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Bacteria are one-celled organisms that lack a nucleus . Bacteria are prokaryotes that maintain their genetic material, DNA, in a long circular molecule. Bacteria also contain DNA in small circular molecules termed plasmids. Gram positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain (safranin) and appearing red or pink. Gram-positive organisms are able to retain the crystal violet stain because of the high amount of peptidoglycan in the cell wall. Gram-positive cell walls typically lack the outer membrane found in Gram-negative bacteria.


Pili connect the bacterium to another of its species, or to another bacterium of a different species, and build a bridge between the cytoplasms of either cells. All pili are primarily composed of oligomeric pilin proteins.That enables the transfer of plasmids between the bacteria. Some bacterial viruses or bacteriophages attach to receptors on sex pili at the start of their reproductive cycle. Despite its name, the sex pilus is not used for sexual reproduction.

                                                        Attachment of bacteria to host surfaces is required for colonization during infection or to initiate formation of a biofilm. A fimbria is a short pilus that is used to attach the bacterium to a surface. Fimbriae are found in both Gram-negative and Gram-positive bacteria. In Gram positive bacteria, the pilin subunits are covalently linked.