This document discusses molecular determinants of pathogenicity and virulence among pathogens. It defines pathogenicity as the potential ability of a pathogen to produce disease, and virulence as the degree of pathogenicity. It describes different types of pathogenicity tests (in vivo, in vitro, in silico) and factors that determine pathogenicity, including host susceptibility, presence of virulence factors, and environmental factors. It also discusses different types of virulence factors such as adhesins, invasins, toxins, and intracellular survival factors. It defines virulence genes and provides examples of adhesins, invasins, and toxins in Salmonella.

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
brs1762@gmail.com

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.