Lecture 2: Prokaryotic cells - Cell Wall

Structure of a typical bacterial cell wall

semi-rigid layer outside PM

critical for structure and protection

Think of the cell wall as a

fence, something can go in and out while providing support

Function of the cell wall

maintains the shape of cell

prevents osmotic lysis

Flow of water if cell is hypertonic and environment is hypotonic

into cell

Flow of water if cell is hypotonic and environment is hypertonic

into the environment, PM shrinks, the cell wall is maintained

True/False: the cell wall can protect against plasmolysis

False

True/false: the cell wall has a major role in regulating entry of material in and out of the cell

False

Coccus

sphere shaped

Bacillus

rod shaped

Vibrio

common shaped

Spirillum

rigid spiral shape

Spirochete

flexible spiral shape

Every bacteria has the same/different cell shape

different

Peptidoglycan (PG) is a

polysaccharide-peptide matrix

PG matrix is

highly crossed-linked and forms multiple layers

More cross-links and layers correspond to

strength

The PG is only found in the domain

bacteria

Each subunits of PG

two amino sugars (NAM and NAG)

tetrapeptide off NAM

In gram neg the third AA is

DAP

In gram pos the third AA is

L- Lys

In the initial formation there is a _____ off the tetrapeptide and it leaves when ____ and is there to ____

D-Analine

extra binding to another subunit

protects against peptidase

Sugar subunit connection

beta-1,4 glycosidic bonds connect NAM and NAG in the peptidoglycan structure.

Gram-negative subunit connection

direct linkage between D-Ala and DAP

Gram-Positive subunit linkage

indirect peptide bridge between D-Ala and L-Lys

The inter bridge of Gram + subunits

vary in type and number of AA

PG stands have a _____ shape

helical

Cross-linking of stands relationship to strength

more linkage = more strength, elastic

Cross-linking allows for elasticity results in

preventing osmotic lysis and allowing it to be porous, globular proteins are able to enter

Cell wall subunits are produced in

the cytoplasm

Bactoprenol

membrane protein transports subunits made in cytoplasm across the PM

Autolysins

bacterial enzymes break PG cross-links to allow the new subunits to enter the cell wall for growth

Penicillin-binding proteins (PBP) enzymes

transglycosylation and transpeptidation

Transglycosylation

add subunits to existing strands of PG (opening gaps in helix to make new connections)

Transpeptidation

cross-links layers of the cell wall determine the shape of bacteria

Disruption of the cell wall may result in

osmotic lysis

Lysis is beneficial when there is

an infection

Lysozyme

an enzyme found in tears, saliva, other body fluids (main line of defense)

breaks glycosidic bonds (beta-1,4 glycoside bonds)

Antibiotics types

beta-lactam family and vancomycin

Beta-lactam family

Ex: penicillins and cephalosporins

very similar to terminal D-Ala of PG precursor

binds to PBP blocking transpeptidation

increases the activity of autolyzing

cell wall weakens, resulting in osmotic lysis

Vancomycin

binds directly to the precursor

blocks transpeptidase from cross-linking subunits, weakening cell wall

Gram-positve have a ___ layer of PG

thick, 50%-90% of wall is PG

Teichoic acid

part of the cell wall that is covalently linked to the PG only Spans only PG

Lipoteichoic acid

covalently linked to PG, spans from PM to PG

Functions of teichoic acids

anchors wall to PM

links PG layers for strength

are neg-charged molecules that influence the passage of ions through the wall

Proteins on outer surface of gram neg include

enzymes, adhesin, and invasins, but differs with strains and species

Enzymes

carry out particular reactions

Adhesin

adhere to other cells/subunits

bind receptors on host cells and surfaces

Invasins

allows things to enter the host cell

Gram-negative cells have a _____ PG layer

thin, 10%-20%

True/False: Gram-positive cells do not have any peptide inter-bridges

true

Gram-negative cells a ____ membrane

outer

Outer membrane

is an highly asymmetrical lipid bilayer with an inner and outer leaf

Inner leaf

on the inside phospholipids (hydrophobic)

Outer layer

outside, lipopolysaccharide (LPS), hydrophilic

LPS

unique to gram-neg bacteria

Components of LPS

0-antigen, core polysaccharide, lipid A

O-antigen

Polysaccharide chain extending outward from core

varies in composition bw species

induces an immune response by the host

Core polysaccharide

conserved between species

Lipid A

2 glucosamine sugars (phosphate and fatty acids attached)

anchors LPS to OM

conserved between species

During infection of Gram-negative bacteria

gram-neg is lysed releasing LPS

When LPS is released

Lipid A is exposed to the body and acts as an endotoxin

Endotoxin causes the body to experience

fever, low BP, vascular collapse (septic shock)

OM impermeability

impermeable to charged, polar compounds, and nonpolar compounds

The outer surface of OM is

highly charged, a barrier to hydrophobic compounds

Lipid A fatty acids are ____ thus____

saturated

packed tightly, leaving no space for hydrophobic compounds to slip past

Nutrient transport across OM can be done by

porins, OM receptors, periplasm

Porins

Diffusion of small polar molecules

OM receptors

specific transport

Periplasm

gelatinous material in periplasmic space that contains enzymes for nutrient breakdown and binding proteins for the transfer of nutrients across PM

Braun’s lipoprotein

most abundant protein in OM (+), covalently binds OM to PG

Surface proteins of Gram-neg bacteria

translocation of large materials

enzymes

adhesin

invasin