Topic 2-L5 - Bacterial Outer Membranes, periplasm and more Flashcards
The outer membrane
Gram negative bacteria have a second membrane outside the cell wall this is very different from the cytoplasmic membrane
- outer leaflet
Outer leaflet
Composed of molecules called LPS
It’s the predominant feature of the outer membrane
Lipopolysaccharide (LPS) three parts
(1) lipid A (within membrane)
(2) core polysaccharide
(3) O-specific polysaccharide (outermost component)
Lipid A
Hydrophobic tails anchor Lipid A in OM
- base of LPS o Phosphates confer a negative charge
Important molecule sensed by the immune
system. Also known as “endotoxin”. Can lead
to a potent and unchecked immune response – can cause septic shock if detected in
bloodstream.
Are lipid A and phospholipids the same
NO
O-specific polysaccharide (“O antigen”) are
Polysaccharide comprised of diverse sugar subunits connected and
branched in different ways
- Repeating combination of sugars with variable numbers of repeat
O-specific polysaccharide (“O antigen”) are highly
Variable - species have hundreds of versions
The contemporary view is that the outer leaflet of the OM is mostly (or
almost exclusively)
LPS with very little phospholipid.
The inner leaflet is comprised of _____________. It is therefore___________.
phospholipid. asymmetrical
The outer membrane is anchored to the
Cell wall
Braun’s lipoprotein serves to
connect the OM to cell wall
Small lipoprotein (protein modified to be linked to lipids) –
lipid anchors in OM, other end of protein covalently attached to peptidoglycan.
- Very common in Gram negative cells.
The OM contains
Porins
Porins are
protein channels that serve as channels for entrance/exit of small molecules
- can be specific or non specific
Functions of the outer membrane
- mechanical strength (ionic bonds between LPS molecules)
- serves as barrier (still permeable)
- protects cell wall
- enables periplasmic space
Periplasmic space (“periplasm”)
Space between cytoplasmic/outer membranes of Gram negative
bacteria
Periplasmic space function
Buffer between environment and cell
Periplasmic space (“periplasm”) is much smaller space between cell membrane and the cell wall in
Gram positive
Periplasmic space (“periplasm”) FUNCTIONS and PROTEINS
- Break down macromolecules for uptake as nutrients
- High affinity binding protein for nutrients
- Detoxify harmful compounds
- Protein folding - disulfide bond formation
S-layers
Rigid/permeable monolayer of protein or glycoprotein – self- organized into a repeating structure that encompasses the cell
- outermost layer
S-layer functions
- same functions as cell wall
- protective layer
- keep external proteins from accessing OM or cell wall
Capsules and slime layers functions
- adhering to surfaces
- protection from host immune cells (bacterial pathogens)
- protection from water loss/dehydration
Capsules are organized into a
matrix and attached to the cell
slime layers are
loosely attached, less organized
Pili (singular = pilus) are
protein filaments that extend from the surface of a cell
Where are pilus found mostly
Gram -
Different pili have different
Functions
Conjugation pili
important for conjugative (transfer of genetic material between bacterial via a pilus bridge)
Fimbria is a term used for a pilus that
mediates attachment (to a surface or another cell)
