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MICR3330 > 18 - Biofilms & Quorum Sensing > Flashcards

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
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2
Q

Sessile

A

Microbes living attached to surfaces

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3
Q

Planktonic

A

Free floating bacteria

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4
Q

What % of human infections are associated with biofilm

A

65-80%

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5
Q

Medical devices that may be colonised with biofilms

A
  • Catheters
  • Prostheses
  • Contact lenses
  • Heart valves
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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
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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
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8
Q

Secondary colonisers

A

May be normal flora (e.g Strep. mitis) or pathogens (e.g. Actinobacillus)

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9
Q

Corn cob formation

A

Binding of streptococci to a filamentous bacterium such as Fusobacterium nucleatum

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10
Q

Calculus

A
  • Salivary calcium and phosphate leads to calcified plaque mass
  • Bacterial toxins within calculus can lead to chronic inflammation
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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

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12
Q

Most complex biofilm

A

Plaque, with about 500 species

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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)
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14
Q

What are sessile biofilm bacteria more resistant than planktonic bacteria to

A
  • Antibiotics
  • Antibody/complement mediated lysis
  • Phagocytosis
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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
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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)
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17
Q

Antibiotic tolerance

A

The ability of a microorganism to survive, but neither grow nor die, in the presence of an antibiotic

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18
Q

Antibiotic that only works if cells are dividing

A

Penicillin

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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
  1. Initial attachment (sad genes)
  2. Microcolony formation
  3. Maturation
  4. Detachment of bacteria from biofilm
  5. 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
Q

Microcolony formation

A
  • Cells move towards each other via a positive feedback loop
  • Type IV pili are required for this motility on a solid surface
26
Q

Microcolony formation positive feedback loop

A

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
Q

Twitching motility by type IV pili

A
  • 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
Q

Extension of pilus

A

Polymerisation of pilus subunits into pilus base

29
Q

Retraction of pilus back into cell

A

Depolymerisation of pilus subunits at base

30
Q

Maturation of biofilm

A
  • 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
Q

Three polysaccharides that are expressed in high density

A
  • Alginate (capsule) genes
  • Ps1
  • Pe1
32
Q

Alginate

A
  • Overproduced by mucoid strains often isolated from CF lungs
  • Contributes to structural stability of biofilm
33
Q

Ps1

A
  • Key role in biofilm initiation (adhesion) and maturation
  • Interacts with extracellular DNA to form a web of fibres
34
Q

Pe1

A

Involved at early stage of biofilm formation and helps to structure mature biofilm

35
Q

Examples of quorum sensing regulated functions

A
  • Bioluminescence
  • Biofilm production
  • Sporulation
36
Q

How does quorum sensing signal high bacterial density

A
  • 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
Q

Luxl/LuxR system

A
  • 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
Q

LuxR

A
  • Transcriptional regulator protein
  • Binds to AHL, when bound, LuxR binds to specific promoters and activates transcription of target genes such as capsule genes
39
Q

Each bacterial species produces a unique AHL

A

Thus only members of same species respond to signal

40
Q

. Detachment of bacteria from biofilm

A

Can occur by natural shear forces (abrasion, erosion etc)

41
Q

Dispersal of bacteria from biofilm

A
  • 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
Q

Role of detachment of bacteria from biofilm and dispersal of bacteria from biofilm

A
  • Transmission of bacteria from environmental reservoirs to human hosts
  • Spread of infection within a host
43
Q

Quorum quenching (QQ)

A
  • 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
Q

Two main categories of quorum quenching molecules

A
  • Structural analogues of AHLs
  • Enzymes that hydrolyse homoserine lactone ring
45
Q

What does AHL stand for

A

Acyl-homoserine lactones

46
Q

Example of QQ in medicine

A

Halogenated furanones isolated from red algae (a natural QQ defence mechanism, structurally similar to AHLs)

MICR3330 (29 decks)

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