17 - Bacterial Adhesins and Colonisation

Question 1

Colonisation

Answer 1

• Establishment of stable bacterial population in host
• First critical step in establishment of disease
• Mucosal epithelium primary sites of colonisation (e.g. respiratory tract)

Question 2

Two possible outcomes after colonisation

Answer 2

1. Pathogen is confined to epithelial surfaces (e.g. V. cholerae in intestinal tract)
2. Pathogen crosses epithelial surfaces, invades deeper tissues, some may cause systemic infection (e.g. S. enterica –> systemic typhoid fever)

Question 3

Mucous

Answer 3

• Secreted by goblet cells & subepithelial glands
• Major component is mucins
• Glycosyl chains of mucins
• Mucins bind to and trap bacterial pathogens

Question 4

Mucins

Answer 4

Family of filamentous proteins

Question 5

Glycosyl chains of mucins

Answer 5

• Protect peptide from bacterial degradation
• Receptors for bacterial surface ligands

Question 6

Mechanical removal of bacterium by host innate defences

Answer 6

• Eye: Flushing action of blinking, tears
• Oral cavity: Flushing action of salivary flow
• RT: Sneezing, coughing
• GIT: Peristalsis and excretion
• UT: Flushing action of urination

Question 7

Requirements of successful colonisation

Answer 7

1. Avoid being trapped in mucous layer
2. Penetrate mucous layer to reach epithelial cell surface
3. Adhesion to epithelial cell receptors via adhesions

Question 8

How can bacteria penetrate mucous layer to reach epithelial cell surface

Answer 8

• Via motility (flagella)
• Via mucinases (dissolve mucous layer)

Question 9

Bacterial adhesins

Answer 9

• Specialised surface structures that bind to specific host receptors
• “lock and key” interactions occur between adhesins and receptors

Question 10

Epithelial cell receptors

Answer 10

• Glycoproteins
• Glycolipids
• Microbial adhesin binds to sugar moiety of receptor

Question 11

Lock and key adhesion by pili

Answer 11

Bacterial cell at a distance from epithelial cell overcomes electrostatic repulsion (Bacterial & epithelial cells are negatively charged)

Question 12

Lock and key adhesion by afimbrial adhesins

Answer 12

Tight binding of bacteria to cell may follow pilus adhesion, as depolymerisation of pili brings bacterium closer to cell

Question 13

Pilus morphology

Answer 13

• 5-7nm diameter
• 0.5 to >10nm long
• Peritrichous or at one pole
• varied morphology
• a few to several hundred per cell

Question 14

Pilus shaft/rod

Answer 14

• Repeating protein subunits (pilins) assembled into helical array
• PapA

Question 15

Specialised tip structure

Answer 15

• PapG adhesin at tip of Pap pili
• PapF, E, K proteins form a thin fibril at the end of the pilus

Question 16

Fibrillae morphology

Answer 16

• Finer fibrous organelles 2-3nm in diameter
• Long multidomain proteins attached at cell surface
• Found widely in G+ve and G-ve but less well studied than pili

Question 17

Receptor specificity of pili

Answer 17

Determines tissue colonised by pathogens

Question 18

Receptor of UPEC PapG protein at pilus tip

Answer 18

• Globotriasylceramide
• Tissue specific as uroepithelial cells have this receptor

Question 19

Advantages of anti-virulence antimicrobials

Answer 19

• Inhibit bacterial virulence instead of bacterial growth
• Hope to avoid resistance development (resistance would not provide survival advantage, therefore wouldn’t be selected for)
• No effect on microbiota

Question 20

Strategies of anti virulence antimicrobials

Answer 20

• Anti adhesion agents
• Anti quorum sensing
• Anti biofilm
• Antitoxin
• Anti T3SS

Question 21

Adhesins as vaccine candidates

Answer 21

Ab blocks adhesin binding pocket - inhibits binding of pathogen to epithelial receptor

Question 22

Stable, high affinity analogues of receptor

Answer 22

• Competitively inhibit binding of adhesin
• Prevent colonisation of epithelium
• Prevent infection

Question 23

Inhibitors of pilus assembly (pilicides)

Answer 23

• Pap & type 1 pilus subunits form helical rod
• Assembly is via action of conserved periplasmic proteins (pilin chaperones)
• Chaperones interact with C-terminal residues of pilin subunits (essential for pilus assembly)
• 2-pyridones resemble PapG C terminus and inhibit chaperone binding
• Thus pilus is not assembled

Question 24

Problems and challenges with anti adhesin vaccines and drugs

Answer 24

• “cocktail” vaccines or multi epitope fusions may be necessary (multiple adhesins are expressed in a time- and tissue- specific manner during the course of an infection)
• Rapid and precise diagnosis is needed
• A drug may be non specific and interfere with microbiome
• May have weak activity in vivo

Question 25

Does adherence always guarantee colonization

Answer 25

• NOOOO
• Pathogens may have adhesins that recognise host receptors but may still fail to colonise host tissue

Question 26

Two major mechanisms through which beneficial bacteria provide colonization resistance to pathogens

Answer 26

• Indirect mechanisms: microbiota-stimulated host immunity
• Direct mechanisms

Question 27

Direct mechanisms of beneficial bacteria providing colonisation resistance

Answer 27

• Antagonise/compete with pathogen
• Directly compete for same niche( compete for nutrients)
• Secrete antimicrobial peptides and toxins e.g. bacteriocin

Question 28

Bacteriocins

Answer 28

• Small, antimicrobial proteins secreted by normal flora
• E.g. Streptococcus mutans (normal flora) secrete bacteriocins that inhibit other streptococci