18 - Biofilms & Quorum Sensing Flashcards
1
Q
Biofilms
A
- Organised community of microbial cells
- Enclosed in extracellular polysaccaride substances (EPS)
- Adhere to a living or non living surface
- Channels allow nutrients and O2 to reach most of the biofilm community
2
Q
Sessile
A
Microbes living attached to surfaces
3
Q
Planktonic
A
Free floating bacteria
4
Q
What % of human infections are associated with biofilm
A
65-80%
5
Q
Medical devices that may be colonised with biofilms
A
- Catheters
- Prostheses
- Contact lenses
- Heart valves
6
Q
Dental plaque
A
- Biofilms are initiated by adherence of primary colonisers (normal flora) to glycoprotein receptors on tooth surface via adhesins (e.g. fimbriae/pili)
- Adherence of secondary colonisers to primary colonisers via co-aggregation
- Process continues –> complex, multispecies biofilm = plaque
7
Q
Co-aggregation
A
- Adhesin of secondary colonisers recognises CHO receptor on surface of primary colonisers
- OR adhesin of primary colonisers recognise CHO receptor on surface of secondary colonisers
8
Q
Secondary colonisers
A
May be normal flora (e.g Strep. mitis) or pathogens (e.g. Actinobacillus)
9
Q
Corn cob formation
A
Binding of streptococci to a filamentous bacterium such as Fusobacterium nucleatum
10
Q
Calculus
A
- Salivary calcium and phosphate leads to calcified plaque mass
- Bacterial toxins within calculus can lead to chronic inflammation
11
Q
Process of development of dental plaque on enamel surface
A
A: Attachment of coccal bacteria
B: Bacteria multiply to form microcolonies
C: Bacteria embedded in matrix
D and E: Bacterial diversity increases, rods and filaments appear
F: corn cob formations appera
12
Q
Most complex biofilm
A
Plaque, with about 500 species
13
Q
Healthy tooth
A
- Plaque levels low, early colonisers predominantly gram +ve aerobic cocci
- High plaque levels causes anaerobic environment which leads to shift in bacterial flora (to gram -ve anaerobic rods, including opportunistic pathogens)
14
Q
What are sessile biofilm bacteria more resistant than planktonic bacteria to
A
- Antibiotics
- Antibody/complement mediated lysis
- Phagocytosis
15
Q
Extracellular capsular material of biofilm bacteria
A
- Diffusion barrier to antibodies, complement and antibiotics
- Inhibits macrophage binding
- Protects cell from hydrolytic enzymes released by phagocytes
16
Q
Phenotypic changes within biofilm that induce antibiotic tolerance
A
- Decreased antibiotic diffusion through capsular material
- Increased expression of specific genes in biofilm (e.g. antibiotic efflux pump genes upregulated)
- Bacteria deep in biofilm are in metabolically inactive state (antibiotic target is inactive so cell is less susceptible)
17
Q
Antibiotic tolerance
A
The ability of a microorganism to survive, but neither grow nor die, in the presence of an antibiotic
18
Q
Antibiotic that only works if cells are dividing
A
Penicillin
19
Q
Molecular mechanisms of persister cells that have differentiated into a dormant state
A
- Reduced cellular energy levels via inhibition of ATP production
- Inhibition of DNA replication
- Reduction of translation via disruption mRNA, tRNA, rRNA or ribosome assembly
20
Q
Cystic Fibrosis
A
- Genetic disorder
- Excessive, viscous mucous production in airways impairs clearing by mucociliary escalator
- Susceptibility increases to opportunistic infections of RT, such as Pseudomonas aeruginosa, Burkholderia cepacia, Staphylococcus aureus
- Chronic, not cleared by antibiotics or immune response
21
Q
Pseudomonas aeruginosa biofilms in CF patients
A
Lower respiratory tract colonised by biofilm
22
Q
Stages of development of biofilms
A
- Initial attachment (sad genes)
- Microcolony formation
- Maturation
- Detachment of bacteria from biofilm
- Dispersal of bacteria from biofilm
23
Q
Sad genes
A
Defined by mutations causing loss of adhesion or Surface Adherence Deficiency
24
Q
What do sad genes encode
A
- Flagellar synthesis (Fla- mutants cannot form biofilms, therefore motility essential for initial adherence to surface)
- Type IV pili
- Ps1 exopolysaccharide production
25
Microcolony formation
- Cells move towards each other via a positive feedback loop
- Type IV pili are required for this motility on a solid surface
26
Microcolony formation positive feedback loop
Cells deposit a trail of Ps1 exopolysaccharide which causes other cells that encounter it to remain longer, generating a positive feedback that directs the cells to form a microcolony at Ps1-rich sites
27
Twitching motility by type IV pili
- Extension of pilus
- Binding of pilus tip to substrate
- Retraction of pilus pulls cell along
- Thus bacteria crawl along surface of substrate a tiny distance
- Bacteria alternate this twitching with “slingshot” movement also mediated by
type IV pili
28
Extension of pilus
Polymerisation of pilus subunits into pilus base
29
Retraction of pilus back into cell
Depolymerisation of pilus subunits at base
30
Maturation of biofilm
- Quorum sensing genes are expressed
- Allows bacterium to sense it is living in a dense population
- In high density (microcolony) bacteria increase expression of three polysaccharides (EPS)
31
Three polysaccharides that are expressed in high density
- Alginate (capsule) genes
- Ps1
- Pe1
32
Alginate
- Overproduced by mucoid strains often isolated from CF lungs
- Contributes to structural stability of biofilm
33
Ps1
- Key role in biofilm initiation (adhesion) and maturation
- Interacts with extracellular DNA to form a web of fibres
34
Pe1
Involved at early stage of biofilm formation and helps to structure mature biofilm
35
Examples of quorum sensing regulated functions
- Bioluminescence
- Biofilm production
- Sporulation
36
How does quorum sensing signal high bacterial density
- In Gram negative bacteria the LuxI/LuxR system uses AHL as the signal molecule (autoinducer)
- Gram positive bacteria have a different QS system, where autoinducer is a peptide
- A third system is active in both Gram
negs and Gram pos (allows interspecies communication)
37
Luxl/LuxR system
- LuxI synthesises AHL
- AHL freely diffuses out of cell
- Concentration of AHL in environment increases in proportion to cell density
- AHL freely diffuses into cells, when it reaches a threshold level in cells it induces gene expression
38
LuxR
- Transcriptional regulator protein
- Binds to AHL, when bound, LuxR binds to specific promoters and activates transcription of target genes such as capsule genes
39
Each bacterial species produces a unique AHL
Thus only members of same species respond to signal
40
. Detachment of bacteria from biofilm
Can occur by natural shear forces (abrasion, erosion etc)
41
Dispersal of bacteria from biofilm
- Occurs by active participation of the bacteria
- Bacteria start to produce matrix degrading enzymes (alginate lyase plus other hydrolases, DNases)
- Dissolves alginate etc. in EPS which releases planktonic cells
42
Role of detachment of bacteria from biofilm and dispersal of bacteria from biofilm
- Transmission of bacteria from environmental reservoirs to human hosts
- Spread of infection within a host
43
Quorum quenching (QQ)
- Inhibit quorum sensing thus reduce biofilm formation and reduce virulence
- Resistance to these agents will not arise, as there would be no survival advantage (quorum quenching agents do not kill bacteria)
44
Two main categories of quorum quenching molecules
- Structural analogues of AHLs
- Enzymes that hydrolyse homoserine lactone ring
45
What does AHL stand for
Acyl-homoserine lactones
46
Example of QQ in medicine
Halogenated furanones isolated from red algae (a natural QQ defence mechanism, structurally similar to AHLs)
