Simple Diagram of Bacteria

Bacteria / September 13, 2019

Gram_negative_bacterial_flagellumBacterial Motility - flagella and nanotechnology

The diagram above shows a model of a close-up view of the basal components of a bacterial flagella.
These are the wheels at the roots of the flagella as mentioned in the introduction to bacteria, if you have
not read this introduction then click here to learn the basics of bacterial structure. The diagrams below add
some labels to this structure. Note that only a tiny portion of the filament is shown here, since this is the
long helix-like propeller that we saw previously. Remember the flagellum rotates like a propeller to propel
the bacterial cell along, as shown by the arrows in the diagram below.

Above: the flagellum emerges from the bacterial surface or wall (or cell envelope)` which consists of
three main layers: the outer membrane (OM), the periplasm (P) which contains a mesh of very strong
fibrous material called peptidoglycan (or murein) and an inner cytoplasmic membrane (CM). (Click
here to learn about cell membranes). Note how different this arrangement is from an animal cell which
has a single cell membrane rather than this double membrane structure. Beneath these layers is the cell
interior (the intracellular compartment) or protoplast (consisting of cytoplasm and nucleoid) and external
to these layers is the extracellular (external) environment, such as the water the bacterium is swimming
in. (Additional layers may exist outside the OM, including a slime capsule, but we shall look at these
possibilities later). This type of wall structure is particular to some types of bacteria called Gram
negative bacteria, but other wall structures occur, as we shall see later. The root of the flagellum
consists of a series of rings (made of proteins) that anchor the structure in the cell envelope.

Above: the flagellar basal complex with some outer structures (Mot proteins) removed to show the
internal structure of the motor. The labels are shown in the figure below:

Above: the flagellar motor rotates, causing the flagellum to rotate (and the cell to rotate in the opposite
sense). Notice the scale bar: the line illustrates a real-life length of 20 nanometres (20 millionths of a
millimetre!) so we are dealing with a minute machine - a nanomachine! These minute electric motors
evolved by natural means, on Earth, long before human beings existed.

The basal structure consists of a series of rings connected by a rod, the rings are the L ring (embedded in
the lipid bilayer of the outer membrane), the P ring (embedded in the periplasm), the S and the M rings
(the rotating motor or rotor) and the C ring. The L and P rings act as a bushing (a bushing is a ring-like
structure that constrains moving mechanical parts, in this case the rotating rod, and may also be lubricated
to reduce friction). The motor proteins (Mot) conduct electric current carried by positively charged
protons ('positive electricity' as opposed to negative electricity in which the current is carried by negatively
charged electrons as in a metal wire) from the periplasm into the cell cytoplasm. The electric charge is
thought to flow into the M (motor) ring where it is converted into rotary mechanical motion, causing the
M-ring to rotate. The M ring is attached to the rod, causing the rod to rotate. The M ring acts against the
fixed Mot (stator) ring, which rotates slowly in the opposite direction to the M ring - slowly because it is
fixed to the bacterial cell wall and so causes the whole bacterial cell to rotate in a direction opposite to the
M ring and rod. The S ring is now known to be part of the rotor, along with the M ring, and forms a socket
for the rod, but is not part of the stator. Confusion may arise when the stator ring is referred to as the

The rod is attached to the filament via a

flexible hook (which acts as a universal joint, transferring rotary
motion to the filament via the hook associated proteins (HAPs)). The filament is actually much too long
to show more than a tiny segment of it in these diagrams, it is about 20 nanometres in diameter, but 10 -
15 micrometres (10 - 15 thousand nanometres) long, which is longer than a typical bacterial cell which is
about 2 micrometres long. The filament is made up of about 30 000 subunits of a protein called flagellin
and is a corkscrew or helix shape. (The flagellin is arranged into typically 11 strands that are twined
together). This shape is important, mutants with straight filaments are immotile - the filament is the
propeller driven by this remarkable microscopic electric motor! (The cell body may contribute to thrust in
some forms in which the body is also helical).bacterial flagellum schematic The helical filament is hollow, and flagellin is transported
from inside the cell, through the C ring (which has a hole in its centre) and along the filament, in its hollow
core, to its tip to which they are added - the filament constantly grows, as it must do to compensate for
breakages. A cap protein forms the tip and stabilises the filament.

Above a diagram of the bacterial flagellum showing the detailed structure of the flagellum basal complex
(in a Gram negative bacterium). The units of protein FliG (about 25-45 units) form a ring extension to
the M-ring (the M ring is shown in section here). Units of the proteins FliM (about 35 units) and FliN
(about 110 units) form the 45 nm diameter C ring, which together with the M and S rings forms the rotor.
This rotor drives the rod, which is a rotor-shaft connected through the centre of the L and P rings to the
hook. The proteins MotA and MotB form a ring of 10 studs embedded in the cytoplasmic membrane,
forming the stator, which is tethered by connections to the rigid Peptidoglycan layer (PG). The L
and P rings act as bushing.The C ring is made up of the proteins FliM and FliN and the Mot proteins
consist of two subunits: Mot A and MotB. The hook associated proteins HAP3 and HAP1 are also known
as FlgL and FlgK respectively. The rod is made up of a variety of proteins and the protein FliF spans the
region between the M and S rings. It is thought that as protons (H+ or hydrogen nuclei) move through
the Mot ring complex, MotA undergoes a shape change, exerting a torque (rotary force) on FliG which
connects to the M ring.

The basal complex anchors the flagellum into the surface of the cell. There are two structural variations
according to the type of bacterial wall it is anchored in. Gram positive bacteria (stain purple with Gram's
stain) like Bacillus possess a thick peptidoglycan wall (about 80 nm) overlying the bilipid cytoplasmic
membrane (CM). In this case the basal body has three rings, the 26 nm diameter M (membrane) ring
embedded in the CM, the S (supramembrane) ring or socket attached to the inner surface of the
peptidoglycan wall by techoic acids and the C (cytoplasmic) ring. The S ring is an extension of the
M-ring, to which it is attached, and both are composed of the same ring of protein FliF, thus the M and S
rings are sometimes considered to be a single double-ring, the MS ring. A rod (7nm diameter) passes
into the S ring socket and its other (distal) end attaches to the hook. More details of these structures are
illustrated below:

Above: illustrating the flow of protons (carrying positive electric charge) across the Mot ring from the

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