Lecture 3. Communities, Quorum Sensing, Biofilms Flashcards
Because microbes live in a community in their natural state, what does this mean to the bacteria and how they live?
Exponential growth means before long there are many cells
20 generations = 5 x 10⁵ cells
In a mixed culture, which bacterias cells will dominate in the environment?
The one with the faster doubling time
After 7 hours, a bacteria with a doubling time of 20 minutes will have a million cells vs 30 minutes having 8,000
What influences the abundance of different organisms in the community?
The environment they find themselves in
In microbial communities, what does a positive interaction mean?
Toxic product of one organism may be substrate for another
What is the main example of a positive interaction?
Cyanobacteria, fix carbon in water and double and grow based on how much they fix and how much light they capture in order to fix the carbon
What happens when cyanobacteria are kept in a pure culture?
Die off after ~8 weeks because they run out of energy and produce toxic products that end up killing them
What happens when a heterotrophic bacteria is present in a culture of cyanobacteria?
Both bacteria can survive for months
Cyanobacteria tend to produce waste products that are toxic to themselves. Heterotroph bacteria use and recycle these waste products to produce products that the cyanobacteria use to survive (as long as light is present or any other limitations)
What are Windogradsky columns?
Made by mixing soil/sediment with water and adding some carbon (cellulose) and nutrients
Creates a whole ecosystem within a tube
Microbes fix carbon and recycle all nutrients so allowing these columns to grow for many years
Carbon fixation by cyanobacteria
Can have layers of aerobes and anaerobes
In microbial communities, what are examples of negative interactions?
Antibiotic production by fungi, targets other microbes (always resistant to own antibiotics) - Penicillium strain produces an antibiotic inhibiting bacteria
Competition - sulphate reducing bacteria outcompete methanogens for substrates
Add acetate, little methane produced, inhibit sulphate reduction, lots of methane produced - These organisms are competing for substartes
How do microbial communities affect each other in molecular terms?
They “sense” each other using chemical signals
How many are we? - Quorum sensing (Staphylococcus aureus, talking to itself, cells interacting from the same source)
What are we doing? - Biofilm formation (80-90% of all microbes on the planet are found in biofilms)
How does quorums sensing work?
Detection of an autoinducer (gene that induces own production/a compound that will induce the infection of itself) - signal molecules that allows cell to sense population size (“quorum”)
Some processes require a certain density of cells to be activated (switch from attaching to invading form) such as production of a toxin
Once autoinducer reaches a threshold then cells respond - Changes expression and function
What are most autoinducers?
Acyl-homoserine lactones
Autoinducing peptides with a thiolactone ring
How does quorum sensing function in Staphylococcus aureus?
Staphylococcus aureus is a major opportunist pathogen
Also forms biofilms in wounds and on implanted medical devices (Staphylococcus always ends up on wounds, but normally harmless)
Switch from a biofilm-forming to an invasive phenotype is driven by quorum sensing
One quorum sensing mechanism is the Agr (accessory gene regulator) regulatory system
What is Agr (accessory gene regulator)?
A two component regulatory system based around an operon with 2 promoters
What does Agr constitutively express?
AgrD – Autoinducing peptide (AIP) - produces quorum sensing molecule
AgrB – Transmembrane protein, secretes mature AIP
AgrC – AIP receptor, binds AIP then phosphorylates AgrA
AgrA – Response protein, activates P3 after being phosphorylated by AgrC producing RNAIII
What happens when the S. aureus cell density concentration is low?
Concentration of AIP is low
AgrC inactive so no phosphorylation of AgrA to activate P3 and produce RNAIII
Cell expresses many surface adhesins, which enhances biofilm formation
What happens when the AIP (autoinducing peptide) reaches its threshold value due to biofilm maturation increasing the cell density?
At a threshold AIP activates enough ArgC which phosphorylates ArgA
ArgA-P induces expression of RNAIII (regulatory RNA)
RNAIII acts as a regulatory RNA controlling many genes
What is the role of RNAIII in S. aureus?
Represses adhesion (represses cell wall compartments) - adhesisns
Induces elements that drive invasion (cytotoxins etc)
Also the mRNA holder of hld (δ-hemolysin) - damages red blood cells, also helps the invading staphylococcus seek out iron
What does the activation of the agr system switch S. aureus from and in to?
Activation of of agr system switches S. aureus from an adhesive colonising commensal to an invasive aggressive pathogen
What does RNAIII regulate?
Regulates some genes directly: Many of these are other regulatory genes which are the starting point for a major regulatory network
Also directly regulates α-hemolysin expression
Forms a complex secondary and tertiary structure: 14 hairpins bends and a globular shape brings 3’ and 5’ ends close together
This binds to hlaA mRNA at the shine-delgarno region allowing its translation
A rare example of RNA activating translation of a mRNA molecule
What are biofilms?
Structured clusters of cells, enclosed in a self-produced polymer matrix and attached to a surface
Cells and tissues solid surfaces
Estimated that 80% all microbial biomass is in biofilms - most microbes attach to surfaces or other cells to allow for the cell to survive
Enable pathogens survival in the environment and in hosts
What can a biofilm on contact lenses cause?
Keratitis - can damage eye sight
Particularly caused by Ps. aeruginosa and Staph. epidermis
What can a biofilm on urinary catheters cause?
Bacteriuria - can cause major complications
Particularly caused by E. coli , Ps. aeruginosa and Proteus mirabilis
What can a biofilm on joint replacements cause?
Septicemia or device failure
Particularly caused by Staph. epidermis and Staph. aureus
