Pathogenicity, Virulence and H. Pylori Virulence Factors

Pathogenicity and Virulence

• A pathogen is any organism that can cause disease.
• Opportunistic pathogens are part of the normal flora but cause disease when they access other tissue sites, or the host is immunocompromised.
• Pathogenicity is the ability of a pathogen to cause disease.
• Virulence is the degree of harm inflicted on the host.
• Virulence is a relative term.

H. Pylori Virulence Factors

• Flagella provides bacterial mobility and chemotaxis to colonize under the mucosa.
• Urease neutralizes gastric acid, causing gastric mucosal injury via ammonia production.
• Lipopolysaccharides adhere to host cells and cause inflammation.
• Outer proteins adhere to host cells.
• Exotoxins, specifically vacuolating toxin (vacA), cause gastric mucosal injury.
• Secretory enzymes like mucinase, protease, and lipase cause gastric mucosal injury.
• A Type IV secretion system injects effectors into host cells.
• Effectors like cagA cause actin remodeling, IL-8 induction, host cell growth, and apoptosis inhibition.
• H. Pylori has modified LPS and flagellin for immune evasion.

Infection

• Infectivity is the ability to create a discrete point of infection.
• Infectious dose 50 (ID50) is the number of pathogens needed to infect 50% of inoculated hosts.
• ID50 varies with the pathogen.
• Invasiveness is the ability to spread to adjacent tissues.

Lethal Dose and Cytopathology

• Lethal dose 50 (LD50) is the dose that kills 50% of experimental animals within a specified period.
• Cytopathology refers to cellular changes that can be observed in tissue culture to determine death rates, rather than using entire organisms.

Extracellular vs. Intracellular Pathogens

• A pathogen must contact a host and survive within it to cause disease.
• Extracellular pathogens grow outside host cells in tissues and fluids.
• Intracellular pathogens grow and multiply within host cells.
• Facultative intracellular pathogens reside within host cells or in the environment and can grow and replicate in pure culture without host cell support.
• Obligate intracellular pathogens only replicate when inside host cells.

Importance of Resources and Evolution of Pathogens

• The host provides the pathogen with protection, nutrients, and energy.
• Infectious agents evolve mechanisms to access and exploit hosts and move to new locations or hosts when necessary.

The Coincidental Evolution Hypothesis

• This perspective suggests that virulence is not always a direct adaptation against the host.
• Virulence can be a side effect of microbes evolving to thrive in harsh environments or to fend off other microbes.
• Examples: Streptococcus pneumoniae vs. Hemophilus influenzae and Vibrio cholerae.

Events in Infection and Disease

• Transmission occurs from a previous host or reservoir to a new host.
• The virulence of the organism, the number of invading organisms, and the presence of adhesion and invasion factors affect success.
• An organism outcompetes the resident microbiota for resources and survives host defense mechanisms.
• Disease occurs when the organism produces molecules that directly damage host cells and/or stimulates host immune cells to destroy infected tissue.

Course of Infectious Disease

• Incubation period: The period after pathogen entry, before signs and symptoms appear.
• Prodromal stage: Onset of signs and symptoms, but not clear enough for diagnosis.
• Illness period: The disease is most severe, with apparent signs and symptoms.
• Convalescence: Signs and symptoms begin to disappear.

Transmission of Infectious Diseases

• Direct and indirect are methods of transmission.

Transmission and Virulence

• A pathogen's virulence can be influenced by its mode of transmission and its ability to survive outside the host.
• Transmission alone is not enough for infection to occur.
• Tropism requires a pathogen to contact appropriate host tissue, determined by specific cell surface receptors.

Adherence and Colonization

• Adherence structures bind complementary receptor sites on the host cell surfaces.
• These structures are classified as virulence factors.
• Examples include pili, fimbriae, membrane and capsular materials, and specialized adhesion molecules.

Invasion

• Penetration into the host tissues can be active or passive.
• Passive penetration is unrelated to the pathogen. (i.e. skin lesions, insect bites)
• Active penetration happens through lytic substances that alter host tissue.

Staphylococci Virulence Factors

• Hemolysins: Alpha hemolysin destroys erythrocytes and causes skin destruction; Beta hemolysin destroys erythrocytes and sphingomyelin around nerves.
• Hyaluronidase: Spreading factor that breaks down hyaluronic acid between cells, allowing for penetration and spread of bacteria.
• Lipases: Break down lipids.
• Panton-Valentine leukocidin: Inhibits phagocytosis by granulocytes and can destroy these cells by forming pores in their phagosomal membranes.
• Phenol soluble modulins: contribute to biofilm formation and survival within phagosome.
• Protein A is antiphagocytic and competes with with neutrophils for the Fc portion of specific antibodies
• Proteases: Break down proteins.
• Toxic shock syndrome toxin-1 (a superantigen): Associated with fever, shock, and multisystem involvement of toxic shock syndrome.

Microbial Adherence Mechanisms

• Adherence structures bind complementary receptor sites on host cell surfaces and are classified as virulence factors. -Pili -Fimbriae -Membrane and capsular materials. -Specialized adhesion molecules on bacterium's cell surface.

Strategies to Evade Host Immune Response

• Produce decoy proteins to bind available neutralizing antibodies.
• Change cell surface proteins by mutation, recombination, or downregulation.
• Produce capsules that resemble host tissue components.
• Produce proteases that degrade host proteins.
• Produce special proteins that interfere with the host's ability to detect and remove them.
• Cause host cell fusion.

Biofilms

• Biofilm bacteria are protected from nutrient deprivation, predators, environmental shifts, antimicrobial agents, and host immune cells.
• Pathogenic bacteria within biofilms exchange plasmids, nutrients, and quorum-sensing molecules, altering their behavior.
• Biofilms makes bacteria less sensitive to antibiotics and more resistant to host defense mechanisms.

Slime Producers

• Produced by pathogenic strains of Staphylococcus.
• Slime is a viscous extracellular glycoconjugate.
• Slime allows bacteria to adhere to smooth surfaces and form biofilms.
• Slime inhibits neutrophil chemotaxis, phagocytosis, and other immune functions.

Exotoxins

• Exotoxins are soluble, heat-labile proteins.
• They are secreted into surroundings as the pathogen metabolizes.
• Exotoxins often travel from the site of infection to other tissues or cells where they exert their effects.
• These are usually synthesized by specific bacteria that have toxin genes in their plasmids or prophage DNA.
• Typically induced by quorum sensing.

Types of Exotoxins

• AB toxins have two subunits: an A subunit (responsible for toxic effect) and a B subunit (binds to specific target cell).
  • Examples: diphtheria, cholera, botulism, pertussis.
• Specific host site exotoxins (e.g., enterotoxins like cholera).
• Membrane-disrupting exotoxins (e.g., EPEC).

Diphtheria Exotoxin Mechanism

• Caused by Corynebacterium diphtheriae.
• Gram-positive bacterium.
• Exotoxin inhibits protein synthesis and is responsible for pathogenesis.
• Resistant to drying.
• Airborne transmission via nasopharyngeal secretions.
• Diphtheria vaccine is a toxoid (inactivated toxin).

Pathogenic E. coli Strains

• Enterotoxic E. coli (ETEC) produces one or both enterotoxins responsible for diarrhea and is distinguished by its heat stability.
• Enteroinvasive E. coli (EIEC) multiplies within intestinal epithelial cells and produces an enterotoxin.
• Enteropathogenic E. coli (EPEC) causes effacing lesions by destroying brush border microvilli on intestinal epithelial cells.

Superantigens

• These stimulate about 30% of T cells of the immune system.
• This causes the T cells to overexpress and release pro-inflammatory cytokines.
• It results in failure of multiple host organs, allowing time for the microbe to disseminate.
• Staphylococcus aureus exfoliative toxins A and B cause loss of surface layers of the skin in scalded-skin syndrome.

Methicillin-Resistant Staphylococcus aureus (MRSA)

• S. aureus isolates resistant to β-lactam antibiotics (penicillins and cephalosporins).
• Community-acquired (CA)-MRSA affects healthy individuals not recently hospitalized.
• It is associated with serious and fatal infection, but may also be acquired in a health-care setting (nosocomial infection).

Endotoxin – Lipopolysaccharide

• Lipopolysaccharide (LPS) is in Gram-negative cell walls and can be toxic to specific hosts.
• It is called endotoxin because it is endogenous, bound to the bacterium, and released when the microorganism lyses.
• The toxic component is the lipid portion, lipid A.
• Endotoxic septic shock manifests as fever, shock, weakness, diarrhea, inflammation, and intestinal hemorrhage, and can be fatal.

Endotoxin Testing

• The presence of endotoxin is detected by the Limulus amoebocyte lysate (LAL) assay.