Bacterial adhesion, invasion and survival within the host

Question 1

Obligatory steps in infection

Answer 1

Attachment / Invasion
Multiplication (and Dissemination) within the host
Evasion of host defences
Damage to the host - essential or collateral?
Transmission to new host

Question 2

Host barriers to colonisation:Skin

Answer 2

Natural physical barrier to infection
Commensals act to prevent colonisation
Fatty acis pH 5.5
Antimicrobial products produced by sebum and commensals

How do bacteria overcome
Physical damage – Cuts/Burns
UV damage
Bites
Insect

Question 3

Host barriers to colonisation:Respiratory tract

Answer 3

Large particles filtered by nasal hairs
Small particles reach lungs
Very small particles can reach the alveoli

Foreign cells moved back out in mucous through ciliary action – Mucociliary clearance

how do bacteria overcome
Adhesion through presence of adhesins
Stop cilia from moving
Production of ciliostatic substances

Question 4

Host barriers to colonisation:Urogenitary tract

Answer 4

Urinary tract is flushed every few hours – prevents colonisation
Strong sphincter muscle prevents access to the bladder

Question 5

Host barriers to colonisation:GI tract

Answer 5

Oropharyngeal region
Saliva creates a flushing action and contains antibacterial substances
Stomach
Extremely acidic, only specialised organisms can colonise here (e.g. Helicobacter pylori)
Small Intestine
Large Intestine
Flow through the intestine provides flushing force
Mucus and commensals provide a mechanical barrier
Commensals compete for nutrients and produce antimicrobials

Question 6

Steps of infection

Answer 6

1.Bacterial attachment (Adherence)
Colonisation – Bacterial attachment and growth at a site following adherence
Adhesin - A surface molecule binding to a receptor
Receptor - A substratum molecule complementary to an adhesin

Surface structures
Flagella
Agents of motility
Important for colonisation
Moving to to an attachment site
Moving through mucus layer
Fimbriae (Pili)

Intimate attachment – Gram negative outer membrane proteins
Promote more intimate attachment

Gram positive cell wall proteinsMSCRAMMs

Question 7

What are adhesins, and how do they function in bacterial adherence to host cells?

Host receptors

Answer 7

Adhesins are bacterial surface proteins or structures that mediate attachment to specific receptors on host cells.

Integrins
Sialic acid moieties
Carbohydrate moieties

Question 8

Bacterial invasion

Answer 8

1.adheres to the surface of the cell
through fimbriae, autotransporters.
2. Activates a specific gene call SPI1
These produce Virulence factors that allow the bacteria to go in inside.
They then switch there genes off to avoid being killed by lysosomes.
3. Switch on another set of genes SPI2 these genes form this kind of injector system, this injector that injects something into the membrane that prevents it from fusing with things (NO) that the cell uses to destroy that bacteria.
4. They then multiply in size and leave the cell

Question 9

Bacterial survival
Macrophages- host defence

Answer 9

Macrophages internalize pathogens through phagocytosis, entrapping them into organelles called phagosomes. Phagosomes then fuse with lysosomes to mature into phagolysosomes, acquiring an acidic and hydrolytic lumen that kills the pathogens.

Inhibition of Phagolysosome Fusion:
SPI 2 prevents lysosome fusion