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The picture shows a cutaway of the plague bacterium, Yersinia pestis, with many of the main components illustrated. The cell is drawn to scale and in this diagram measures 700nm in diameter and 2100nm in length (excluding the outer lumpy coat). The image above is 300 pixels across and the original is 2,277 pixels across and lacks all labels.GENOME: The genetic material is a skein of circular DNA localised as the nucleoid (orange brown mass in middle of cell). The nucleoid lacks a nuclear membrane (a defining characteristic of prokaryotic cells). In Yersinia pestis, the nucleoid chromosome has 4,650,000 base pairs. There are also various types of plasmids shown. Plasmids are separate pieces of circular DNA that replicate independently. Yersinia has 3 types of plasmid:pFra 96,200 bases (shown as yellow globules) 1-2 copies per cell (encodes the F1 polysaccharide that forms the lumpy surface feature). This plasmid is fixed to the line of cell division and so is shown in the middle of the cell in this diagram. pYV 70,300 bases (shown as green globules) 4-8 copies per cell pPst 9,600 bases (shown as blue globules) 100-200 copies per cell This means that the chromosome is roughly 48 times as big as the biggest plasmid and this is the approximate scale to which they are drawn. The plasmids are shown as being roughly spherical. Plasmids are formed from closed rings of DNA that presumably supertwist into a bundle. The sequence of the genome has recently been determined (NATURE, Vol 413, 4 October 20901, p523527). genome sequence of Yersinia pestis CYTOPLASM The bacterial cytoplasm is shown filled with ribosomes. These are somewhat smaller than their eukaryotic counterparts and many are shown linked into polysomes. CELL WALL: The outer membrane comprises an inner phospholipid layer and an outer lipopolysaccharide layer. The middle orange layer is peptidoglycan which lies just exterior to the plasma membrane (blue). This is not like the typical Gram-negative enterobacterial cell wall in that it lacks O-side chains due to a disrupted O-antigen gene cluster (it is a so-called "rough mutant"). CAPSULE: Under some circumstances, Yersinia pestis is surrounded by a loose capsule. The illustration shows the "lumpy" F1 antigen that preferentially forms at 37 degrees C. The capsule is cut away to reveal the underlying cell and its contents. For more information, please see: Scanning Electron Microscopic Study of Virulent Yersinia pestis and Yersinia pseudotuberculosis Type 1, T.H. Chen & Sanford S. Elberg, INFECTION & IMMUNITY, Mar. 1977, p 972-977NEEDLES: Type III secretion needles are shown radiating from
the cell. These are probably produced at 37 degrees C. Such needles have been demonstrated
in the closely related species Yersinia enterocolitica. They have not yet been demonstrated
in Yersinia pestis but it is a reasonable hypothesis that they are there (along with
the capsule) at 37 degrees C, the temperature in mammals. It is generally thought
that it is contact with cells that triggers the formation of these needles. For more
information on these needles, please see:
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