively be utilized by single cells, collectively defending against antagonists, and optimizing population survival by differentiating into distinct cell types.<ref name=shapiro1/> For example, bacteria in biofilms can have more than 500 times increased resistance to antibacterial agents than individual "planktonic" bacteria of the same species.<ref name=costerton1/>

One type of inter-cellular communication by a molecular signal is called quorum sensing, which serves the purpose of determining whether there is a local population density that is sufficiently high that it is productive to invest in processes that are only successful if large numbers of similar organisms behave similarly, as in excreting digestive enzymes or emitting light.

Quorum sensing allows bacteria to coordinate gene expression, and enables them to produce, release and detect autoinducers or pheromones which accumulate with the growth in cell population.<ref name="pmid11544353">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>



Many bacteria can move using a variety of mechanisms: flagella are used for swimming through fluids; bacterial gliding and twitching motility move bacteria across surfaces; and changes of buoyancy allow vertical motion.<ref name=Bardy>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Flagellum of Gram-negative bacteria. The base drives the rotation of the hook and filament.

Swimming bacteria frequently move near 10 body lengths per second and a few as fast as 100. This makes them at least as fast as fish, on a relative scale.<ref>Dusenbery, David B. (2009). Living at Micro Scale, p. 136. Harvard University Press, Cambridge, Mass. ISBN 978-0-674-03116-6.</ref>

In bacterial gliding and twitching motility, bacteria use their type IV pili as a grappling hook, repeatedly extending it, anchoring it and then retracting it with remarkable force (>80 pN).<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal

"Our observations redefine twitching motility as a rapid, highly organized mechanism of bacterial translocation by which Pseudomonas aeruginosa can disperse itself over large areas to colonize new territories. It is also now clear, both morphologically and genetically, that twitching motility and social gliding motility, such as occurs in Myxococcus xanthus, are essentially the same process."

Flagella are semi-rigid cylindrical structures that are rotated and function much like the propeller on a ship. Objects as small as bacteria operate a low Reynolds number and cylindrical forms are more efficient than the flat, paddle-like, forms appropriate at human-size scale.<ref>Dusenbery, David B. (2009). Living at Micro Scale, Chapter 13. Harvard University Press, Cambridge, Mass. ISBN 978-0-674-03116-6.</ref>

Bacterial species differ in the number and arrangement of flagella on their surface; some have a single flagellum (monotrichous), a flagellum at each end (amphitrichous), clusters of flagella at the poles of the cell (lophotrichous), while others have flagella distributed over the entire surface of the cell (peritrichous). The bacterial flagella is the best-understood motility structure in any organism and is made of about 20 proteins, with approximately another 30 proteins required for its regulation and assembly.<ref name=Bardy/> The flagellum is a rotating structure driven by a reversible motor at the base that uses the electrochemical gradient across the membrane for power.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> This motor drives the motion of the filament, which acts as a propeller.

Many bacteria (such as E. coli) have two distinct modes of movement: forward movement (swimming) and tumbling. The tumbling allows them to reorient and makes their movement a three-dimensional random walk.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> (See external links below for link to videos.) The flagella of a unique group of bacteria, the spirochaetes, are found between two membranes in the periplasmic space. They have a distinctive helical body that twists about as it moves.<ref name=Bardy/>

Motile bacteria are attracted or repelled by certain stimuli in behaviors called taxes: these include chemotaxis, phototaxis, energy taxis, and magnetotaxis.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> In one peculiar group, the myxobacteria, individual bacteria move together to form waves of cells that then differentiate to form fruiting bodies containing spores.<ref name=autogenerated1 /> The myxobacteria move only when on solid surfaces, unlike E. coli, which is motile in liquid or solid media.

Several Listeria and Shigella species move inside host cells by usurping the cytoskeleton, which is normally used to move organelles inside the cell. By promoting actin polymerization at one pole of their cells, they can form a kind of tail that pushes them through the host cell's cytoplasm.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

Bacteria sections
Intro  Etymology  Origin and early evolution  Morphology  Cellular structure  Metabolism  Growth and reproduction  Genetics  Behavior  Classification and identification  Interactions with other organisms  Significance in technology and industry  History of bacteriology  See also  References  Further reading  External links  

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