Transcript The outer membrane IS a barrier to large solutes such as glucose

Other Extracellular Layers
Outer membrane
Capsule
Sheath
Cell Appendages
Filamentous, small: Fimbriae, Pili, & Spinae
Filamentous, large: Flagella
Outer Membrane
Most common in Gram negative bacteria, often associated
with a fairly thin peptidogylcan layer
A second lipid bilayer, but NOT water and ion impermeable
(permeable largely due to the presence of channel
proteins called porins). The outer membrane IS a
barrier to large solutes such as glucose.
Often is an asymmetric bilayer, where the outer leaflet contains
the unusual lipid LPS (LipoPolySaccharide). LPS can be
shed from the outer membrane and is a major endotoxin
Cell Layers in a Gram Negative Bacterium
Lipopolysaccharide (LPS)
Structure
Capsules
A thick layer of secreted polysaccharide which surrounds the cell
generally a defensive structure, often involved in pathogenicity
Protein Jackets
Like capsules are secreted by the cell and built around the cell
Much more complex than capsules mostly made of protein
Often associated with multicellular bacteria
Fimbriae, Pili, & Spinae
Frimbriae & Pili both refer to filamentous projections from the cell
surface made of protein (Frimbriae is the more general term)
There is an ongoing attempt to classify different types of pili:
Type I --- adhesive
Type II --- involved in general secretion of proteins to the
outside of the cell
Type IV --- defined by assembly pathway
and so on . . .
Spinae, as the name implies are spine-like projections composed
of protein often used to increase the cell’s surface area
Two Types of Bacterial Motility
Swimming: ability to move through liquids
--- usually employing a spiral propeller (flagellum)
--- some bacteria can move over surfaces with flagella
--- all bacterial flagella are genetically similar
(Horizontal Gene Transfer)
Surface (Gliding) Motility: movement over surfaces
--- cells lack flagella
--- gliding cells generally cannot swim
--- multiple systems have arisen among
different groups of bacteria
Styles of Flagellation
Swimming
Bacteria
equatorial
(R. sphaeroides)
peritrichous
(E. coli, S. typhimurium)
polar
(A. serpens)
polar in at least one archaeon
(Halobacterium salinarium)
All bacteria have
essentially the same
flagellar genes
(homologous)
The Archea have an analogous
structure that is not genetically
similar to the bacterial flagellum
Flagella
Used for bacterial locomotion (usually swimming)
requires tremendous energy input (up to 20% of cellular
energy budget for multiply flagellated cells)
can serve as a model system for intracellular signaling
Flagellar Factoids
-“flagellum” is a misnomer from early microscopy, the filament is
helical and appears to move in a wave-like manner as it
rotates, it is actually quite rigid
- each flagellar filament is composed primarily of one protein,
flagellin (FliC), in about 1 million copies, flagellin is also an
important antigen for the immune system
- the flagellar filament is a passive structure, basically a propeller
- the flagellum is driven entirely at it’s base by a complex of proteins
that function as a proton driven rotary motor
The flagellar motor is fueled by the trans-membrane ion gradient
~ 300 rev/sec
outer membrane
peptidoglycan
(hundreds/rev)
inner membrane
MotA/B Torque
Generators (Stator Complex)
CheY- P
The flagellar motor is reversible
CCW: run
CW: tumble
E. coli responds to chemical gradients by biasing its random walk
No stimulus
Gradient of chemoattractant
[Asp], e.g.
Chemotaxis
- the ability to sense and respond to extracellular
concentration gradients of solutes
Input
Chemoreceptors
CheY – P
- CW signal
- 3 sec. lag
bacterial cell
Output:
motor bias,
CCWCW
rotation