cellular and molecular mechanism of bacterial and viral virulence

Cellular and Molecular Mechanism of Bacterial and Viral Virulence

dr. Tristia Rinanda, M.Si Bagian Mikrobiologi Fakultas Kedokteran UNSYIAH

Terminology Pathogen

= A microorganism capable of causing diseases

Opportunistic pathogen =

An agent capable of causing disease only when the host’s resistance is impaired

Non pathogen =

A microorganism that does not cause disease

Terminology Infection

= Multiplication of an infectious agent within the

body

Pathogenicity =

The ability of an infectious agent to cause disease Virulence

= The quantitative ability of an agent to cause

disease/degree of pathogenicity

VIRAL VIRULENCE

Viral Pathogenesis

Sum of the effects on the host due to virus replication and host’s immune system Involving several steps/factors: Viral dissemination in host (entry and primary

replication, spread, tropism, shedding and transmission) Viral virulence Injury induced by viruses Host determinant (host susceptibility)

Iceberg Concept of Viral Infection

Viral Dissemination Viral attachment and entry Replicate at the primary site of

entry (local spread) Systemic spread Presence of virus in a blood

viremia Viral attachment and spread

the role of tissue tropism Shedding: To maintain a viral infections in

population of host Usually occurs from the body

surface = viral entry Ability to survive in environment

Local Infection

Systemic Infection

Mechanism of spread

Hematogenous VS Neuronal

Viral genetic determinant Divided into 4 classes: Ability to replicate Ability to modify host’s immune system Ability to spread in the host and among host Produce directly toxic genes products

Viral virulence

Ability to replicate: Exhibit reduced or no replication in cell host or many cultured cell types Non coding sequences HCV, poliovirus

Viral virulence

Ability to modify host’s immune system Virokin secreted proteins that mimic cytokines, growth factors or extracellular immune regulator Viroreceptor homolog of host receptors or cell surface immune molecules

Apoptosis Virus akan menghambat apoptosis Direct Indirect

Viral virulence

Ability to spread in the host Specific genes encoding specific proteins specific recognition to host cells specific pathogenesis/virulence Example: reovirus type 1 dan 3, bunyavirus (La Cross virus) plaques

Viral virulence

Toxic viral proteins NSP4 Proteins from Rotavirus Plays important role on budding proccess toxic to host cellsintestinal mucosa increasing chloride secretion

Viral virulence

Injury induced by viruses

Caused temporary and permanent damage due to several process: Direct effects to virus replication Consequences of host adaptive and innate

immune system or both

Direct Effects by cytolytic virus Increased permeability of cell membranea

leakage Fused cell syncytia (paramyxovirus) Vacuolization, polyokaryocytosis, inclusion bodies herpesvirus

Pathogenic effects of non cytolytic virus Virus doesn’t destroy the cell, but reduced the

growth hormones (lymphocytic choriomeningitis virus)

Injury induced by viruses

Indirect Effect of viral infection Cellular response damaged due to proteolytic

enzymes, reactive radicals and cytokines Immunopathological lesion associated with T

cells (CD8+ and CD4+) and B cells Autoimmune disease molecular mimicry Ex. Herpes simplex infection caused stromal keratitis

(UL6 similar to IgG2)

Injury induced by viruses

Host genetic determinant Responsible genes for immune system Determining host susceptibility to viral disease Genetic determinant polymorphism in MHC1 dan

MHC II Host proteins involve in replication, transcription and

translation Host cell receptor

Non genetic determinant age, sex, pregnancy, malnutrition, cigarette, temperature, mental status

Host Determinant

BACTERIAL VIRULENCE

Basic concept identifying the cause of infectious diseases

Koch’s postulate molecular Koch’s postulates

Host-pathogen interaction Virulence factors vaccines

development

Terminology

Identifying bacteria that cause disease

Koch’s Postulates molecular Koch’s Postulates molecular guidelines Koch’s postulates : classification of bacteria as

pathogens, non pathogens and opportunistic pathogens

Important stages of Bacterial Pathogenesis

Transmission site of entry, vectors Infection process Virulence factors Regulation of virulence factors

Transmission

Asymptomatic infection enhancing the possibility of transmission Some bacteria cause disease in human, exist primarily in animal, incidentally infect human Clinical manifestation transmission routes Most frequent portals of entry mucous membranes meet the skin

The infectious process

Bacteria attach /adhere to host cells primary site of infection colonization multiplication spread bloodstream (bacteremia) suitable tissue multiplication

Virulence Factors Adherence

factors/adhesins Invasion of host cells

(Invasins) Capsule Motility Toxins Enzymes Antiphagocytic factors

Antigenic heterogeneity The requirement for iron Intracellular

pathogenicity Biofilm formation Resistance to antibiotics

Adherence Factors/adhesin

Specificity of adherence tissue tropism, species specificity, genetic specificity within the species Mechanism of adherence may involve two steps:

1)Non specific adherence (docking) reversible attachment of bacteria to eukaryotic cells, due to several conditions: Hydrophobic interaction Electrostatic attraction Brownian movement Atomic and molecular vibration Trapping by biofilm polymer

Adherence Factors/adhesin

2)Specific adherence (anchoring) reversible permanent attachment Complementary receptors and adhesin molecules Lock and key system irreversible attachment

Adhesin

Kenneth Todar, 2009

Invasion to host cells

Production of extracellular substances breaking down primary or secondary defenses of the body INVASINS Consist of spreading factors, hemolytic enzymes, extracellular digestive enzymes, toxins with short range effects related to invasion

Invasins

Kenneth Todar, 2009

Bacterial Toxins

Endotoxins (Lipopolysaccharide and peptidoglycan) and exotoxins (proteins) LPS Lipid A (toxicity), polysaccharide (antigenicity), O antigen (antigenicity) Peptidoglycan much less potent than LPS Fever, leukopenia, hypoglycemia, hypotension, DIC

Endotoxin VS Exotoxin

Kenneth Todar, 2009

BACTERIAL TOXIN Kenneth Todar, 2009

Motility

Flagella, actin-based motility Flagella function in attachment, biofilm formation, colonization of host tissues, export of substances associated with virulence Induce strong immune system and manifest antigenic variation

Capsules

Most pathogenic bacteria polysaccharides capsules Poorly immunogenic, antiphagocytic protect the microbe from intracellular killing Immunomudulatory effects S. aureus and E. fecalis

Antiphagocytic factors

Evade phagocytosis or leucocyte microbicidal mechanism absorbing normal host components to their

surfaces Polysaccharide capsules Pili

Show antigenic heterogeneity protection to the same type, no cross immunity Toxin that inhibits chemotaxis by leucocytes

Intracellular pathogenicity

Live and grow in hostile environment in PMNs cell, macrophages and monocytes, due to abilities: Avoid entry to phagolysosome, live in cytosol of the

phagocyte Prevent phagosome-lysosome fusion, live within

the phagosome Resistant to lysosomal enzymes, survive within the

phagolysosome Escaping oxidative burst

Live within non-phagocytic cells

Macrophage Function

Antigenic heterogeneity

Surface structure of bacteria considerable antigenic heterogeneity Serologic classification of bacteria Frequent switching of antigenic forms evading the host’s immune system

The requirement for Iron Iron is the most studied nutrient essential to infectious process Wide oxidation-reduction potential metabolic function Human and animal Abundant amount of iron >>> intracellular Free iron ( ferric compound)

Most bacteria Low affinity iron assimilation system (using polymeric form of iron) Some bacteria high affinity iron assimilation system (using

siderophore) Availability of Iron affects the virulence of pathogens

The requirement for Iron

Bacterial Biofilm Biofilm aggregate of interactive bacteria attached to a solid

surface or to each other and encased in exopolysaccharide matrix Single or multiple species Related to persistent and difficult to treat human infection Biofilm formation: Colonization of the surface (using flagella, pili)secrete low level of

molecules (quorum sensing signal) concentration of signals increase treshold is reached bacteria respond and change their behavior (changing activation of genes) Activated genes : influence metabolic pathways and production of

virulence factors Biofilm matrix : protect bacteria from host’s immune system,

diffusion barrier for some antimicrobials

Bacterial Biofilm

Pathogenicity Island

Located in bacterial chromosomes or plasmid Contain of large genome, encode genes which contribute to virulence, survival under stressful condition Pathogen that undergo gene transfer; plasmid, phaga, transposon acquired by horizontal gene transfer

Antimicrobial resistance

Genetic determinant spontaneous mutation and horizontal gene transfer Formation of biofilm

Regulation of bacterial virulence factors

Pathogenic bacteria adapted to saprophytic or free living states Evolving complex signal transduction systems to

regulate the genes important for virulence Environmental signals control the expression;

temperature, iron availability, osmolality, growth phase, pH, specific ions, nutrient factors.

Virulence Factors

Hsing Ju Wu et al, 2008)

Hsing Ju Wu et al, 2008)

Hsing Ju Wu et al, 2008)

Hsing Ju Wu et al, 2008)

http://www.mgc.ac.cn/VFs/

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