Bacterial Pathogenesis II

The host have two defence strategies: Non-specific and specific 

Bacteria have evolved strategies to counteract these defences. 

Virulence factor and virulence strategies allow bacteria to cause an infection. 

Virulence factors/ strategies 

Enable bacteria to 

• colonise the host
• survive the host immune system
• invasion and dissemination in the host
• acquire nutrients and growth within the host
• release and spread within the host

Often virulence factor expression is correlated to different stages of growth. 

Through horizontal gene transfer and mutations. 

Pathogenicity Islands

• distinct region of the DNA in bacteria but absent in non-pathogenic strains 

Evolution of Salmonella 

• non-pathogenic E.coli and Salmonella are closely related 

Evolution of Yersinia pestis 

Y. pestis - Plague. Strict pathogen with entry by flea bite or aerosols. Systemic infection

Y. Pseudotuberculosis - Yersiniosis. Found in the environment and gain its entry through oral digestion. Gastroenteritidis (occasionally systemic)

Y. enterocolitica - Yersiniosis. Found in the environment and gain its entry though oral digestion. Gastroenteritidis (rarely systemic)

There are three steps in evolution of Y. pestis

1, add DNA

2, stir

3, reduce

Genes have been acquired from other organisms 

• two extra plasmid that enable transmission by fleas and dissemination from intra-dermal site of infection
• Hms (pigmentation locus) for flea colonisation
• iron transport 

At least 3 major genome arrangement have occured - two inversions and one translocation. The genome is in the early stages of reduction - 208 pseudogene (genes which have lost their protein coding ability) have been identified. 

All of this enable the bacetria to survive in flea and transmit, gain dissemination from flea bites, gain resistance to complement lysis and lost its mobility and adhesion.

Evolution of Legionella

Its a aquatic bacetrium. Biofilm and Protozoa are important for Legionella. 

Legionella growing in protozoa are provided with:

• nutrients
• protection from unfavoured environment
• vechicle for their transmission into new habitats - human

Interactions with protozoa is a driving force in its pathogenicitiy evolution.

Infection processes

1, gain entry and adhere and colonise the host              4, spread to another site in host

2, gain access to nutrients and mutiply                         5, tissue damage

3, evade host immune system 

Route of transmission

• biological vectors
• aerosol
• body fluids - blood, saliva
• nosocomial inefctions - infection in favour of hospital environment

Route of entry 

• break of skin
• respiratory, urogential and gastrointestinal tract
• eyes
• surgical intervention

Reservoir

The place where the microorganism live such as human and animals. It must be able to meet all the requirements for pathogen to survive and growth

Zoonosis

An infection acquire from an animal source - possibility of transmission to human 

Transmission of yersinia pestis is through zoonotic infection

Transmission of Legionella is through environmental (man-made)

Transmission of Campylobacter is through food supply

Attatchment and colonisation 

Allow bacetria to resists washing action of fluids and replicate in number.

1, hair like structure called pili

2, non-piliated structure

3, soluble factor

4, biofilm

Salmonella attachment

• each genome encodes ~ 12 fimbrial operones
• they display diversity in adhesins 
• production is determined by temporal and spatial regulations - contributes to host adaptation and modulates pathogenicity of salmonella in different host. Contributes to immune evasion

S. Aureus attachment 

A soluable coagulase protein is secreted which complxes with prothrombin. This activates the protease activity of thrombin which converts fibrinogen to fibrin. The bacteria is encapsulated into fibrin clot. This helps the bacteria in the colonisation process and protect it from phagocytic and host immune defence system. 

Note that not all bacteria invade. It is optional.

Salmonella invasion 

This is mediated by internalised (injection) of toxins which induce extensive actin polymerisation. 

• SPI-1 is translocated
• SopE and Co. - extensive active polymerisation
• mature memebrane ruffles
• complete reversal actin - depolymersation (SptP)

Temporal reguulation of Salmonella virulence effector function

• antagonistic SopE and SptP effector are translocated into infeced cells - same time, same level
• their activities must be controlled since SopE must function before SptP

SopE contain internal sequence which targets it for Proteasome dependent protein degradation - SptP is in excess to overcome the effect of SopE. 

Zipper mechanism - associated with internalins 

Listeria monocytogene uptake is mediated by bacetrai ligand or host receptor interactions. This exclusive rely on activiation of signalling cascades initiated upon host receptor engagement. Delivery of virulence factor into the host. 

Internalins are multigene in Listeria. 

The multiple internalins established host and tissue tropisms during infection because InIA binds to E-cadherin - mediated infection by oral route in human. 

Advantages of bacteria that can invade host cells

• they can access deeper tissues
• the infected cell act as transportation for the bacteria to spread to another site of the host
• they can better access nutrients and are protected from complement an Ab
• persistance

3, nutrients acquistion and multiplication 

Gaining of nutrients is essential for multiplication. Siderophore is synthesised, a molecules which have high binding affinity to various forms of Fe

L. monocytogenes are well adapted to cytosolic growth. Their generation time is 40-60 minutes  compared to 40 minutes generation time in rich broth. Hpt are only found in pathogenic strain of L. monocytogenes which is essential for efficient replication in cytosol. Hpt is regulated by PrfA - a transcriptional factor that activates the virulence genes. 

4, Evade of host defence system

This contributes to persistance. The bacteria is prevented from detention by the host immune response. This is achieved by

• changing its surface appearance
• capusolation to avoid phagocytosis
• avoiding ost recognition
• avoiding to be killed following phagocytosis

It maniplates the host immune responsive 

Type IV pilus variations in Neisseria 

Phase variaton - random and reversible inter-conversion between on and off state of gene expression.

• slipped-strand miss-paring is due to poly-C-mediated phase variation of pilC turns of pilC production 
• pilC is the pilus tip adhesin but also requred for pilus biogenesis 
• in the absent of pilC, pilin subunit cannot assemble into pilin 
• the primary subunit (piIE) is produced as pro-form 
• during tis transportation to the surface, it can be process into two alternatives: 

> typical pili which polymerises into P~pili 

> Ps pili which is soluable secreted form of pili

When bateria changes its surface structure so that the host immune system cannot detect it until a new Ab that recognise it is synthesised - by means of changes in AA or chemical structure. 

The major mechanisms of antigenic variation of pilus occurs when sole pilin expression cassette (piIE) recombines with one of silent cassette (piIS) genomes. The piIS lack  a promoter and do not code for the N-terminal hydrophobic region in the present of piIE. The new variation of piIE polypeptide that differs in the C-terminal region are frequently generated. 

S. aureus and anti-phagocytosis 

It avoid the complement system: 

• mannose-binding lectin to array of carbohydrates groups on bacteria cell surface - lectin pathway is activated. 
• C3 convertase and the attachment of C3b on the bacteria cell surface - alternative pathway
• the Ab serum which recognises the cell-surface component  such as teichoic acid, peptidoglycan and surface proteins - classical pathway 

1, It is capsulated - nehe plasmin nutrophil receptor access to bind complement and Ab is compromised

2, Staphylokinase is sectreted - activates the plasmin serine protease which cleaves surface bound C3b and IgG

3, protein A on the surface of S. aureus binds to IgG by Fc receptor

4, extracellular fibrinogen-binding protein is seceted - binds to C3 --> this blocks the C3b from being deposited on the bacteria surface

5, biofilm growth - makes the bacetria resistance to phagocytosis

S. enterica Typhi immune evasion

Up-regualtes Vi capsule inhibits LPS recognition by TLR4

Down-regulates flagella to prevent activations of TLR5 

• Anti-phagocytosis and pro-inflammatory immune supressor
• plasmid-borne Yersinia T3SS is in anti-phagocytosis and immune supression 

Host cell mimicry

• Lipopolysaccharides (LPS) - this is essential for physical integrity and funtioning of the outer membrane. 
• The LPS contain oligosaccharides that mimic the host sugars - camouflage and serum resistance 

Intercelluar lifestyle of Salmonella and Chlamydia 

• Salmonella inhibit endocytic trafficking - SCV do not fuse with late endosomes ly lysosomes
• Avoids ROI and NOS dependent killing

• Delays apoptotic cell death
• Assemble meshwork of F-actin and microtubles around the SCV

Results: repliacation of intercelluar bacteria within SCVs - Salmonella containing vacuoles 

Chlamydia development cycle is T3SS dependent.

Legionella and T4SS --> Legionella containing vacuoles (LCVs) - this recruits host proteins involved in vesicles trafficking in proteins secretary pathway 

Manipulation of host immune responsiveness - the formation of granuloma during the infections of Mycobacterium tuberculosis.