Structure and functional anatomy of bacteria Flashcards
What does a prokaryotic cell look like?
- circular DNA free in the cytosol
- ribosomes in the cytosol
- capsule around the outside provides a stickly adhesive surface
- multiple pilli in different directions
- Cell wall (different peptidoglycan compositions)
- cell membrane
Prokaryotes: What is the capsule?
- The slime layer
- Polysaccharide & Polypeptide rich
- sticky adhesive layer that allows attchment to surfaces
- May provide protection
- Protect against desiccation (for hydration)
- (Environmental persistence)
- protects against phagocytosis by masking biomarkers detected by immune cells
- Not all bacteria have this layer (not always present)
- The slime layer can extend beyond the microorganisms themselves
describe the ability of bacteria to supress phagocytosis
They mask biomarkers – reduces immune response
Streptococcus mutans:
Describe the role of biofilms as a physiological advantage to bacteria
What can be done to reduce the formation of biofilms when putting in a central venous line?
Biofilms form on clean shiny surfaces, perfect for bacteria to bind e.g.
- Teeth (plaque)
- Catheters: Central venous line- at the site at which the catheter penetrates the skin, there is a interaction between the plastic penetrating the skin and the microbes on the surface
- Use sterile wound dresings, disinfect the area to reduce the formation of biofilms
- plastic
Sterilisation necessesary
Interior bacteria persist, titration (reducted availability of the drug)- antibiotic can only kill external bacteria
A big problem; complex communities can form
Describe the bacterial cell wall
- Complex, semi-rigid structure→peptidoglycan provides movement and flexibility as well as strength
- Characteristic shape of the cell, bacteria are under considerable osmotic pressure
- Surrounds the fragile cell membrane (phospholipid bilayer)
- Prevents osmotic rupture
- Contributes to disease mechanisms→Helps bacteria to evade host immune responses
- can contribute toward toxic shock syndrome
- Almost all bacteria have a cell wall
Describe the structure of peptidoglycan (PG)
Composed of N-acetylglucosamine (NAG) and N-acetylemueramic acid (NAM) repeating units linked by glycosidic bonds
These are alternating molecues that form chains with cross links that have peptide crossbridges
Overall:
Carbohydrate backbone (N-acetylglucosamine (NAG) and N-acetylemueramic acid (NAM) + and amino acid side chain
Describe gram negative cell walls
- lipopolysaccharides (LPS)
- periplasm which has peptidoglycan layer
- outer membrane which is anchored to the thin peptidoglycan layer
- transmembrane proteins
- plasma membrane
- lipoproteins
Describe gram positive cell wall
- smooth exterior
- peptidoglycan layer leafleted with lipoteichoic acid (anchors peptidoglycan to plasma membrane)
- membrane proteins distributed
Describe the Cell (Cytoplasmic) Membrane
- Encloses cytoplasm of the cell
- Mainly phospholipids (amphipathic- hydrophobic + hydrophilic)
- Lipid bi-layer (fluid mosaic model)
- Polar head►hydrophilic
- Non-polar tail ►hydrophobiic
- Protein channels and porins through membrane
- (fluid mosaic model- dynamic system)
Cytoplasmic Membrane: Main functions
- (i) Permeability barrier:
prevents leakage and functions as a gateway for transport of nutrients into and wastes out of the cell
- (ii) Protein anchor:
site of many proteins that participate in transport, bioenergetics and chemotaxis
- (iii) Energy conservation:
Site of generation and dissipation of the proton motive force
Describe the composition of Cytoplasm
What is contained within the cytoplasm?
What physiological characteristic contribute to survival?
Internal matrix of the cell:
- 80% H2O
- Proteins, carbohydrates, lipids
Contained within the cytoplasm:
- DNA
- Ribosomes
- Inclusion bodies (storerooms within bacterial cells that contain proteins, fats and carbohydrates
- -function as energy reserves which contribute to survival)
What is the structure and function on flagella?
Provides motility
4 arrangements relating to poles:
- Monotrichous (Mono = one)
Single polar flagellum
- Lophotrichous (Lopho = tufted)
Two or more at one/both ends
- Amphitrichous (Amphi = both)
Single polar flagellum at each end
- Peritrichous (Peri = all around)
Distributed over entire cell
Describe the differences in flagellum structure in gram positive and gram negative bacteria
State the location of each ring
How do they work to cause whipping of the flagellum?
- Gram positive, no L ring, larger P ring that goes right out through the peptidoglycan layer
- C ring and MS rings present within cytoplasmic membrane
- P ring which is very small and anchored into the peptidoglycan
- L ring present within outer membrane
- Hook protein that creates a kink in the tail made of flagellin which rotates (whips)
- The rings turn, driven by the proton motive force (proton translocation across the electrochemical gradient) that causes turning and whipping of the flagellum
Describe monotricious and peritricous bacterial motility
A series of runs and tumbles
- CW = clockwise
- CCW = counter clockwise
- If monotricious= roation of polar flagellum in CCW direction that will cause the organism to run in whatever direction its facing. The organism will then tumble (due to brownian motion) within the liquid/matrix (whatever the bacterium is present in) and when it stabilises there is another run in whatever direction it is facing
- If peritrichious= as its runing the flagella bundle together as they all whip in a CCW direction then when the run has finished (as the electrochemical gradients resets) they tumble due to (Brownian motion) then the flagella just float in solution. Whatever direction the bacterium is facing, rotation in a CCW causes another run and the organism moves in a different direction
What is Chemotaxis?
What happens when bacteria are heading towards an attractant?
- A Chemotaxis is response to chemical stimulus (also phototaxis (moevement towards light), aerotaxis (movements towards oxygenation), osmotaxis (moving towards a particular solute) and hydrotaxis (moving out of salty conditions)
- Runs are extended if they are moving towards the attractant and shortened if they are moving away
- Positive/negative chemotaxis (runs towards or away)
- Series of runs and tumbles where runs are extended if in direction of a chemical attractant (chemoreceptors) if not there is just random walking
What are Fimbrae & Pili?
- Fimbrae (Fimbril)
Protein hook that allows adherence e.g. enteric bacteria
Found all over the bacterial cell
Pathogenicity determinant= different pathogenic E.Coli have different arrangements of fimbrea that allow them to attach as they travel through the digestive tract
E. coli
- Pili (Pilus)
Made of protein called Pillin
1-2 pilli per cell
Join bacterial cells during transfer of DNA (bacterial sex - HGT -Horizontal gene transfer)
Describe the properties of the bacterial genome
- Chromosome►Single long circular molecule of dsDNA
No envelope (Prokaryotic cell)
- Plasmid:
Can exist in multiple copies
Replicate independently
Not critical for bacterial survival but can provide an advantage e.g. antimcrobial resistance genes or genes for efflux pumps
Transferable
What is conjugation?
Explain the process
Bacterial sex
- Donor cell (with plasmid- called F+) and recipient (F-) cell come together and the pillus forms
- The cell pairs are stabilised and the F plasmid nicked in one strand
- Transfer of one strand from F+ cell to the F- cell when pillus opens up
- Synthesis of the complementary strand begins in the recipient cell - Single stranded break in plasmid (origin of transfer) allows it to pass through the pore in a single stranded fashion
- Completion of DNA transfer and synthesis
- Cells separate both are F+
What are the 3 main modes of horizontal gene transfer?
- Conjugation
- Transformation
- Transduction
Describe the process of transduction
Occurs via viruses (bacteriophages) so are specific to bacterial machinery and they can insert genetic informtion into bacteria
Describe transformation
Chemically disrupt the bacterial cell wall, this allows DNA present in the environment to be taken up by other bacteria
Describe prokaryotic ribosomes
Involved in protein synthesis
2 subunits
30S (small subunit)
50S (large subunit)
=70S in total
Eukaryotic ribosomes are 80s
Bacterial ribosomes are different to eukaryotic ribosomes. What antibiotics targets using these differences?
Target for antibiotics:
Streptomycin
Neomycin
Tetracyclines
Macrolides
What are inclusion bodies?
- Reserve deposits
- Volutin ►Inclusion body of Phosphate
- Glycogen ► Storage Polysaccharide
- Lipids also form in inclusion bodies
Describe the properties of bacterial endospores
What are the 2 bacteria known to sporolate most readily?
What are the names characterised by postion of spore formation
An endospore is a dormant, tough, and non-reproductive structure produced by some bacteria
- Gram positive bacteria
Clostridium and Bacillus are known to sporolate most readily
- Position of spore formation:
Terminally (at one end)
Sub-terminally (near one end)
Centrally
Size of spores can vary massively
- can be bigger (Clostridium) or smaller (Bacillus) than bacteria
- Not a reproductive process: 1 spore - 1 vegetative cell
Describe sporulation- “into spore”
Stage I: Asymetric cell division; commitment to sporulation
stage II: Vegative cell produces prespore and the prespore is engulfed
stage III: cortex formation (becomes very hard)
Stage IV: coat formation around the outside, Ca2+ uptake, small, acid-soluble spore proteins (SASPs), dipicolinic acid
stage VII and VII: Maturation and cell lysis that frees up the endospore
Describe the process of Germination- “out of spore”
Going back to the vegetative cell
- Germination
- Swelling
- Shedding of spore wall
- Outgrowth of vegetative cell
- Cell division
Describe Toxins
Exotoxin:
Mostly produced by Gram positive bacteria
Produced inside bacteria as part of growth
Released
Endotoxin:
Constituent of cell wall of Gram negative microbes
Only when the bacteria is killed is the toxin released
Describe Exotoxins
- Protein molecules:
Active (A) sub-unit
Binding (B) sub-unit
- Mainly produced by Gram positives
- Highly specific
Highly soluble in blood ►Rapid transmission
- Rapid onset of symptoms
Explain Classic Exotoxin Producers
- Clostridium botulinum:
Releases botulinum toxin (botulism) which Inhibits the release of acetylcholine
Muscles can’t contract (flaccid paralysis)
- Clostridium tetani
Inhibits removal/breakdown of acetylcholine (inhibits acetylcholinesterase)
muscles permanently contracted (Spasmic paralysis)
Explain endotoxins in detail
What are the sympotoms of toxic shock syndrome?
- Present in outer cell wall of Gram negative bacteria
- Lipid portion of LPS ►also called Lipid A
- Released only upon bacterial cell lysis
Symptoms of endotoxins:
Same regardless of species
Severity variable
Fever, weakness, general aches,
LPS toxic shock syndrome:
Describe antigens
These are the different features of a bacterial cell:
capsule (K antigen)
LPS (O antigen)
Fimbriae (F antigen)
Flagella (H antigen)
What is the only bacterium that lacks a cell wall?
mycoplasma
Can conjugation be between bacteria of different species?
Yes
This allows them to change and adapt very quickly
