Bacterial Pathogenicity

What is pathogenicity?

The ability of organism to cause disease

What is virulence?

The degree of harm caused by a microorganism

• Virulence depends on infectivity, invasiveness, and degree of damage. These all depend on its virulence factors

What are the virulence factors?

1. Adhesion

2. Invasion

3. Acquiring nutrients

4. Host defence

5. Toxins

How do you measure virulence?

Infectious Dose

• ID₅₀ = The dose to infect 50% of hosts

Lethal Dose

• LD₅₀ = The dose to kill 50% of hosts

Whether an infection leads to disease depends on…

• Virulence

• Number of bacteria

• Host resistance

What are the 2 methods of transmission?

1. Direct host-to-host transmission

2. Indirect host-to-host transmission - facilitated by living or inanimate objects

Which is a more common portal of entry; the skin or mucosal surfaces?

Mucosal surfaces are more favourable because they’re warm, moist, and have more nutrients

Skin could occur due to hair follicles, wounds, insect bites

Name the different direct transmission routes

• Respiratory route

• Body contact

  • STDs

  • Skin infections

  • Through damaged skin

• Faecal-oral route

  • Through contact or through gastrointestinal pathogens

• Body fluids

• Vertical transmission

  • Prenatal, perinatal, postnatal

  • Germline (through viral DNA)

What pathogens directly transmit via the respiratory route?

Mycobacterium tuberculosis

Influenza virus

What pathogens directly transmit via body contact?

Through damaged skin e.g. staphylococcus aureus

What pathogens directly transmit via faecal-oral route?

Gastrointestinal pathogens e.g. Salmonella enterica

In indirect transmission, there are vehicles, and vectors. Name the vehicles for indirect transmission:

• Soil

• Contaminated water e.g. Vibrio cholerae

• Contaminated food

• Fomites (inanimate objects) e.g. Clostridioides difficile

In indirect transmission, there are vehicles, and vectors. Name the vectors for indirect transmission:

• Warm blood animals

• Anopheles spp.

• Rat flea

Why is adhesion important for bacteria?

Bacteria must adhere to host cells or tissues to colonise

Bacteria must also survive in host environment and compete with normal microbiota

Adherence to surfaces depends on adhesins

What are the main types of bacterial adhesins?

Proteins and polysaccharides.

What are the protein types of bacterial adhesins?

Fimbrial proteins and other surface proteins.

What are the polysaccharide types of bacterial adhesins?

Capsule components and teichoic/lipoteichoic acid.

What do bacterial adhesins interact with, and what types of interactions occur?

They interact with host cell-surface receptors via protein–protein or protein–carbohydrate interactions.

Host receptors for bacterial adhesins could be:

• Membrane proteins

• Glycolipids

• Extracellular matrix proteins

What are the adhesive properties of E.coli?

• E.coli frequently cause UTIs

• P-pili / type I pili bind to sugar moieties of glycolipids on epithelial cells lining urinary tract

After adherence, how may pathogens invade further into tissues or cells?

Via:

• Extracellular invasion (barriers of tissues are broken down)

• Intracellular invasion (microbes penetrate cells and survive intracellularly)

How does extracellular invasion work?

• Allows access to niches in tissue that aid in proliferation and spreading

• Achieved through production of enzymes that:

  • attack extracellular matrix

  • degrade carbohydrate-protein complexes between cells

  • disrupt cell surface

How does intracellular invasion work?

Some pathogens penetrate cells and survive intracellularly

Others use it as a means of proliferation or spreading

• Phagocytic cells invaded through phagocytosis

• Non-phagocytic cells invaded using systems that induce a phagocytosis-like process

How do pathogens survive phagocytosis?

• Some reside in the phagolysosome

• Some reside in unfused phagosome (like Salmonella spp.)

• Some destroy or escape from the phagosome and live in the cytosol

How do pathogens invade non-phagocytic cells?

Bacterial proteins recruit host proteins to induce phagocytosis

A secretion system is used by some gram -ve bacteria (salmonella, pseudomonas)

Invasion proteins are injected

Host signalling is activated and actin is recruited

Draw a diagram of bacterial pathogen invading either extracellularly or intracellularly?

What are biofilms?

• Pathogens must find a niche that is optimal for growth and colonisation

• It often involves the formation of biofilms on biotic or abiotic surfaces

• Bacteria attach to the surface, grow, and become enveloped in matrix

What do biofilms do?

A biofilm protects from phagocytosis, antibiotics, and disinfectants

They have high bacterial density: production of virulence factors through quorum sensing (the biological process of cell-to-cell communication that allows bacteria to detect and respond to population density by changing gene expression)

What is the main nutrient that is limiting for bacteria?

Iron

• In aerobic conditions, iron is oxidised to its ferric form (FeIII) which has low solubility

• In the body, most iron is complexed to proteins, so is not freely available

How do bacteria uptake iron?

• Direct contact using cell surface proteins (e.g. transferrin binding protein, haemoglobin binding protein)

• By secreting small compounds (siderophores) with very high affinity for iron that capture iron from host proteins or the insoluble ferric salts

What are siderophores?

• Produced when concentration of iron is low

• Low MW, high affinity for iron

• Compete for free or bound iron

• Transport iron into cell

The defense mechanisms in the body are:

1. Physical barriers

2. Innate immune system

3. Adaptive immune system

How do pathogens evade host defence?

• Evading complementation

• Resisting phagocytosis

• Intracellular survival

• Evading host antibody response

How do pathogens evade complement (avoid being destroyed by the complement system - opsonisation, inflammation, direct killing)?

• The thick polysaccharide capsule prevents complement activation.

• The LPS O-antigen helps bacteria evade complement because its elongated O-antigen chains prevent complement activation

How do pathogens resist phagocytosis?

• Prevent effective contact with phagocyte e.g. biofilms, capsules or specific proteins

• Affect phagocyte migration - S. pyogenes peptidase cleaves complement factor C5a

• Destroy phagocytes - Using toxins

How do pathogens survive inside cells?

• Intracellular life protects from e.g. phagocytes, complement, antibodies, some antibiotics

• Some pathogens are phagocytosed but survive

  • survive phagolysosome

  • prevent formation of phagolysosome

  • destroy or escape from phagosome and live in cytosol

• Some pathogens invade non-phagocytic cells

How do pathogens evade the host antibody response?

They bind host proteins, making them appear less foreign. E.g. S. pyogenes M protein.

They also produce surface proteins which bind antibodies in the wrong orientation, preventing effective opsoninsation, e.g. S. aureus and S. pyogenes

Opsonisation vs Anti-opsonisation

What are the effects of microbial products?

They can cause host damage, induce inflammation, aid invasion, acquire nutrients and evade immune responses.

What are exotoxins, endotoxins and toxoids?

Exotoxins are actively secreted; endotoxins are structural and released during lysis; toxoids are inactive/low-activity toxins used in vaccines (e.g. tetanus).

How are exotoxins transmitted?

• Ingestion of pre-formed exotoxin

  • food poisoning (e.g. botulinum toxin)

• Colonisation followed by exotoxin production

  • infection of tissue followed by toxin production

  • damage can be local or toxin can spread though e.g. blood

What are the main characteristics of bacterial exotoxins?

Usually proteins and therefore heat-labile. They can be extremely potent, e.g. botulinum toxin.

What are the four main types of exotoxin and their effects?

• Neurotoxins → nervous system → paralysis/respiratory failure

• Enterotoxins → GI tract → diarrhoea/vomiting

• Cytotoxins → cell damage → necrosis/organ failure

• Superantigens → massive T-cell activation → fever/shock

What are superantigens?

• Produced by e.g. staphylococci, eg toxic shock syndrome toxin (TSST) and several other enterotoxins

• Much higher proportion of T cells respond as compared to normal antigens

• Corrupts immune system, leading to massive non-specific inflammatory response, tissue damage, and multi-organ failure

What is endotoxin and what are its effects?

Lipid A of LPS in Gram-negative bacteria; heat-stable and released during lysis.

Causes fever (pyrogenic), activates complement/clotting and can cause toxic shock.

Antibiotic treatment may lead to the release of LPS

How do bacteria regulate the expression of virulence factors?

They sense environmental conditions and alter gene expression.

Early: produce adhesins.

Later: produce exotoxins and reduce adhesin expression to evade host defence.

What is quorum sensing and how does it work?

It is cell–cell communication using an autoinducer (AI).

As cell density increases, the autoinducer accumulates and binds an R protein, activating transcription of specific genes.

There is a high concentration of autoinducers only at high cell density

Biofilms and quorum sensing

In biofilms there is a high cell density; biofilm formation often leads to expression of virulence factors through quorum sensing

So how is virulence regulated?

• Environmental signals

• Quorum sensing