BMS2052 - W2: Bacterial Pathogenesis

Define colonisation, infection and disease

Colonisation: presence of a microorganism on/in host, with multiplication of the organism but not necessarily exerting an impact on the host.

Infection: colonisation by an infectious agent. This has the potential to develop into a disease

Disease: clinical signs and symptoms, pathology

Factors that influence infection and disease

• pathogen factors (e.g. virulence factors)

• host factors (e.g. immune function, pregnancy)

• environmental factors (e.g. water quality)

Name the types of microbes from good to bad

commensals, opportunist, pathogens

Pathogens - what are virulence factors?

Virulence factors: Substances, products, appendages or strategies that are essential for survival, infection and pathogenicity

What are the 5 steps that most pathogens must do to cause disease?

1. enter the body (transmission)

2. colonise the host

3. evade hose defenses

4. multiply and disseminate

5. cause damage to the host

How can virulence factors be acquiared?

• mutation

• horizontal gene transfer

Horizontal gene transfer

Horizontal gene transfer helps bacteria adapt

Recipient bacteria becomes a transformant, transductant or transconjugant

Transformation

• uptake of foreign DNA

Transduction

• bacteriophage can inject its genetic info into bacteria which can be incorporated in bacterial chromosome

• bacteriophage can take up genetic info from a bacteria and inject into another bacteria (transconjugant)

Conjugation

• plasmids can transfer from one bacteria to another

What are plasmids

Bacteria usually have bacterial DNA and one or more plasmids

Plasmid:

• Extrachromosomal double stranded circular DNA

• Self-replicating

• many plasmids are associated with virulence as virulence factors can be encoded on these plasmids

• can be transferred between bacteria (conjugation)

Conjugation

Transduction

Lysogenic Phage: Integrates into the bacterial genome (prophage)

Lytic phage: Infects and rapidly kills the bacterium through it’s replication

• basically turns the bacteria into a replication machine to create more bacteriophages

• bacterial DNA can accidentally get incorporated into some of the bacteriophages produced - can inject bacterial DNA into another bacteria (transduction)

Transposons

Fragment of DNA that can move around, in and out of a bacterial chromosome, plasmids and bacteriophages

• Can transfer antibiotic resistance genes, toxin encoding genes and other virulence factors

Pathogenicity islands

Pathogenicity islands are elements that have been horizontally acquired, are stable (now are just part of the chromosomes)

• on a bacterial chromosome that contains genes encoding for virulence factors

• pathogens can have more 1 pathogenicity island

• permanently integrated

• Can be detected by the different G+C ratio of the DNA = different origin to the rest of the genome

Genes from mobile elements can accumulate in gene clusters. These clusters of genes on chromosomes can work together to mediate virulence = pathogenicity islands

Regulation of virulence

Virulence factors are not being made all the time (waste of energy) as they are only required at specific times and in specific locations

• bacteria detect external signals to determine when to express virulence factors

• essential for energy conservation

• can be switched on/off (e.g. flagella, capsule production)

What factors influence successful infection?

• Portal of entry (need to be at the correct niche)

• host specificity (some bacteria can infect many animal types, while others cannot)

• bacterial load (number of bacteria entering the body)

colonisation

Pathogens (or normal microbiota) establish themselves in a host

• must first overcome physical and innate immune barriers

• must outcompete normal flora

Adhesion - needed for colonisation

1. contact with host cell (non specific electrostatic attraction)

2. binding to host cell (surface interactions with host molecules, mediated by virulence factors termed adhesins)

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Adhesins:

• fimbrial adhesins (long extensions that interest with host cell surface)

  • functions: attachment to surfaces, motility, DNA transfer, biofilm formation

  • fimbriae - thinner and shorter fibers than pili

  • adhesion via tip of pilus

• afimbrial adhesins (membrane proteins that interact with components on surface of host cell)

  • establishes very close and strong adhesion with host cell

• capsule

  • polymeric structure surrounding the cell wall

  • composed of polysaccharides

  • protects bacterial cell, facilitates adhesion (to each other and other surfaces)

• different adhesin types (which respond to different molecules) contributes to pathogen’s ability to colonise different niches (e.i. bladder vs kidney) and animals.

Invasion: What is it?

What does invasion mean?

• to disseminate to deeper tissues or invade a previously uncolosnised site/ tissue

• to invade a particular cell

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Advantages:

• avoid immune recognition

Disadvantages:

• cell autonomous defense

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Obligate intracellular bacteria: Needs that condition to survive

Facultative intracellular bacteria: does not necessarily need the condition to survive

Invasion mechanisms: How do bacterial pathogens get inside our cells?

Zipper:

• exploits host cell pathways normally used for adhesion

• induces receptor immobilisation

• bacteria is engulfed (similarly to endocytosis)

• very close interaction with host cell surface

e.g. Listeria monocytogenes

• gram +ve, food-borne pathogen

• internalisation requires the actin cytoskeleton and bacterial protein Internalin A (an invasin) → interacts with E-cadherin which normally forms tight junctions, but Internalin A causes clustering of these E-cadherins to facilitate entry

Trigger:

• via secretion system, pathogens inject effector proteins into cells to manipulate cell cytoskeleton → ruffling

e.g. Salmonella

• mediated by a type II secretion system. Needle like structure injects into the host cell

How can pathogens overcome the innate immune system?

• evade phagocytosis

• resist killing

Phagocytosis:

Phagocytes destroy microbes through ‘cellular eating’

• can recognise pathogens through pattern recognition receptors (recognise elements on the bacteria), C3b receptor (component of compliment), Fc receptor (arm of an antibody sticking out on the bacterium)

Strategies to evade phagocytosis: Extracellular pathogens

Direct evasion of phagocytosis

• kill phagocytes that come in contact

  • bacterial cell scan produce proteins that interact with specific components on phagocyte surface → create pore in the phagocyte surface → destruction of phagocyte

• Capsules

  • can resemble host components (things that are usually found in human body) → not recognised by immune system

  • can mask underlying structures (hides the components like PAMPS which are recognised by immune cells) → avoid complement binding, recognition → prevent phagocytosis

Interfering with opsonins (substances that bind to microorganisms to facilitate phagocytosis e.g. antibodies, complement, mannose-binding protein/lectin) - bacteria can prevent themselves from being coated with opsonins

• bacterial surface proteins can bind to C4BP or FH, leading to degradation of complement components

• bacterial proteins can inactivate C3

• secreted bacterial proteins can degrade complement proteins

• Bacterial surface proteins that bind to the Fc region of antibodies (causing the antibody to bind in the wrong direction than it’s supposed to) can prevent complement binding = block phagocytosis

Resist Killing: intracellular pathogens

Resist killing by phagocytes

Once inside a phagocyte/ epithelial cells they can:

1. get out of the phagsome/ vacuole into the cytosol

2. prevent trafficking of vacuole to lysosomal niche (redirect to diff environment)

3. able to survive in lysosome niche

Overcoming adaptive immunity

• direct immunosuppression

• expression of weak antigens

• antigen modification

• antigen diversity

Multiplication and dissemination:

What do bacteria need to do to be able to multiply and disseminate?

What do bacteria need to do to be able to multiple and disseminate?

• overcome immune system

• outcompete normal microbiota

• nutrients for cell division, growth, dissemination

Example of nutrient acquisition:

Iron acquisition strategies:

• siderophores

  • excreted into extracellular environment

  • can interact with iron at very high affinity, forming complex

  • complex interacts with receptor on surface of bacteria → internalised

• express receptors for host iron capture proteins (capture proteins which capture iron for host)

• express high affinity iron binding receptor for haemoglobin (capture haemoglobin)

• express toxins to release host iron in local environment which can be taken up by bacteria

Tissue damage:

• Direct toxicity - mediated by the bacterial toxins

• Induction of cytokines (immune mediated)

• Induction of immunopathology (immune mediated)

Bacterial toxins

Directly mediate damage to humans

2 main categories:

• Exotoxins - secreted out of the bacteria or released by bacterial lysis

• Endotoxins - within the bacteria, usually cell wall components

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Endotoxins:

• within the bacteria, usually cell wall components

• can be released into local environment when a bacterial cell dies and cell wall breaks down

• can be produced by gram positive and gram negative bacteria

• e.g. LPS, PG, LTA

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Exotoxins:

• secreted out of the bacteria or released by bacterial lysis

• can be produced by gram positive and gram negative bacteria

• usually produced by extracellular pathogens rather than intracellular pathogens (as bacterial do not want to kill its host)

Can be classified according to:

• site of action (where the toxin acts)

  • intracellular

  • extracellular (toxin has to be taken up into the cell)

• Tissue specificity

  • Enterotoxin (gut)

  • neurotoxin (nerve tissue)

  • cardiotoxin (heart)

Can be further classified as:

• Heat-liable toxins (proteins) - can be denatured (inactived) by heat

  • Type I - superantigens (act to overstimulate our immune system)

    • causes overactivation of nonspecific T-cell production which can send the body into shock and lead to organ failure (VERY TOXIC!!)

  • Type II - membrane disrupting toxins (act on cell membrane)

    • damage cell membrane causing cell lysis

    • can create pores (loss of nutrients, etc. → cell death), cleave phospholipid bilayer → lysis/ cell death

  • Type II - A-B toxins (have an active component + binding component that binds to a receptor on host cell for interalisation)

    • gets internalised into host cells

    • usually made up on an A and B subunit (can have more subunits)

    • causes internal cellular function being disrupted, often leading to cell death

• heat-stable toxins (non-proteins) - cannot be denatured (inactivated) by heat