Microbiology: L20-21 - Human-Microbe Interactions & Pathogenic Bacteriology

What are the 4 mechanisms of bacterial pathogenesis?

• Colonization

• Physical damage due to bacterial growth

• Toxins

• Inflammation or immune-mediated damage

Pathogenesis

The process by which bacteria cause disease in a host, involving mechanisms like colonization, toxin production, & immune evasion.

Adherence

The ability of bacteria to attach to host cells or surfaces, which is crucial for colonization & infection

Non-covalent adherence factors

Molecular interactions that enable bacteria to attach to host surfaces without forming covalent bonds, often involving fimbriae, pili, & other surface structures.

Tiissue Tropism

The preference of certain pathogens to infect specific tissues or organs within a host

What mediates specificity?

Adhesins & receptors

Sites of microbial shedding:

Locations where microbes are expelled from the host, such as respiratory secretions, feces, or skin.

• this helps in the transmission of pathogens

Virulence Factors

Molecules produced by pathogens that enhance their ability to cause disease, including toxins, adhesion factors, and invasion factors.

Examples of virulence factors

Include exotoxins, endotoxins, & capsules

LD50

The dose of a toxin or pathogen that is lethal to 50% of a population, used to measure its potency.

How can the LD50 test be used to define the virulence of a pathogen?

The LD50 test quantifies the potency of a pathogen by determining the amount required to kill half of a test population, allowing comparisons of virulence across different microorganisms.

Attenuation

The process of reducing the virulence of a pathogen, making it less capable of causing disease, often used in vaccine development.

What circumstances might contribute to the attenuation of a pathogen?

Include genetic mutations, prolonged culture in non-host environments, & vaccination against the pathogen

Exotoxins

Proteins secreted by bacteria that cause damage to host cells & tissues through various mechanisms

• include:

  • A-B toxins

  • Superantigens

  • Cytolytic proteins

• come from external sources

• heat sensitive (labile)

• usually relies on enzymatic activity

• relatively high potency

• high degree of specificity

Why do exotoxins typically have enzymatic activity?

Bc they are proteins & function to catalyze biochemical reactions that disrupt cellular processes

Why do endotoxins not have any enzymatic activity?

Bc they are lipopolysaccharides found in the outer membrane of Gram-negative bacteria, which do not catalyze biochemical reactions.

Why are endotoxins have relatively little potency?

Bc they are part of the bacterial cell structure & are released only when bacteria die, leading to a less intense immune response compared to exotoxins

Why are exotoxins deadlier in smaller amounts relative to endotoxins?

Bc they are highly potent proteins that can elicit a strong immune response even at low concentrations

Pyrogenicity

The ability of a substance to induce fever

Why do endotoxins cause fever?

Bc they triggering the release of pyrogens from immune cells, which then stimulate the hypothalamus to raise body temperature as part of the immune response.

Why do exotoxins only occasionally produce fever?

Bc they do not consistently trigger the release of pyrogens, making fever a less common response.

Endotoxins

Components of the outer membrane of Gram-negative bacteria, primarily lipopolysaccharides, that trigger strong immune responses & can lead to severe inflammation & shock

• extremely heat stable

• no enzymatic activity

• weakly toxic & rarely fatal (low potency)

• low degree of specificity

A-B Toxins

A type of exotoxin consisting of two subunits, where one binds to the target cell and the other delivers a toxic effect

• include:

  • enterotoxins

  • diphtheria

  • botulinum

  • tetanus

Primary role of B-subunit of A-B toxins:

Mediation of high-affinity binding to specific cell surface receptors & entry into the cell

• often glycolipids or proteins

Primary role of the A-subunit of A-B toxins:

Delivery of the toxic effect into the target cell, often through enzymatic activity

• once inside the cell, it will begin to modify critical cell targets

Glycocalyx

A protective layer surrounding some bacterial cells, composed of polysaccharides & proteins, which aids in adhesion and immune evasion.

Superantigens

A class of exotoxins that cause excessive stimulation of T-cells, leading to an over exaggerated immune response and potential toxic shock

• associated with diseases like toxic shock syndrome & systemic inflammation

Cytolytic Proteins

A type of exotoxin that damage the host cell membrane, leading to cell lysis and death, often playing a role in bacterial virulence

• use proteases, lipases, etc to break down cellular components

Exotoxin Transport Mechanisms

• 1) General mechanism of A-B toxin binding & uptake

• 2) Receptor mediated endocytosis of diphtheria toxin

Enterotoxins

A type of A-B toxin that specifically affects the cells of the intestinal mucosa, causing diarrhea, vomiting, & significant loss of water from the body

• multiple types, but cholera is a prominent example

Diphtheria Toxin

A potent A-B toxin produced by the bacterium Corynebacterium diphtheriae, which inhibits protein synthesis in host cells leading to cell death and tissue damage

• characterized by sore throat, fever, & swollen lymph nodes

Cholera Toxin

A potent enterotoxin produced by the bacterium Vibrio cholerae, causing severe diarrhea & dehydration by disrupting ion transport in intestinal cells

• poses extremely high risk of dehydration

• characterized severe weakness

What do the A & B subunits of cholera toxin do?

A-subunit:

• activates G_aS receptors, locking AC in an active state

B-subunit:

• binds to GM1gangliosides on the intestinal epithelial cells

How does cholera induce massive water & salt loss?

Through the activation of epithelial AC by cholera toxin (B)

• this blocks Na+ movement into the cell & induces Cl- movement to the lumen

• the movement of these ions to the lumen ‘draws’ water out from the intestine

Individuals w/ cholera can lose up to how much water/day?

~ 5 liters

Botulinum Toxin

A rare but serious illness caused by a toxin produced by the bacterium Clostridium botulinum

• classified as an A-B toxin

• extremely potent

• effects stimulatory ACh neurons, blocking the its release

• produces a ‘flaccid’, non-contracted state

Uses of botulinum as a treatment:

• Dystonia (excessive muscular twitching)

• Chronic cramping

• Strabismus (cross-eyed-ness)

• Blepharospasm (uncontrollable blinking)

• Hyperhidrosis

• Plastic surgery (Botox)

Why the primary cause of botulism differ between adults & young children?

Due to the differences in gut microbiota and development;

• infants are at risk due to the immaturity of their intestinal flora, allowing C. botulinum spores to thrive.

Typical causes of botulism in adults:

• Improperly canned food

• Honey → which may contain possibly infectious spores

Tetanus Toxin

A neurotoxin produced by Clostridium tetani, which blocks the release of inhibitory NTs, notably GABA & glycine, across the synaptic cleft

• causes muscle stiffness & spasms by blocking neurotransmitter release → effective the ‘opposite’ of botulism

What can be used in the treatment of tetanus?

Botulinum toxin, due to its ability to induce muscle relaxation

What serves as the target for botulinum toxin?

Efferent motor neurons

What do the A & B subunits of tetanus toxin do?

A- subunit:

• cleaves synaptobrevin (VAMP), blocking GABA/glycine release → uncontrolled muscle spasms/contraction

B-subunit:

• binds GD2 gangliosides on motor neurons, undergoing retrograde transport to the CNS

General Tetanus

Most common form of tetanus (~ 80 % of cases)

• characterized by muscle stiffness and spasms, primarily affecting the jaw & neck

• spasms triggered by external stimuli such as noise or touch, leading to lockjaw (trismus) & potential respiratory failure

Cephalic Tetanus

A less common form of tetanus characterized by cranial nerve involvement, leading to symptoms such as facial muscle spasms, difficulty swallowing, & altered mental status

• is often associated with wounds to the head, ear infections, or dental procedures

• characterized by cranial nerve palsy & flaccid paralysis

Local Tetanus

A rare form of tetanus involving localized, persistent muscle spasms & stiffness at sites of injury, without generalized symptoms

• typically occurs in response to localized contamination of a wound

• may resolve or progress to generalized tetanus

Neonatal Tetanus

A severe form of tetanus occurring in newborns, typically due to unsterile umbilical cord cutting or improper care of umbilical stumps

• leads to muscle rigidity & spasms, significantly increasing mortality rates in affected infants

T-cells

A type of white blood cell that plays a central role in the immune response, including the identification & destruction of infected or cancerous cells

Antigen

A molecule or molecular structure that is recognized by the immune system, particularly by antibodies or T-cells, & which can elicit an immune response.

Antigen Presenting Cells (APCs)

Cells that process & present antigens to T-cells, initiating an immune response

• include dendritic cells, macrophages, and B cells.

MHCII Molecules

Molecules that present antigens to CD4+ T-cells, crucial for initiating helper T-cell responses

Cytokines

Small proteins that mediate communication between immune cells, influencing their activities and responses in immune reactions.

Phagocytes

Cells that engulf & digest pathogens + cellular debris, playing a key role in the immune response

• part of the innate response

• pathogens are engulfed in vesicles called phagosomes

• can present antigens to T-cells to activate the adaptive response

How do superantigens over-activate the immune system?

By binding directly to MHCII molecules and T-cell receptors, leading to widespread activation & excessive cytokine release

Lipopolysaccharides (LPS)

Components of the outer membrane of Gram-negative bacteria, recognized by the immune system

• can trigger strong inflammatory responses.

Why do gram-positive bacteria never produce endotoxins?

Bc they lack an outer membrane & lipopolysaccharides, which are necessary for endotoxin production.

Host Specificity

Refers to the preference of certain pathogens to infect specific host species or types of cells

• influenced by factors such as receptors &immune responses.

• for a given pathogen, rates of susceptibility may vary among hosts

Why might susceptibility vary among hosts?

Due to differences in immune responses, genetic variations, & the presence of specific receptors that pathogens recognize & exploit

What are the most potent biological toxins?

The exotoxins produced by microorganisms

• each exotoxin affects specific host cells, causing specific impairment of a major host cell function

What are the key features shared by all exotoxins?

• All are secreted

• Made of protein

• Heat labile

• High potency & specificity of action

• High immunogenic potential

What makes A-B toxins unique among exotoxins?

• 2-component structure

• Intracellular mechanism of action

• Receptor-mediated specificity

Are bacterial growth & infections in the host always necessary for the production of toxins?

No, toxins can be produced by bacteria even in the absence of infection, as they may be secreted into the environment or host tissues

• toxins can also be consumed

What are the 2 types of host defense mechanism?

• Constitutive (Innate) immunity

• Inducible (Adaptive) immunity

Constitutive Defenses

General, non-specific, mechanism common to healthy individuals

• aka ‘innate defense’

• present from birth

• does not require prior exposure to pathogens

Inducible Defense

Mechanisms that are ‘activated’ through exposure to a pathogen & involve immune system-mediated defenses

• Ex) antibody production

Access Environment

The environment accessible to microbes for survival & interaction with the host, influencing their pathogenicity and immune response.

Compatible Environment

An environment that supports microbial growth & reproduction while minimizing host defense responses,

• facilitates pathogenic interactions.

Examples of constitutive/non-specific defenses:

• Skin

• Lysozyme in tears + other secretions that dissolve cell walls

• Flushing of the urinary tract

• Mucus & cilia in the upper respiratory tract

• Mucus & phagocytes in the lungs

• Cilia in the nasopharynx

How is skin an innate physical barrier?

By providing a tough, protective layer that prevents pathogen entry, while also secreting antimicrobial substances & facilitating immune responses to potential infections

• produces antimicrobial fatty acids

How does the flushing of the urinary tract serve as an innate defense mechanism?

Facilitates the removal of potential pathogens from the urinary system, thereby reducing the risk of infection.

How does a rapid pH change in the digestive tract serve as an innate defense mechanism?

It creates an unfavorable environment for many pathogens, inhibiting their growth and survival.

How can the normal flora w/in the lumen of a given area provide protection against infection?

Competes with pathogens for resources and attachment sites, produces antimicrobial substances, & helps to stimulate the host's immune response

• compete for space & resources

If the normal flora of the lumen is compromised, what can happen?

Pathogens can proliferate, leading to increased risk of infection & disease

Host factors in bacterial infection:

• Age

• Stress

• Diet

• Hygiene

Compromised Hosts

Individuals with weakened immune systems due to factors like illness, age, or stress, making them more susceptible to infections.

Example of compromised hosts w/in the hospital:

• Patient pre/post surgery

• Cancer patients

• Transplant patients

• Patient taking anti-inflammatory medications

Examples of compromised hosts outside the hospital:

• Smokers, heavy drinkers & IV drug users

• Individuals w/ persistently poor sleep & nutrition

• Individuals dealing w/ chronic infections (AIDS/HIV patients)

What forms when a phagosome fuses w/ a lysosome?

Phagolysosome

Phagosome

A membrane-bound vesicle that engulfs pathogens or particles during the process of phagocytosis, which then fuses with a lysosome to form a phagolysosome.

Phagolysosome:

A structure formed when a phagosome fuses with a lysosome, containing enzymes that digest engulfed pathogens.

Pathogen Associated Molecular Patterns (PAMPs)

Molecules associated with groups of pathogens & which are recognized by the immune system as ‘non-self’, triggering an immune response

• ‘small, conserved molecular motifs’

Pattern Recognition Molecule (PRMs)

Proteins that recognize PAMPs on pathogens, playing a crucial role in the immune system's ability to detect & respond to infections.

Antigen Processing

The process by which antigens are broken down into peptide fragments & presented on the surface of antigen-presenting cells (APCs) for recognition by T cells.

Antigen-Specific, Antibody-Mediated Immunity

The immune response that involves the production of antibodies specifically targeted to antigens

• primarily elicited by B cells.

B-cells

A type of white blood cell that plays a key role in the immune response by producing antibodies

• essential for humoral immunity, recognizing specific antigens & differentiating into plasma cells.

Cell-Mediated Immunity

A type of immune response that does not involve antibodies but instead relies on the activation of T-cells to combat intracellular pathogens & tumor cells.

Antigen Destruction

The process by which the immune system eliminates antigens through various mechanisms, including phagocytosis and cytotoxicity.

Means of escape from host defenses:

• Capsules

• Intracellular growth

• Antigenic growth

• Inhibition of immune system activity

• Degradative enzymes

How do capsules aid in the avoidance of host defenses?

• Mimicry (of host molecule)

• Phagocytic evasion

• Protection in phagocytes

Antigenic Activation

The stimulation of the immune response by antigens, leading to the production of antibodies and activation of immune cells.

How does the inhibition of immune system activation aid in the avoidance of host defenses?

Prevention of complement activation & phagolysosome fusion

How might a pre-existing infection compromise an otherwise healthy host?

May preoccupy resources that would otherwise be used to prevent a second infection

Antigen Binding Site

The specific region on an antibody or antigen that interacts with an antigen, crucial for immune recognition and response.

Light Chain

A component of antibodies consisting of two identical proteins that pair with heavy chains, contributing to the antigen-binding sites

Heavy Chain

The larger polypeptide chain in an antibody that contributes to its structure & function, paired with light chains to form an immunoglobulin molecule.

How do vaccines work?

By stimulating the host immune system, directing it toward a pathogen or toxin

• dependent upon on the immune response of the host

Primary Immune Response

The initial immune response triggered upon first exposure to a specific antigen, characterized by a slower onset & the production of antibodies and memory cells.

Secondary Immune Response

The more rapid and robust immune response that occurs upon subsequent exposures to the same antigen, involving a faster production of antibodies and effector cells.

Killed Bacteria/Inactivated Virions

Microbial agents that have been killed or inactivated to stimulate an immune response without causing disease

Attenuated Strains (bacterial or viral strains)

Live microbial agents that have been weakened to stimulate an immune response without causing disease.

• non-pathogenic strains