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Summary Prokaryote morphology
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Summary of the different morphological features of bacteria and archaea, including cell wall composition, flagellum and pili
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Prokaryote morphology- In prokaryotes, DNA is grouped into a nucleoid. Some species also have plasmids,
which are small loops of DNA.
Structures external to the cell wall
- Glycocalyx (capsule) – a polysaccharide or protein on the external surface. This is
used to adhere a prokaryote to a surface and is also I reveal in the formation of
biofilms. Some species such as Rickettsia also have a slime layer, which is
unorganised and loosely attached to the cell wall.
- Flagella – these are long appendages, made up of a filament, hook and basal body.
They rotate to propel the organism (taxis – positive and negative). Flagella are
composed of flagellin, a globular protein arranged in intertwined chains, forming a
helix around a hollow core. Flagellin antigens (H antigens) are used in the serological
identification of bacterial pathogens and vary between gram-negative and gram-
positive bacteria.
- Bacteria have four main flagella arrangements:
• Monotrichous – one flagellum, species such as Vibrio cholerae
• Amphitrichous – one flagellum at either end of the cell, e.g., spirochaetes
• Lophotrichous – several flagella at either end of the cell, such as Spirillum
• Peritrichous – flagella all around the cell body, e.g., Escherichia coli
- Axial filaments – endoflagella are protected by the outer sheath, which helps to
maintain shape, and is particularly important in spirochaete bacteria.
Fimbriae and pili
- These are short and thin protein appendages (smaller than flagella)
- Used to adhere cells to the surface.
- Can be used in floatation, increasing buoyancy.
- Cells only have a few pili, but many fimbriae.
- Pili play a key role of joining cells in conjugation, the transfer of genetic material
from cell to cell.
- Pili are also used for certain types of cell movement.
- Fimbriae are involved in biofilm formation.
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