Virulence Factors Mediate Pathogenic Activities of all Microbes
Adhesins - promote attachment to host cells
and tissues, which allows bacteria to contact and colonize host surfaces
pili - Escherichia
coli and Neisseria gonorrhoeae use these to attach to urethral cells; Salmonella and Escherichia
coli use them to attach to intestinal cells; fungi that infect in their yeast form use
capsules for attachment
capsules - Escherichia
coli uses these for attachment to intestinal cells
hemagglutinins - bacteria (Salmonella and Bordetella) and many viruses
use these to attach to various host cells
type
III secretion system - Escherichia coli uses this
to aid attachment to intestinal cells and introduce
toxins, etc. into host cells (type III secretion system - Escherichia
coli uses this to aid attachment to intestinal cells
spikes (peplomers) -
used by viruses to attach to cells they then infect
holdfasts
suckers are used by some protozoa (Giardia) and some helminths (Echinococcus, Schistosoma)
mouthparts with "teeth" are used by helminths (especially tapeworms) to attach
to host surfaces
hyphal tips - fungi have molecules at the tips of their hyphae that attach to host cells
adhesive disc - Giardia uses
a ventral adhesive
disc to attach to intestinal cells
apical
complex - some protozoa (Apicocomplexa such as Plasmodium,
Toxoplasma)
use their apical complex to attach to host cells
Entamoeba
histolytica binds to intestinal cells using lectins to attach to polysaccharides
on intestinal cells
Invasins - promote entrance into and/or movement through tissues
or cells, which allows bacteria to infect the host
digestive enzymes - break down barriers to infection
fibrinolysin -
enzyme produced by Staphylococcus aureus and Streptococcus pyogenes that dissolves
blood clots, thus preventing bacterial isolation and promoting spreading
hyaluronidase -
enzyme produced by bacteria like Staphylococcus aureus and Streptococcus pyogenes that
digests connective tissue, allowing them to spread through tissues more readily
hemolysins - enzymes
produced by bacteria like Clostridium perfringens and Staphylococcusaureus that
dissolve red blood cells, causing anemia and limiting oxygen delivery to infected tissues
type III secretion
system - Salmonella uses this to invade intestinal
cells by inducing phagocytosis by host cells
hyphal extension - fungi (Aspergillis, Candida, Stachybotrys) can invade the
tissues by growing hyphal elements into them
"boring" devices - protozoa, including Apicocomplexa such as Plasmodium,
Toxoplasma and
helminths, such as Schistosoma,
have specialized devices that allow them to "bore" into cells or through tissues; in the Apicocomplexa,
it is called an apical
complex
Evasins - protect pathogen from host defense factors, especially
phagocytes, which allows bacteria to evade the host defense system and makes it
more possible for them to grow within the tissues
capsules -
fungi, like Cryptococcus, that infect in their yeast form use these to avoid phagocytosis;
bacteria like Haemophilus influenzae, Neisseria meningitidis and Streptococcus
pneumoniaeuse these to avoid
phagocytosis ... here's how phagocytosis is supposed to
work (movies courtesy of ASM MicrobeLibrary,
Author: James A. Sullivan, CellsAlive)
catalase - enzyme
produced by Candida and other fungi as well as Staphylococcus aureus and other
bacteria to degrade peroxides, thus promoting survival inside phagocytes
coagulase - enzyme produced by Staphylococcus aureus and Yersinia pestis that
promotes blood clotting (coagulation), thereby walling-off the site of infection and protecting
these bacteria from phagocytosis; this also causes the characteristic appearance of the skin
in black plague
M protein - this surface protein is produced by Streptococcus pyogenes to inhibit
surface complement activation (which would cause opsonization)
leukocidins - Staphylococcus aureus, Streptococcus
pyogenes and other bacteria use these to kill phagocytes by damaging
their membranes
anti-phagocytic factors - various factors allow protozoans such as Leishmania and bacteria such
as Chlamydia, Listeria, Mycobacterium, Rickettsia, and Salmonella to
survive within phagocytes; fungi use hyphae and pseudohyphae as
antiphagocytic factors
Intracellular growth - allows many bacteria, fungi and protozoa to
"hide" from the host defense system
immune system blockades - viruses and some bacteria can produce (or trigger the host
cell to produce) molecules that shut down the immune response to the parasite
Toxins - this is a general category that includes any molecule that can
promote damage to the cells or tissues of the host
toxic molecules produced by bacteria and fungi include:
exotoxins - toxic proteins that are secreted by living microbes
bacterial examples
botulin - Clostridium botulinum neurotoxin causes flaccid
paralysis in botulism
tetanospasmin - Clostridium tetani neurotoxin causes simultaneous
contraction of opposing muscles, resulting in tetany, which is painful
and makes tetanus a
life-threatening disease
enterotoxin - Escherichia
coli, Staphylococcus
aureus, Vibrio
cholerae and other bacteria produce this toxin, which causes
intestinal cells to pump water and salts (electrolytes) from the bloodstream
into the intestine, causing diarrhea that leads to dehydration, shock,
and even death in the worst cases
diphtheria toxin - Corynebacterium diphtheriae produces
this exotoxin, which damages cells of the heart, kidneys and central nervous
system by inhibiting protein synthesis, thus resulting in diphtheria
fungal example - gliotoxin this exotoxin causes the bright red inflammation
seen in Candida infections
endotoxin - lipopolysaccharide (LPS) portion of the outer membrane of Gram-negative
bacterial cell walls which is released when the bacteria disintegrate and causes fever and/or endotoxin
shock, depending on its concentration in the bloodstream
animal viruses - consequences of virus infection
of a animal host cell include:
Lytic infection - viral nucleic
acid initiates destructive replication cycle in which progeny virions are produced and
the host cell is destroyed
Picornavirus
replication
Herpes
virus replication
Persistent infection - host cell remains
alive and produces progeny virions at a slow rate, but for a long time
Latent infection - there is a delay between
infection of the host cell and generation of progeny virions
Transformation - the viral nucleic acid triggers
neoplastic changes in the host cell which "immortalilze" it and cause uncontrolled growth,
which can lead to tumors (cancer) in the host
benign tumor - noninvasive ... tumor cells, often enclosed in a "capsule" of
host tissue, do not spread to other tissues
malignant (metastatic) tumor - invasive ... tumor cells spread to other
tissues and proliferates there
protozoa can be directly toxic to cells - Entamoeba histolytica is cytotoxic to
intestinal cells as a result of its ability to bind to them, then secrete molecules that generate
holes in the intestinal cells' cytoplasmic membranes, resulting in the diarrhea seen in amebic
dysentery
toxicity can also be triggered by viruses and other intracellular parasites ... as a
result of various types of damage to the internal structures, external structures (modification
of erythrocyte membranes in malaria) or
functional molecules of host cells
damage or toxicity can be triggered as a result of an inappropriate immune response by
the host, as seen in schistosomiasis