Bacterial cell walls L6 Flashcards
what are the membrane bound cellular compartments
there is not any
where are the nucleic acids
free in cytoplasm
what is a nucleiod
DNA organised into structure associated with proteins
what surrounds the cytoplasm
rigid cell wall
what is the basic ‘building block’
peptidoglycan – murein (structure of amino acid, but not used in protein synthesis, used to build cell wall structure)
what is peptidoglycan (murein) made of
repeating dimers of two sugars
- N-acetylglucosamine (NAG)
- N-acetylmuramic acid (NAM)
where are the dimers of peptidoglycan assembled
cytoplasm
what is attached to the NAM residues
side chains of peptides
- added to dimers before export
- some amino acids in peptides are bacteria-specific – Mesodiaminopimelic acid
what bond joins the two sugars in murein
beta 1,4-glycosidic bond
what is the beta 1,4-glycosidic bond structure like
very flat –leads to making long linear chains
what is the beta 1,4-glycosidic bond sensitive to
lysozoyme
what is the first thing that acts against bacteria
lysozoyme
what form do enzymes not recognise
d forms
what form do enzymes recongise
L forms
why aren’t d forms used in our systems
quite toxic
what is peptidoglycan
long linear chains of repeating sugars
what are crosslinks
peptide bonds join two chains give overall rigidity of structure
how does the structure grow if it is a big cage like structure
need to grow from within structure, NOT adding onto ends – add into middle
what is the function of autolysin
create controlled cuts in peptidogylcan chains, insertion of disaccharide units
where does growth occur in the bacterial wall
growing points have controlled activity of autolysin, happen at very defined places
what is cell elongation
add in new dimers, dimers made in cytoplasm with peptide on, transported out, added on where cell wall cut
what are bactoprenols
specific transport molecules, ability to exist in cytoplasm, insert spontaneously into membrane
what is the function of bactoprenols
all flip, what was on inside on outside
new molecules can be flipped out and introduced into growth of new peptidoglycan
what is attached to bactoprenols and what do they cause
two phosphate groups attached to them – pick up two dimer molecules transported into membrane
what causes transpeptidation
penicillin binding protein
what does transpeptidation cause
allow chemical attack between reactive NH2 group and C-N bond so forms one peptide all the way across linking the two chains
what does penicillin do to the cell
weakens cell wall structure and leads to cell lysis
prevents transpeptidation
what do antibiotics do
stop cross links being formed, have long chains of sugar, but not turned into 3D mesh so cells weaker and eventually die
what colour are gram positive bacteria after purple dye added and washed with alcohol
retain crystal violet/iodine complex (purple) dye stays after washing
what colour are gram negative bacteria after purple dye added and washed with alcohol
Stain removed by alcohol from cell wall of Gram-negative bacteria
Counterstain (fuchsin) used to colour cells so they are visible
Gram-negative, colourless cells as not bound to cell – if coloured with fuchsin stay pink
what is a gram positive cell wall made of
- 2D cross-linking of chains creates 3D sheets of PG in a rigid matrix
- Glycine interbridges allow more flexible cross linking
- Forms one giant scaffold-like molecule
- Termed the murein sac or sacculus
how does peptidoglycan chains extend
can put in repeating glycine units, so get glycine inter-bridges so the chains can extend in more directions – can go between layers and between chains, can build
peptidoglycan in in 3D sheet
why do gram positive bacteria remain purple
dye trapped in peptidoglycan structure as complex is made, too big so can’t get out again
what is the second major component of gram positive cell wall
teichoic acid - up to 50%
what are the polymers of gram positive bacteria made of, what are they made into
glycerol phosphate or ribitol phosphate – these are then linked into long chains
what is the length and the types of side chain groups
varies depending on species
how can our body tell different gram positive bacteria apart
varied side chains
scanning these molecules, if they have same pathogen with recognised pattern of sugars antibodies have seen before, will active immune system to deal with pathogen
what are some teichoic acids linked to and what bond
covalently linked to N-acetylmuramic acid
some are attached to glycolipid in the underlying cell to form lipoteichoic acid
what covers the gram positive bacteria
smooth carbohydrate outer layer
what is the role of lipotechoic acid
anchoring PG layer to the cell-lipid bit anchored down in membrane, structural role – connects membrane and cell wall together
which bacteria have teichoic acids
gram positive bacteria
what is a form of a major antigenic determinants of cell surface
teichoic acid
what holds the bacterial cell wall together
lipoteichoic acids that are anchored in the membrane will be cross linked to peptidoglycan
what is the pseudoperiplasm and its function
gel like matrix
similar function to periplasm – transport enzyme function all occurs at this interface
where are the cross links in gram negative cell wall
only occurs in one plane
what is the gram negative bacteria cell wall like
thin layer of peptidoglycan around cell
what do the gram positive and negative bacteria have in common
peptidoglycan, little chemical resemblance between two bacterial structures
where is the peptidoglycan in gram negative bacteria
thin sheet between two membranes
where and what is the periplasm or periplasmic space
space between the two membranes
what is the gel made of that fills the periplasm
- proteins (hydrolytic enzymes, sensors, transporters, antibiotic resistance proteins)
- small metabolites (enzymes broken things down into these)
- oligosaccharides small sugars (create osmotic pressure buffer creates strength in cell wall)
is peptidoglycan strong
not that strong
what gives the bacterial cell wall strength
periplasm
how can the immune system can tell different bacteria apart
carbohydrates extends to outside, is the variable region
how does something enter a gram negative bacteria
has to go across outer membrane then inner membrane
porins – sit in outer membrane and allow exchange of molecules, they’re hollow, so lots of small molecules freely diffuse
what is the structure of the gram negative bacteria
- Inner leaflet consists of ordinary phospholipids
2. Outer leaflet consists of a bacteria-specific molecule called lipopolysaccharide (LPS)
what covers the gram negative bacteria
lipid rough outer surface
what is LPS like to humans and animals
highly toxic to man and animals
- endotoxin (= part of cell)
- induces fever and shock even in minute amounts (endotoxic shock)
if you have wrong antibiotics
receive second toxic shock if receive wrong
what is an endotoxin
that forms part of the cell naturally that’s toxic to us if were exposed to it
how does the gram negative bacteria recognise infection
immune system targets bacteria using those outer molecules and starts to break down cell wall structure, in doing so releases LPS can cause endotoxic shock (not infection is poisoning)
what does LPS consist of
- Lipid A
- Core polysaccharide
- O-antigen (outer part
what are other cell wall structures
acid-fast e.g. mycobacterium tuberculosis
gram positive organism structure
lipid bilayer cell membrane covered by single porous peptidoglycan layer
gram negative organism structure
surrounded by two membranes
outer membrane functions as a permeability barrier
mycobacteria structure
thick mycolate-rich outer covering act as an exceptionally efficient barrier
how are acid fast bacteria identified by
staining characteristics
e.g. they do not destain with acid and alcohol once stained with arylmethane dyes
what acid do acid fast bacteria cell walls have
- long chain, branched fatty acids
- mycolic acids and short chain fatty acids form a pseudo outer membrane, result in unusual staining characteristics of cells
what happens to gram positive, negative and acid fast bacteria in dye + washed
Gram-positive and Gram-negative de-colourise when washed with acid and alcohol, but acid-fast bacteria will stay stained
Dye fast – dye stuck to surface, will not come off
what is acid fast bacteria cell wall like
waxy, hydrophobic and high lipid content
responsible for hydrophobicity of these cells
up to 60% of dry weight of organisms may be mycolic acids (thick layer)
what is acid fast bacteria cell wall linked to and by what
linked to PG by arabinogalactan
how is mycobacteria distinguished
type of mycolic acid
what properties does the acid fast bacteria have, what may this cause
adjuvant
may be responsible for development of delayed type hypersensitivity
how is the general immune system stimulated
adjuvant property, when infected get DTH, as presence of these molecules now making the immune system hyperalert to any other molecules its faced with
how are mycolic acid and peptidoglycan linked
arabinogalactan
how is the cell shape determined
by cell wall
what is the bacteria interior like
very fluid interior
what is the individual shape of cell outlined by
peptidoglycan, which has been laid down in particular way
how is peptidoglycan laid down
in spirochete bacteria laid down to get cork screw shape, nucleic acid in cell labelled up with fluoroscent dye, if anything yellow is a spirochete
what happens if bacteria are gently treated with lysozymes
strip outer cell wall, need to do this in osmotically balanced buffer NO turgor pressure
can see as time go on sphereoplasts form, is spirochete but DNA is converted from spiral shapes just into these spheres
what are the other factors involved in cell shape
penicillin binding proteins in cell wall regulate pattern of PG biosynthesis
Other key shape determining protein = MreB
what is the function of the penicillin binding proteins in cell wall
- involved in controlling pattern of interbridges
- expression of PBPs regulated by morphogene (BolA)
what is the function of MreB
- forms actin-like cytoskeleton in some bacteria cells
- forms filaments, laid down in spiral-shaped bands around inside of cell under cytoplasmic membrane
what happens in the cell division of FtsZ
FtsZ accumulates
Ring smaller as forms boa constrictor – pinches off so BPBs move inwards - septum grows inwards
after cell division loose FtsZ ring
divide in different planes
FtsZ ring always appears in same plane – controlled event so get chains
what is the crescentin function
forms a track along cell – prevents as much peptidoglycan being put at bottom then to the top = cresent
what happens if remove the cell wall from prokaryotes
most prokaryotes cannot survive without a cell wall
how are spheroplasts formed
lysozyme treatment in osmotically balanced buffer
where are mycoplasmas
naturally occurring cell-wall free bacteria
many pathogenic, living inside eukaryotic cells in osmotically-controlled environment
what are the mycoplasmas membranes like
have tougher membranes to resist osmotic shock
what cell wall do mycoplasmas have
really regulated so don’t need cell wall
how do mycoplasmas gain structural integrity
accumulating molecules from their host cells
why could mycoplasmas have lost their ability to synthesise cell wall
due to long association with hosts
= Obligate pathogens, needs to be inside host to replicate
how much of the mycoplasma membrane is proteins
approximately 2/3 of membrane consists of proteins- strength
- all lipoproteins anchored to membrane
what are the proteins in mycoplasma wall function
> majority have a structural role
> others have enzymatic activity, e.g. transport
what do lipids consist of
phospholipids, glycolipids and neutral lipids
what are the lipids like in prokaryotes
unique among prokaryotes in having a requirement for cholesterol in their membranes
how does the mycoplasma make lipids
totally incapable of synthesizing own lipids, use the hosts
what are archaea cell walls made of
pseudopeptidoglycan
- repeating units of NAG & N-acetyl talosaminuronic acid
- has β(1,3)
- contains L-amino acids rather than D-aa seen in eubacteria
what is the β(1,3) linkage for in the archaea cell wall
so is resistant to lysozyme
what do some archaea lack
PG or pPG (pseudopepdidoglycan)
what provides the cell wall with structure and strength
glycoprotein or protein
what is the most common wall type among archaea
S-layer or paracrystalline surface layer
- proteins arranged in a in regular array on cell surface
- generally have hexagonal symmetry
where are S-layers found
additional layers on some Eubacteria
archaea
what type of bacteria is caulobacter crescentus
gram-negative
what contains S-layer protein (RsaA)
Caulobacter crescentus
archaea
what is on the Caulobacter crescentus cell surface
S-layer proteins exported on to cell surface in large quantities
what happens if remove S-layer from bacteria
Exploited this as these bacteria can live without this layer
how do bacteria make outer layer
bacteria have to produce a lot of protein and transport out – ABC transporter specifically recognizes RsaA protein pick it up and transport assemble into S layer
what is LPS
major surface antigenic determinant
what is lipid A
phospholipid containing glucosamine instead of glycerol. Toxic component of LPS
what is the core polysaccharide
contains some unusual sugar residues but a fairly constant structure
what is the outer part antigen
polysaccharide side chains with variable composition, forms major antigenic determinant
