8. Microbial biofilms Flashcards
Define ‘biofilm’
- community of microbial cells
- encased within a matrix of polymers
- associated with an interface
What kind of interface are biofilms found at?
- can be air-liquid
- or liquid-solid
Biofilms have emergent properties. What does this mean?
- not simply a sum of microbes within them
Explain planktonic biofilms
- in liquid
- homogenous
- single species
- nutrient rich
Explain surface-associated biofilms
- cultures on agar plate
- limited heterogeneity
- single species
- nutrient rich
Explain natural biofilm
- in wildlife
- heterogenous
- mixed species
- limited nutrients
4 stages of biofilm growth
- attachment
- colonisation
- development
- dispersal
In the mouth, how are most cells transported?
- saliva
- sloughing of epithelial cells
What happens in attachment of a biofilm?
- cells swim or passively reach surface
- non-specific and specific attachment forces
What happens in colonisation of biolfilm?
- adherent cells start to grow
- cell-cell communication
What happens in development of a biofilm?
starts to form 3D structure
What happens in dispersal?
active or passive sloughing
Pseudomonas aeruginosa is a … pathogen
opportunistic
How does pseudomonas aeruginossa link to biofilms?
- commonly found in water systems
- usually harmless but can cause UTIs, wound infection or lung infections in CF patients
- can cause outbreaks in hospitals e.g ear piercing clinics
- forms thick biofilms in labs and is studied as a model in biofilm processes
Explain why biofilms are heterogenous
- biofilms are 3D
- open structures with channels permeating the matrix
- they’re heterogenous as cell distribution is not uniform and microcolonies are often seen
- nutrients and waste products not uniformly distributed either due to matrix impeding mass transfer
Biofilms contain a range of different niches. Explain
- anaerobic bacteria can grow in some parts containing less oxygen
- more oxygen rich parts would encourage growth of aerobes
Macromolecules such as … form the basic structure of a biofilm
- polysaccharides
- proteins
- nucleic acids
Small molecules can be trapped in the … of biofilms e.g …
- matrix
- nutrients
- metals
- signalling molecules
How to prepare a biofilm for EM scanning?
- have to dehydrate and remove water
What percentage of human infections can be attributed to biofilms?
65-80%
Examples of human issues caused by biofilms
- CF
- dental disease
- chronic wounds
- kidney stones
- endocarditis
- atherosclerosis
Diagnostic criteria for biofilm infections
- pathogenic bacteria associated with surface
- direct exam of infected tissue shows aggregated cells in cell clusters encased in a matrix (of bacterial or host origin)
- infection confined to particular site on host
- recalcitrance to antibiotic treatment despite demonstrated susceptibility of planktonic bacteria
Infections are harder/easier to treat when in a biofilm
harder
Who investigated Pseudomonas aeruginosa?
Bill Costerton
What did Bill Costerton do?
- followed observations that catheter infections are often recalcitrant to antibiotics
- used model of artificial urine flowing over discs of catheter material
- inoculated with Pseudomonas aeruginosa (common cause of UT catheter I)
- tested antibiotic sensitivity in biofilm bacteria and compared with planktonic cells
How does ‘antimicrobial resistance occur?
- genetic changes
- transmitted to progeny during cell division (vertical transmission)
- transmitted between resistant and sensitive strains of species or between species (horizontal transmission)
How does antimicrobial tolerance occur?
- environmental adaptation e.g biofilm formation or change in growth rate
- transient reduction in antimicrobial sensitivity (not directly through cells but traits can be passed on)
- responsible for treatment failure
Why are biofilm bacteria so tolerant to antimicrobials?
- slow growing
- presence of dormant ‘persister’ cells
- sequestration from immune system (important for bacteriostatic antibiotics that work with host immunity)
- elevated expression of efflux pumps
- poor penetration of antibiotics
Studies on synchronous populations of cells show that antibiotic sensitivity varies with …
stage of growth
How does growth rate link to antibiotic resistance?
- growth as in cell wall synthesis may play a role but not just growth rate
- slow growing cells in biofilms should be more resistant to RNA synthesis inhibitors
Antibiotics that target cell walls are less effective against biofilms than?
planktonic cells
Explain persister cells
- all populations of bacteria (planktonic or biofilm) have a small proportion of inactive/dormant cells
- antibiotics are ineffective against them because they’re asleep
- very difficult to kill and therefore they become a significant proportion of biofilm bacteria
- form small colonies on agar and may be responsible for infections on replacement hips 15-20 yrs after operation
Antibiotics targeting protein or RNA synthesis were … effective against planktonic and biofilm cells
equally
Biofilms are highly resistant to …
phagocytosis
Why are biofilms resistant to phagocytosis?
- inflammatory cells can’t penetrate the matrix which encapsulates bacteria
- even if they can penetrate channels, they don’t digest bacteria effectively
Antibodies penetrate biofilms well/poorly
poorly
Adaptive responses in biofilms can … bacteria e.g?
- protect
- pseudomonas spp. produce rhamnolipids in biofilms which can kill neutrophils
- interactions with oral strep induce production of a complement evading protein
- up regulation of efflux pumps
Role of efflux pumps
- pumps things out of cells
- important in chemo as cancer cells become more tolerant as up regulate the pumps
- pumps drugs out
Antibiotic efflux pumps are up-regulated in biofilms formed by …
- E.Coli
- P.aeruginosa
- Candida albicans
Efflux pumps are effective against all antibiotics?
no
- upregulation of them is not universal
How can you measure mass transfer in biofilms?
- using fluorescently labelled latex beads, dyes or labelled proteins
Diffusion may be impeded by the biofilm matrix but …
- not significant for small molecules
- even large molecules like IgG are weakly limited
- molecules are carried through water channels
For mass transfer in biofilms, what’s more important than diffusion being impeded?
- electrostatic or van der Waal’s interactions
- matrix acts as an ion exchange resin e.g metal cations are trapped in an ionic matrix
- reaction also matters as reactive molecules like peroxide are inactivated by the matrix
Positively charged tobramycin is impeded whereas … is not
ciprofloxacin (neutral)
Reduced penetration occurs in non-mucoid stains. Why?
do not produce the carb alginate
Adding extracellular DNA to biofilms enhances …
protection vs tobramycin
What hypothesis is created with tobramycin and negatively charged DNA?
negatively charged DNA in the biofilm matrix retards tobramycin
A biofilm is not just a trap but a …
3D sticky web
3 stages of bacterial cell cell interactions in oral biofilms
- signalling
- synergism
- antagonism
2 relations of biofilms to dentistry
- cross-contamination (surfaces in clinic, dental unit waterlines, can harvest respiratory viruses)
- dental plaque
What is the predominant mode of bacterial growth?
biofilms
