33.1 Bacterial Structure and Physiology Flashcards
Describe the features that distinguish prokaryotes from eukaryotes.
- Generally smaller and simpler
- No membrane-bound organelles
- Have haploid cells (while eukaryotes have either haploid or diploid)
- Have peptidoglycan in cell walls
- Do not have a nucleus
Do prokaryotes have a cytoskeleton?
Yes, but it is not as well defined as in eukaryotes.
What are the main shapes of bacteria?
- Bacillus -> Rod
- Coccus -> Sphere
- Spirillum -> Spiral
- Spirochaete -> Corkscrew
- Vibrio -> Comma
What is a typical size for prokaryotic cells?
Around 1µm (much smaller thatn eukaryotic cells)
In bacteria, what is there instead of a nucleus?
Nucleoid (a condensation of the DNA)
What is the bacterial capsule?
A polysaccharide layer outside bacterial cell membranes.
What can be found in the bacterial cell envelope?
Peptidoglycans, teichoic acids, mycolic acids, lipopolysaccharides (in the outer membrane).
What is the function of peptidoglycan?
Provides rigid support and helps maintain bacterial cell shape.
Where are teichoic acids found?
In the peptidoglycan of Gram-positive bacteria.
What are the two types of teichoic acids?
- Wall teichoic acid -> Reaches out from the peptidoglycan layer
- Lipoteichoic acid -> Reaches down from the peptidoglycan layer to the lipid layer of the cell membrane
What are teichoic acids and what is their function?
- Highly variable anionic polymers in the peptidoglycan layer of Gram-positive bacteria
- They play a role in:
- Cell shape, growth and division
- Resistance to antimicrobial peptides
In what bacteria is the outer membrane found?
Gram-negative
What is the outer membrane of Gram-negative bacteria made of?
It is a lipid bilayer:
- Inner layer -> Mostly phospholipids
- Outer layer -> Mostly lipopolysaccharides (LPS)
What is the role of lipopolysaccharides (LPS) in an infection?
- Elicits inflammatory responses in humans
- Elicit complement activation via the alternative pathway
- Are involved in virulence and pathogenesis, by for example:
- Allowing attachment and invasion
- Acting via molecular mimicry to appear like host cells
Describe the structure of a lipopolysaccharide (LPS).
- Lipid A (endotoxin)
- Polysaccharide part:
- Core polysaccharide -> Usually conserved within a species
- O antigen -> Repeating polysaccharide of variable length
What is an endotoxin and what is its role?
- It is the Lipid A part of a lipopolysaccharide (LPS)
- It leads to systemic inflammatory response syndrome (SIRS):
- Fever
- Hypotension
- Disseminated intravascular coagulation (coagulation of blood throughout the body)
- Activation of complement and macrophages
In other words, it is responsible for many of the roles of LPS in Gram-negative bacteria.
What are mycolic acids?
Long fatty acids found in cell walls of Mycolate bacteria
What important functions do bacterial surface molecules have during infection?
- Contribute to motility, adhesion, invasion, resistance
- Harbour molecular patterns that trigger immune responses
- Targets for components of the complement system/immune system
What are the bacteria surface structures?
Cell wall, polysaccharide capsule, pili, flagella, lipopolysaccharide, fimbriae.
What are bacterial capsules and what is their role?
- High molecular weight polymers on the surface of bacteria
- They are structurally diverse and can be structural mimics of host molecules
- They are important in resistance to the host’s immune response
- protect from phagocytosis
- protect against desiccation.
What is the clinical importance of bacterial capsules?
They can be targeted by conjugate vaccines.
Where does the bacteria capsule lie relative to the cell wall?
It lies outside of the cell wall.
Give an example of a bacterial capsule providing resistance to the host’s immune response. [EXTRA]
- In Streptococcus pneumoniae (Gram positive), the capsule provides some resistance to phagocytosis.
- In Neisseria meningitidis (Gram negative), the capsule provides some resistance to complement lysis (since the complement pathway results in assembly of the MAC complex on the outer membrane of Gram negative bacteria)
What is the bacterial capsule composed of?
Polysaccharides
What are flagella and what is their function?
- Long, thin filaments involved in bacterial motility (swimming/swarming)
- Enable bacterial movement through rotation of the filament
What are the 3 components of a flagellum?
- Basal body -> Embedded in the inner and outer membrane of a Gram-negative bacterium. It can spin to allow movement.
- Hook
- Filament
What are flagella powered by?
The proton motor force (chemiosmosis).
What makes up the filaments in the flagellum of bacteria?
Flagellin:
- A globular protein that arranges itself in a hollow cylinder to form the filament in a bacterial flagellum.
- It is the principal component of bacterial flagellum, and is present in large amounts on nearly all flagellated bacteria.
What is the clinical importance of flagellin?
- It is a MAMP, since it is a TLR5 ligand
- Therefore, it can be used as an adjuvant (agent that improves the immune response of a vaccine) in vaccines, since it stimulates local inflammation and immune response
What are pili and what is their role?
- Filament-containing structures on the surface of bacteria that are projections of the cell membrane
- They have important roles in adhesion
What are pili made of?
Repeating units of pillin
What is the difference between pili and fimbriae?
- They are essentially the same
- Short pili are also known as fimbriae and are higher in number than long pili
On what bacteria are pili and fimbriae typically found?
Gram negative
Describe the role of the type I pilus of UPEC in UTIs. [IMPORTANT]
Type I pilus causes lower UTIs:
- The lectin domain of the FimH tip adhesin of the type I pilus of UPEC (uropathogenic E. coli) binds to mannosylated uroplakins (found on plaques) in the bladder.
- This allows the UPEC to bind to the bladder, causing the UTI.
- Understanding this structure allows the design of anti-infectives.
What are secretion systems in bacteria?
Protein complexes on the cell membranes of bacteria for secretion of substances
How many different secretion systems in bacteria do we know about?
9
What are secretion systems designed to allow substrate transport across?
The cell membrane(s) of bacteria. In Gram-negative bacteria, there is the inner and outer cell membrane -> extra challenge.
Give some examples of the roles of bacterial secretion systems.
- Cell entry
- Attachment
- Replication
- Anti-phagocytic
Describe the type 3 secretion systems in bacteria.
- Found in many different Gram-negative bacteria
- Type 3 secretion systems transport substrates across 3 cell membranes -> The inner cell membrane, outer cell membrane and the cell membrane of the host
- They have a basal body (like flagella do), needle and translocon
- Essentially they can be viewed as syringes that pump toxins directly into the host cell
- The exact function of the secretion system varies between bacterial species, depending on what substrates are secreted
How can bacteria transfer DNA horizontally?
transformation, transduction, conjugation.
Describe bacterial transduction.
- Bacteriophage (virus) inserts its DNA into bacterium
- Cuts bacterial DNA into small pieces
- Takes up its own DNA and bacterial DNA
- Can then inject bacterial DNA into other cells
- Other cells incorporate new DNA into chromosome
Describe bacterial conjugation.
(Plasmid is self-transmissible)
- Mating bridge (plasmid contains DNA code) forms between bacteria via plius.
- Plasmid -> single strand -> transferred across
- Both cells synthesise a complementary strand –> double stranded circular plasmids.
Describe bacterial transformation.
Bacteria able to actively take up and integrate exogenous DNA
What is the effect of horizontal gene transfer on bacterial phenotypes and virulence?
It can confer properties such as antibiotic resistance, which in turn can increase virulence.
What is the definition of virulence?
The degree of damage inflicted by a pathogen
Compare the ideas of intergenomic and intragenomic variation.
- Intergenomic variation is where DNA is introduced into a bacterium from an external source
- Intragenomic variation is where DNA mutates or is altered within the bacterium itself
Name the different types of intragenomic variation.
- Point mutation
- Phase variation
- Antigenic variation
What is phase variation (in bacteria)? [IMPORTANT]
- It is the on/off switching of genes, without changing the sequence.
- It is heritable, but it can be reversed!
Name two ways in which phase variation can occur in bacteria.
- Inverter elements
- Homopolymeric tracts
How does phase variation occur by invertable elements? Explain using an example.
- Example: The expression of Type 1 fimbriae in E. coli, a fim switch is used to control whether the fimbriae are expressed
- If the fim element is inverted, then the promoter elements in the fim do not line up with the fimbriae gene, so it is not expressed
- FimB encodes an invertase that switches between the inverted and non-inverted alignment
How does phase variation occur by homopolymeric tracts? Explain using an example.
- Example: In Neisseria meningitidis, expression of PorA (an outer membrane protein) is controlled by homopolymeric tracts
- Homopolymeric tracts are lengths of repeated nucleotid, either in the promoter or in the open reading frame (ORF)
- DNA polymerase has relatively low affinity over repeating sequences, so the length of the tract can increase or decrease during replication
- This causes the gene to shift in or out of frame
- This can be reversed in subsequent rounds of replication
- Aside from this, PorA can be interrupted by insertion of mobile DNA within the bacterium, which enters the ORF and stops translation
What is antigenic variation (in bacteria)?
The mechanism by which a bacterium alters the proteins or carbohydrates on its surface and thus avoids a host immune response.
How does antigenic variation occur?
The sequence of a gene coding for a protein can change by:
- Point mutation
- Uptake of DNA by horizontal gene transfer
Give examples of different mobile genetic elements in bacteria.
Plasmids, bacteriophages, transposons, integrons.
What are integrons?
A mobile DNA element in a bacterium that can capture and carry genes, particularly those responsible for antibiotic resistance.
How do integrons work? What is their structure?
Integrons have:
- An integrase that enables the addition of new DNA to the integron
- A promoter that enables expression of the DNA
- A recombination site (and a recombination site on the new incoming casette), so that the new DNA can be added to the integron
This means that a plasmid can contain multiple resistance genes that the bacterium ‘collects’. These can collectively be passed on using the plasmid.
What are plasmids?
- Circular, extrachromosomal DNA found in bacteria
- Transferable between strains
- There are many copies
- Move by transformation or conjugation
- Major contributor to AMR
What are transposons?
- A transposable element is a DNA sequence that can change its position within a genome, sometimes creating or reversing mutations and altering the cell’s genetic identity and genome size.
- Transposition often results in duplication of the same genetic material.
What are bacteriophages?
They are viruses.
Take part in genetic transfer by transduction.
What is MRSA?
Methicillin-resistant S.aureus (MRSA). It is one of the most common antibiotic-resistant bacterial pathogens. PBP altered by MecA gene (encoding PBP2a). Resistant to almost all beta lactams (unable to bind to cell membrane).
Its emergence pre-dates the use of methicillin.
Are blood, lymph and CSF usually sterile or unsterile?
Sterile.
Why are blood and lymph normally sterile?
They have a high level of leukocytes.
Why is CSF normally sterile?
Pathogens cannot cross the BBB and there are microglial defensive cells.
Which three microbiota are the main populators of mucosae?
- Bacteroidetes
- Firmicutes
- Actinobacteria
Which two are the most common bacteria on the skin?
- Staphylococcus epidermis
- Corynebacterium
Which two opportunistic pathogens are found at lower levels in mucosae?
- Clostridium difficile
- Helicobacter pylori
Why are C. difficile and H. pylori not found at higher levels in the mucosae?
They are forced into smaller niches by competition from other commensals.
Define a commensal.
A harmless member of the flora.
Define a symbiont.
A commensual in a mutually beneficial relationship.
What are the two main types of pathogen based on when they cause disease?
- Primary pathogens -> Cause disease as a result of their presence or activity within the normal, healthy host, and their intrinsic virulence is a necessary consequence of their need to reproduce and spread
- Opportunistic pathogens -> Normally commensal and do not harm the host, but can cause disease when the host’s resistance is low.
What are opportunistic bacteria?
Small proportion of potentially pathogenic organisms.
What are the attributes of a pathogen necessary for virulence?
- Tropism
- Replication
- Evasion of immune killing
- Toxicity
- Transmission (between hosts)
- Aquisition of nutrients
What is tropism?
The ability of a bacterium to find and establish a niche (intracellularly or extracellularly).
Why might a host have predisposition to infection?
Disruption in physical barriers and attenuation of active responses, e.g:
- wounds
- catheterisation
- chronic ulcers
- foreign bodies
Diabetes
Immunodeficiency
What does the term ‘immunocompromised host’ refer to?
The ability to respond normally to an infective challenge is in some way impaired.
What is the difference between immunosuppression and immunocompromisation?
Immunosuppression and immunocompromised both refer to having a weakened immune system. However, there is a difference:
- immunosuppression is a result of medication.
- Immunocompromised refers to a weakened immune system due to a medical condition.
What are some examples of disinfection and sterilisation that you need to know?
- Pasteurization
- Autoclaving
- Hypochlorite
- Halogenated phenols
- Gamma-irradiation
What is Pasteurization used for and what are its antibacterial actions?
- Pasteurization is used to extend the shelf-life of food and decrease food wastage without degrading the nutritional value.
- Food or beverages are heated to specific temperatures to destroy pathogenic bacteria w/out affecting nutritional value.
What is autoclaving used for and what are its antibacterial actions?
It is primarily used for sterilizing equipment and supplies by subjecting them to high-pressure saturated steam at 121 degrees Celsius for around 15 - 20 minutes.
What is hypochlorite used for and what are its antibacterial actions?
- Widely used as a disinfectant/ bleaching agent.
- It is an oxidising agent, disrupting essential bacterial activity. It also degrades fatty acids and disrupts cell metabolism.
What is Halogenated phenols and what are their antibiotic actions?
Wide range of applications and effects. Including as a disinfectant/ anti-septic. Actions include disruption of cell wall/ inhibition of energy metabolism and damage to DNA. It is effective against a broad range of bacteria including drug-resistant strains making it invaluable in hospitals/ public health settings.
What is gamma-irradiation used for and what is its antibacterial actions?
Gamma rays used to eliminate all forms of microbial life.
- widely used for sterilization and preservation.
- DNA damage, production of radicals, disruption of cellular functions.
What roles do surface structures have in colonisation and disease?
- Capsules -> Bacterial resistance to phagocytosis and complement
- Flagella -> Motility and activate inflammatory responses
- Pili -> Adhesion to host receptors
- Type 3 secretion systems -> Cell entry/avoidance of phagocytosis
- Toxins -> May be sufficient to mediate disease
