Bacteria; Structure, and cellular differentiation Flashcards
define bacteria
Superkingdom or domain of prokaryotic microorganisms containing all prokaryotes that are not members of the Archaea.
how is bacteria distinguished by archaea?
Distinguished from the Archaea based on molecular genetics and biochemistry.
what are bacteria?
- Small, single celled organisms
- No nucleus
- DNA is in a loop
May have additional plasmids (circular DNA)
bacteria has no
membrane bound organelles
bacteria typically possess cell walls contianing
peptidoglycans (some exceptions)
what rna present in bacteria?
70S rRNA
bacteria has a rudimentary
cytoskeleton
what kind of dna in bacteria?
circular chromosome of DNA (single orign of replication)
hwo do bacteria transfer genetic material
Transfer genetic material by conjugation and by virus mediated transfer.
bacteria are a
…Extremely metabolically and ecologically diverse group of unicellular microorgansms
bacteria are free…
… living in soil and water
bacteria have what forms?
parasitic forms
bacteria are saprophyts of
… plants and animals
bacteria are found in
all enviornments on earth
- Extreme environments (deep sea vents, volcanoes, Artic/Antarctic etc.)
- Soil, air, water
- Plants, animals, humans
bacteria are a Commercially and medically important species in
- disease
- health
- food and drink manufacture
- agriculture + other industries
bacteria often form
colonies
bacteria reproduce by
binary fission to produce identical offspring
bacteria grow…
… rapidly (E. coli can divide every 20 mins under optimal conditions)
some bacteria can produce…
… spores to survive unfavourable conditions (eg bacillus anhracis
some bacteria can produce…
… toxins
why is cell size important? –
- small size provides a high surface area to volume ratio
- nutrients can be taken up quickly,
- waste products disposed of quickly
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bacteria is commonly grouped based on…
… shape
Commonly grouped based on shape:
what are these?
Cocci (spherical)
Bacilli (rods)
Spirilla (spirals)
Vibrios (commas)
Spirochaete (corkscrew)
- Pleomorphic = variable in shape
arrangements of Bacilli?
- Coccobaciullus
- Bacillus
- Diplobacilli
- Palisades
- Streptobacilli
arrangements of cocci?
- coccus
- diplococci
- chains of cocci
- sarciniform
- irregular graplelike clusters
- tetra
spiral bacteira forms?
- helical form (helicobacter pylori)
- corkscrew form (borrelia burgdoferi
- filamentous
- spirochete
additional shapes of bacteria
vibrio
comma form
club rod
helical form
Some bacteria are …
… pleomorphic
Some bacteria are pleomorphic:. this means…
… shape is variable
eg: helicobacter pylori has different morphologies - this species can cause stomach ulcers.
cholera vibrios are
slim, short,
curved rods, about 0.5 × 3 μm in size.
Cells may be linked end to end,forming ..
… S shapes and spirals
vibrio cholera are highly…
motile with a single polar flagellum, non-spore forming.
vibrio cholerae are non…
… spore forming
Cell morphology depends upon
the presence of a rigid cell wall in most bacteria
Cell wall essential in most species to prevent
lysis due to high osmotic pressure in the cytoplasm.
example of some wall-less species?
genus Mycoplasma
Some wall-less species, e.g. genus Mycoplasma. Resistant to …
…. penicillin which targets peptidoglycan cross-links
All bacterial cell walls contain …
… Peptidoglycan, a polysaccharide.
Peptidoglycan consists of two repeating subunits:
What are they?
N-acetylglucosamine (NAG)
N-acetylmuramic acid (NAM)
The NAM subunit has a
tetrapeptide attached
The NAM subunit has a tetrapeptide attached, which can form
peptide bonds with adjacent chains.
This crosslinking makes the structure very
rigid and strong
Gram-positive cell walls are much
thicker
How much of Gram-positive cell walls are peptidoglycan?
Consist of 90% peptidoglycan in up to 25 sheets.
gram psitive cell walls contain…
- Lipoteichoic acid
- Teichoic acid
Lipoteichoic acid is connected to the …
… cell membrane via a lipid anchor
what is Teichoic acid ?
- a glycopolymer covalently attached to NAM residues in peptidoglycan.
teichoic acid contributes to…
… rigidity of the cell walls
Teichoic acids and lipoteichoic acids are responsible for …
…the negative charge of the cell wall in Gram-positive bacteria – their negative charge attracts Mg2+ and Ca2+ ions which are transported into the cell
gran-negative cell walls have a more…
… compelx structure
gram negative bacteria have a…
… thin peptidoglycan layer (5-10% of cell wall)
gram negative bacteria have a membrane outside…
… the peptidoglycan layer (outer membrane)
Outer membrane of gram negative bacteria contains
lipopolysaccharide (LPS)
the function of LPS is mainly …
… structural
what is the main constituent of ourter membrane?
Lipopolysaccharide
what is Lipopolysaccharide?
Complex compound of lipid and polysaccharide
O-antigen and Core are …
… polysaccharide
lipid A anchors into the…
… membrane
Lipopolysaccharide components?
O-antigen
Core
Lipid A
Lipopolysaccharide contributes to…
… net negative charge
Lipopolysaccharide are a physical….
…barrier to cell wall for certain substances
O-antigens trigger
immune responses
What is Lipid A?
Lipid A is an endotoxin
in order to take up hydrophilic molecules from the environment (e.g. ions) bacteria require
a means of transporting these across the membrane
in Gram-negative bacteria the presence of
two membranes makes transport even more difficult
what makes transport difficult in gram neg bacteria?
the presence of two membranes
porin molecules like OmpF provide a means for
- entry of hydrophilic molecules by passive diffusion -
- also provide a means of entry for antibiotics which are hydrophilic
– all tend to have the same beta barrel conformation
treatment with penicillin?
Disruption of cell wall in rod-shaped bacteria (with penicillin) results in the formation of spheroplasts.
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Both Gram negative and Gram positive bacteria have a
‘periplasmic space’ between the membranes of gran neg bacteria and etween th emembrane and wall of gram negative bacteria.
The periplasmic space is involved in
transport of molecules into the cell by means of substrate binding to periplasmic binding proteins and then being transported across the inner membrane
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Proteins associated with the plasma membrane are essential for
processes including:
- Perception of signals from the environment
- Transport of nutrients and ions across the membrane
- Generating ATP via movement of protons
- Formation of wall components
Perception of the environment in bacteria occurs via
2-component receptor systems
In response to a signal (e.g. chemoattractant, pH change or osmolarity) a …
… histidine kinase is activated by phosphorylation
This results in the downstream activation of a
response regulator which will either switch off or switch on genes depending on the requirement of the external signal
The ability to differentiate between different signals depends on
the specificity of the receptorin the periplasmic space
go look at slide 36
Homologues are now known to exist in …
,…. in bacteria for all three major eukaryotic cytoskeleton components (actin, tubulin and intermediate filament proteins
Filaments of MreB (bacterial actin) in Bacillus subtilis visualised using
GFP-tagged MreB
Filaments form a
helical structure which contributes to the familiar rod shape of bacteria – suggested that spherical bacteria evolved from rod-shaped bacteria
The spatial organisation of the cytoskeleton directs the
shape of the cell partly by directing enzymes involved in cell wall lysis and production (e.g. PBP) to certain points of the cell
Tubulin homologue FtsZ is involved in
defining the plane of cell division
Intermediate filament homologue crescentin (CreS) is involved in
curvature of bacterial cells
The glycocalyx is the region outside of the
cell wall
The glycocalyx is the region outside of the cell wall consisting of
loose polysaccharides forming either a capsule or slime layer
capsules can be seen by
staining with Indian ink as in the photograph opposite
what are the functions of capsules?
1) resistance to drying
2) resistance to phagocytic engulfment
3) attachment to host cells (e.g. Vibrio cholera in the intestine
4) nutrient reservoir
Spore formation is common in some groups of bacteria, especially…
… species that live in the soil – Bacillus, Clostridium
Spore formation provides a means for
bacteria to survive adverse conditions e.g. desiccation, freezing, heat or nutrient depletion.
When the medical profession started being concerned about sepsis it was discovered that while most bacteria are killed by heating many
remain viable
Sporulating bacteria are resistant to
temperatures of 100°C therefore boiling is ineffective.
Autoclaves use
steam under pressure to heat equipment above boiling point
A key feature is that the dry air is driven…
… out of the chamber.
Pressure means …
… that heat is transferred to all contents.
121ºC for 20 mins
Exosporium is a
protein
spore coat is a
spore specific protein
cortex is a
peptidoglycan and dipicolinic acid (DPA) cross-linked to Ca2+ ions
because of the two charges on dipicolinic acid and calcium, a …
… chain of crosslinks can be formed – this assists the cell in resisting dehydration further
sporulation is a type of…
… cell differentiation and occurs in distinct phases:
Signal is perceived to
initiate differentiation, e.g. nutrient depletion.
the cell replicates the…
… chromosome
stage 2 of endospore formation?
An asymetrical septum forms and the replicating chromosome is moved across into the forespore.
stage 3 of endospore formation?
The forespore is engulfed by the mother cell plasma membrane
stages 4 and 5 of endospore formation?
a cortex is laid down between the membranes and the forespore dehydrates
stage 5 of endospore formation?
the forespore matures.
stage 7 of endospore formation?
the mother cell lyses releasing the spore
Initiation of sporulation depends upon …
the perception of signals from the environment
Initiation of sporulation depends upon the perception of signals from the environment – when the cell detects that nutrient levels are low (the actual signal is not known) a …
… histidine kinase (KinB) at the membrane becomes phosphorylated (active)
The signal is transduced via three proteins to
Spo0A which acts to control gene transcription
- switches from normal division to asymmetric division
Bacterial genes are regulated by
sigma (σ) factors
Sigma factors are
subunits of all bacterial RNA polymerases that are responsible fordetermining the specificity of promoter DNA binding and efficient initiation of RNA synthesis (transcription).
Bacterial genes are regulated by sigma (σ) factors – the RNA polymerase enzyme does not
bind to the promoter of by itself – sigma factors are required to interact with the promoter regions of a gene
different sigma factors are required to
turn on different sets of genes
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in normal growth Bacillus cells form a
Z ring in the middle of the expanding cell composed of FtsZ (the tubulin homologue)
an early event during sporulation is the
decision to form a septum at one of the poles
SpoEII is essential for
formation of polar septa and is overexpressed along with FtsZ under control of sigma factor
