Molecular determinants of pathogenicity and virulence among pathogens.pptx

The presentation discusses the pathogenicity and virulence of pathogens, their determinants and their interaction with the host. It talks briefly about pathogenicity, virulence, adhesions, invasions, toxins, disease, pathogenesis, pathogenicity islands (PAIs), intracellular, extracellular, bacteria, virus, fungi, prion, metazoan worms, protozoa, tuberculosis, E. coli, Salmonella, Yersinia, Mycobacterium, cytotoxins, enterotoxins, exotoxins, neurotoxins, endotoxins, in-silico, in-Vitro, in-vivo, immunohistology, haemagglutinins, spike proteins, integrins, and phagolysosomes.

1. Molecular determinants of pathogenicity and virulence among pathogens DOI:  10.13140/RG.2.2.23779.94249 Bhoj R Singh Division of Epidemiology, ICAR-IVRI, Izatnagar-243122, India [email protected]

2. Pathogenicity/ Virulence Pathogenicity : the potential ability of a pathogen to produce disease in a host. Virulence : the disease-producing power of an organism which defines the degree of pathogenicity or variability in pathogenicity among different strains within a group or a species of pathogen.

3. Pathogenicity and its tests Types of pathogens : Major types are: Bacteria, Fungi, Viruses, Prions , Parasitic protozoa and metazoan worms. The pathogenicity test ( in-vivo ): This involves the reproduction of lesions following artificial infection in suitable hosts (real or experimental) under SPS conditions. The pathogenicity test ( in-vitro ): Detections of markers of pathogenicity potential of a pathogen (including cell culture assays) in glass (in a laboratory ) without the use of a living system or using components of hosts, like host cells (haemolysis, MTT assays, cell cytotoxicity assay, CPE), organs etc. The pathogenicity test ( in- silico ): Identification of genes/ sequences encoding pathogenicity islands or virulence factors.

4. Pathogenicity determinants Host Susceptibility Host Resistance Presence of Virulence Factors in agent Presence of Host-mediated Pathogenesis, i.e., interaction response interleukins, cytokines, hormones, quorum sensing molecules, growth regulators etc. In certain infections like tuberculosis and leprosy, tissue damage results from the toxic mediators released by lymphoid cells rather than from bacterial toxins. Ability for invasion , intracellular and extracellular persistence , growth, and evasion from host responses, i.e ., virulence markers present in pathogenicity islands (PAI) in pathogens. Environmental interference/ support

5. Pathogenicity testing Whenever a new suspected pathogen (novel or mutant/ variant of existing pathogen) is identified it is necessary to evaluate its disease causing potential. It is usually done in vivo at different scale of testing depending on severity of disease and extent of losses incurred. Pathogenicity of a novel pathogen can be assessed by experimental or observational methods. In experimental method, a statistically feasible population/ sample of host may be tested for altered signs and symptoms of illness/ health. In the second method, it may be necessary to test thousands of individual survivors of disease for altered symptoms induced by the suspected pathogen. In observational methodology Evan’s postulates are used as guideline to assess the causal relationship of a novel agent and the disease.

6. Virulence and Virulence markers/ factors Virulence is described as an ability of an organism to infect the host and cause disease . Virulence markers/ factors are the molecules that assist the pathogen in colonizing the host at the cellular level, including attachment (adhesion), invasion, toxicity and evading host defence . These factors are either  secretory , membrane-associated , cytosolic , coded by chromosomal or transferable (RNA/ DNA of viruses, phages , plasmids etc.) genetic code. Virulence factors are detected using phenotypic, proteomic and genomic approaches. Pathogen cell-associated virulence determinants are easily localized using microscopic methods ranging from light microscopy to electron microscopy. The measurement of pathogen growth within infected tissue is an excellent objective property of pathogenesis and can be determined by several direct and indirect means, impression smear pathology, histopathology & immunohistopathology , pathogen load, real-time PCR etc.

7. Types of virulence factors/ markers Adhesins Fimbrins , flagellins . Multivalent Adhesion Molecules (MAMs) are a widespread family of adhesins found in Gram-negative bacteria, including  E. coli, Vibrio , Yersinia , and Pseudomonas aeruginosa . The most common adhesins are fimbriae, they are multi-subunit structures extending outward from the bacterial cell surface. Fimbriae are classified in part based on the host cell receptor with which they interact. Gram-positive pathogens like Streptococcus  and  Staphylococcus  species, produce head-stalk-type adhesins , which are serine-rich repeat proteins (SSRP ). The spike proteins, hemagglutinin , glycoproteins (gp20 of HIV) found on viruses are examples of a viral adhesin allowing viruses to bind to the sialic acid and other receptors on the membrane of host respiratory and intestinal cells.

8. Invasins Invasins are  proteins associated with the penetration of pathogens into host cells. Invasins promote entry during the initial stage of infection . Most of the intracellular and facultative intracellular pathogens produce invasins . Examples: inv, an OMP interacting with host β1  integrin , is produced by Yersinia enterocolitica and Y . Pseudotuberculosis ;   PagN of E. coli and Salmonella interact with cell surface heparin sulphate  proteoglycans  to invade,  type three secretion system, called T3SS-1 on   Salmonella  pathogenicity island-1 (SPI-1 ), Rck (resistance to complement killing ) are invasins of Salmonella .

9. Intracellular survival The cytosolic factors facilitate the bacterium to undergo quick adaptive—metabolic, physiological and morphological shifts; the capable bacteria survive, and others die once inside phagosome / lysosomes / intracellular vacuoles. The membrane-associated and secretary virulence factors aid pathogens in adhesion and evasion of the host’s innate and adaptive immune responses ( cell-mediated killing, antibody-mediated killing).   Toxins : Secretary virulence factors: Mostly produced by extracellular or facultative extracellular pathogens. Exotoxins and endotoxins Enterotoxins , cytotoxins , necrotoxins , hepatotoxins , nephrotoxins , neurotoxins Siderophores: iron-binding factors enabling pathogens to compete with the host for iron, which is bound to haemoglobin , transferrin, and lactoferrin. Capsules and loose polysaccharides: Help in evading antibody-mediated killing and evading immune response.

10. Virulence and Avirulence genes Virulence genes : Encode for one or other functional molecule required for adhesion, invasion, intracellular or extracellular survival, multiplication and toxicity responsible for pathogenicity. Avirulence ( avr ) genes are genes of the pathogen that govern its specific recognition by particular host genotypes (mostly in plants ). Recognition depends upon the presence of a pair of matching genes, an  avr  gene in the pathogen and a resistance  (R ) or susceptibility (S)  gene in the host. The specific recognition of the pathogen results in the induction of the host's defence response, often a hypersensitivity reaction (HR).

11. Quiz Enlist adhesin , invasin and toxin molecules of Salmonella . Define pathogenicity islands (PAIs) and enlist PAIs of E. coli, Salmonella and Yesinia spp. Most of the members of Mycobacterium strains don’t have defined virulence factors to cause disease; explain their pathogenicity module. Is drug resistance among pathogens can be defined as a virulence factor? Explain.