CHAPTER 15
MICROBIAL MECHANISMS OF PATHOGENICITY
Pathogens
can gain entrance to the human body and other hosts through several portals of
entry. Most pathogens must adhere to host tissues, penetrate or evade host
defenses, and damage the host tissues to cause disease. Some pathogens cause
their harm by the production of microbial waste products which are toxic to the
host. A few microbes, such as those causing dental caries (cavities) and acne,
can act without penetrating the body.
1. Mucous
membranes---many
bacteria and viruses gain access to the body by penetrating the mucous
membranes of the respiratory, digestive, or genitourinary tracts, or by
penetrating the conjunctiva (mucous membrane that lines the eyelids).
The
respiratory tract is the most frequent site of entry. Microbes are inhaled in
drops of moisture or dust particles. The common cold, pneumonia, tuberculosis,
influenza, measles, and smallpox are examples of diseases commonly contracted
this way.
Microorganisms
gain access to the gastrointestinal tract through food, water, and contaminated
fingers and objects put into the mouth. Many of these microbes are destroyed by
the hydrochloric acid and enzymes of the stomach, or by bile and enzymes in the
small intestine. Some pathogens can survive and cause disease, including those
causing polio, hepatitis A, typhoid fever, amoebic dysentery, giardiasis,
shigellosis, and cholera. These pathogens are then eliminated in feces and can
spread to other hosts.
The
mucous membranes of the genitourinary tract are the portal of entry for
pathogens that are sexually transmitted. Some can penetrate unbroken mucous
membrane and others require a cut or abrasion. STD’s include HIV infection,
genital warts, chlamydia, herpes, syphilis, and gonorrhea.
2. Skin is an important defense against microbes,
since most microbes cannot penetrate unbroken skin. Some can enter through hair
follicles and ducts of sweat glands. Hookworm larvae can penetrate intact skin,
and some fungi grow on the skin itself.
3. Parenteral
route--microbes can
gain access to the deeper tissues of the body by way of punctures, infections,
bites, cuts, wounds, surgery, etc.
Most
microorganisms have a preferred portal of entry. If they gain access to the
body by some other route, they often cannot cause their disease. Other microbes
are able to cause harm when entering by several different routes.
In
most cases, fairly large numbers of microbes must enter the body to cause
disease. If only a few enter, they will probably be overcome by the host’s
defenses. Even when large numbers do enter, the numbers usually still must
increase by reproduction of the microbes before there are enough to cause
disease. The trick for the microbes is entering in large enough numbers so that
even though many are wiped out by the defenses of the host, the ones that
survive will be enough to multiply and cause disease.
The
virulence (degree of pathogenicity) of a microbe is often expressed as the LD50
(lethal dose for 50% of hosts). This is the number of microbes that will kill
50% of inoculated susceptible test animals. The dose required to produce disease
(but not necessarily death) is the ID50 (infectious dose for 50% of
hosts).
Once
the pathogen has gained access to the body, it usually must have some means of
attaching itself to the host’s tissues. This attachment is called adherence and
is a necessary step in pathogenicity for most pathogens. This attachment is
accomplished by means of surface molecules on the pathogen called adhesins or
ligands that bind to complementary receptors on the cells of certain host
tissues. Adhesins may be located on the glycocalyx or other surface structures
of the microbe such as fimbriae. Adhesins are most frequently glycoproteins or
lipoproteins. Receptors are usually sugars which are associated with the plasma
membrane. Not everyone's cells have the same receptors and not all cells
belonging to the same body have the same receptors. If either the adhesins or
the receptors can be altered, this can be a way of preventing infection.
Streptococcus mutans, a major cause of tooth decay, attaches to
the surface of the teeth by means of its glycocalyx. Next, Actinomyces uses its fimbriae to attach to the glycocalyx of S. mutans. Together these form the
deposit known as plaque. This mass of microbes and their extracellular
products, stuck to a surface, are called a biofilm. Biofilms may be made up of
several layers, and microbes in deeper layers may be protected from antibiotics
and disinfectants.
Pathogenic
strains of Escherichia coli have
adhesins or fimbriae that adhere to cells in certain regions of the small
intestine. E. coli and Shigella cause host cells to take them
in by endocytosis and then multiply inside them.
Treponema pallidum hooks its tapered end into a host cell.
Listeria monocytogenes produces an adhesin for specific receptors
on host cells.
Neisseria gonorrhoeae also has fimbriae with adhesins which fit
receptors of cells in the genitourinary tract, eyes, and pharynx.
Staphylococcus aureus binds to skin cells in a mechanism similar
to that of viruses.
Most
pathogens must penetrate tissues to cause disease. Factors that contribute to
the ability of bacteria to invade a host:
1. Capsules----some bacteria form an organized
glycocalyx called a capsule around their cell walls which increases the
virulence of the species. The capsule helps resist host defenses by interfering
with phagocytosis. If the human body produces antibodies against the capsule,
this can allow destruction of the bacteria by phagocytosis.
Streptococcus pneumoniae owes its pathogenicity to the presence of a
capsule. Strains without a capsule are avirulent (do not cause disease);
strains with a thin capsule cause a mild pneumonia; strains with a thick
capsule cause a severe or fatal pneumonia. Other bacteria which need a capsule
to aid them in causing disease include Klebsiella
pneumoniae, Haemophilus influenzae, Bacillus anthracis, and Yersinia pestis. However, not all
encapsulated bacteria are pathogenic, and many bacteria without capsules cause
diseases.
2. Components of the cell wall---various
components of the cell wall can contribute to virulence.
a. Proteins--one example is the M
proteins built into the cell walls of Streptococcus
pyogenes. This protein aids in attachment to host cells and also interferes
with phagocytosis. Neisseria gonorrhoeae uses fimbriae and a protein
called Opa to attach to host cells and induce endocytosis.
b. Waxes--the waxy substances in the cell
wall of Mycobacterium tuberculosis helps protect
it from damage inside phagocytic cells so well that it can live and multiply
inside the phagocyte.
c. Enzymes---extracellular enzymes
(exoenzymes) are secreted into host
tissues by bacterial cells.
1) Coagulases---bacterial enzymes that
coagulate the fibrinogen in blood. By triggering the formation of fibrin
threads and therefore a blood clot, the bacteria may isolate themselves from
the defenses of the host. Members of the genus Staphylococcus produce coagulases and this may contribute to their
tendency to form boils.
2) Kinases---bacterial enzymes that
break down fibrin, dissolving clots and enhancing the spread of infections. Streptococcus pyogenes produces an
enzyme known as fibrinolysin or streptokinase. This enzyme has been used
medically to dissolve harmful clots, such as in heart attacks. Staphylococci
produce staphylokinase.
3) Hyaluronidase----an enzyme that
breaks down hyaluronic acid, a polysaccharide that forms the ground substance
for some of the connective tissues of the body. This helps the bacterial
infection spread through tissue. The enzyme may also be used in injections to
promote spread of a drug through tissue. It is produced by some streptococci
and some clostridia.
4) Collagenase---this enzyme breaks
down the protein collagen, allowing spread of infection through muscles and
other body tissues. It is produced by several species of Clostridium and plays a role in gas gangrene.
5) IgA proteases destroy antibodies
(defensive proteins produced by the host)—Neiserria produces these
Antigens
are the part of an invader that triggers an immune response in the host.
Antibodies are the specific proteins produced in response to the presence of an
antigen. When an antibody combines with the specific antigen that triggered its
production, the antigen is somehow inactivated. Some pathogens can alter their
surface antigens so that antibodies are no longer effective. Well known for
doing this: Neiserria, Influenzavirus, Trypanosoma.
By
growing in host cells, bacteria can evade host defenses such as phagocytosis
and antimicrobial substances. Not all bacteria enter host cells; in fact many
of them cause damage without ever penetrating the cell. However, some bacteria
do enter cells. They attach to the host cell by adhesins and this can trigger
signals in the host cell that result in the taking in of the bacterium by the
host cell by a mechanism involving the host cell cytoskeleton.
Bacterial
species that enter cells include Salmonella,
a cause of food poisoning, and Escherichia
coli, which may cause severe diarrhea in children. These microbes attach to
the plasma membrane of the host cell and cause changes in the membrane at the
point of contact. The microbes produce proteins called invasins, that alter the
cytoskeleton and cause the actin filaments of the cytoskeleton to take the
bacterium into the cell. Once inside, these and others such as Shigella and Listeria may continue to use
the actin filaments to move through the
cytoplasm or even to move from cell to cell.
Bacteria
also may make contact with plasma membranes where cells join each other and use
a glycoprotein called cadherin to move from cell to cell.
Many
microbes entering a host are destroyed by lysosomes. If the microbes manage to
avoid this, they may cause damage in four main ways:
1. By using host's nutrients
2. By causing direct damage in the immediate
area of the invasion
3. By producing toxins that may be
transported by blood and lymph to damage sites far from the original invasion
4. By causing the host to react with a
hypersensitivity reaction
Some
pathogens produce proteins called siderophores, which steal iron from transport
proteins and make it available to the microbe.
Pathogens
can enter host cells and multiply, killing the host cells and moving on to new
host cells. Some bacteria can induce host cells to engulf them by a process
resembling phagocytosis, but the bacteria are not then destroyed as usually
occurs. In fact, they can move through the original host cell and cause it to
release them into deeper layers of cells by a form of exocytosis. Some bacteria
release enzymes to allow penetration. Some use their own motility to enter.
This penetration may damage the host cell. Also, waste products of the microbe
may damage the host cell.
This
is the main way bacteria cause harm. Toxins are poisonous substances produced
by certain microbes. The ability to produce toxins is called toxigenicity.
Toxins may be carried far from the site of invasion by the blood or lymph.
Various toxins may cause fever, cardiovascular disturbances, diarrhea, and
shock. They can inhibit protein synthesis, destroy blood vessels, and disrupt
the nervous system. About 220 bacterial toxins are known, and 40% of them
damage eukaryotic plasma membranes.
Toxemia refers to the presence of toxins in the blood.
These
are produced inside some bacteria as they grow, and released into the
surroundings. They are proteins, and many are enzymes that catalyze only
certain biochemical reactions. Many bacteria that produce exotoxins are
gram-positive, but some gram-negative bacteria also produce them. The genes for
the production of the toxin are mostly carried on bacterial plasmids or phages.
Exotoxins are soluble in body fluids, so they easily enter the blood and are
carried throughout the body.
Exotoxins
work by destroying particular parts of the host’s cells or by inhibiting
certain metabolic functions. They are highly specific in their effects. They
may be placed in 5 groups, according to the type of tissue they affect:
1. Cytotoxins---kill a variety of host cells
or affect their function
2. Neurotoxins---interfere with nerve
impulse transmission
3. Enterotoxins---affect cells lining the
gastrointestinal tract
4. Leukotoxins—attack leukocytes
5. Cardiotoxins—attack the heart
Bacterial
exotoxins are the most powerful poisonous substances known. One milligram of
botulinum exotoxin could kill 1 million guinea pigs. Diseases caused by
bacteria that produce exotoxins can be caused by tiny traces of the toxin. The
bacteria themselves do not cause the signs and symptoms of the disease; it is
the poison they produce and release into the body of the host that causes the
disease. For example, the wound infected by Clostridium
tetani may be smaller than a pin prick, but the organisms produce a
neurotoxin that can be fatal.
In
response to the presence of a toxin, the body produces antibodies called
antitoxins, which will combine with the toxin and make it harmless. We can take
active toxins and treat them by heat or exposure to chemicals such as
formaldehyde. This makes them harmless but still able to trigger the immune
response that causes the production of antibodies. The inactivated toxins are
called toxoids, and are used for vaccinations. Diphtheria and tetanus vaccines
are prepared this way.
Exotoxins
are also classified into 3 types based on structure and function:
1. A—B
toxins (type III toxins)—largest group, these have 2 parts, A and B. A
is the active enzyme component and B is the binding component. B binds to a
receptor on the host cell and both parts are transported into the cell. The two
parts separate and A (the enzyme) does the damage.
2. Membrane-disrupting toxins (type II
toxins)—disrupt the plasma membrane and cause lysis of the cell, either by
forming channels through the plasma membrane or disrupting the phospholipids of
the membrane.
a. Leukocidins are membrane-disrupting
toxins that kill phagocytic white blood cells (leukocytes)
b. Hemolysins are toxins that kill red
blood cells.
3. Superantigens (type I toxins)—antigens
that provoke a very intense immune response, so strong that it actually leads
to damage instead of protection. T cells are stimulated to release enormous
amounts of cytokines. Cytokines in smaller amounts are intended to allow
communication between cells of the immune system. In excessive amounts they
cause harm. Symptoms include fever, nausea & vomiting, diarrhea, shock and
even death. Staphylococcal toxins act this way.
Some
major exotoxins:
1. Diphtheria toxin—Corynebacterium diphtheriae produces this toxin when it is carrying
a lysogenic phage which includes the tox
gene. This is a cytotoxin which inhibits protein synthesis as follows:
a. The toxin consists of 2 different
polypeptides combined, A (active) and B (binding). A is the one which actually
causes the harm to the host, but it cannot act without B also being present.
b. Polypeptide B binds to surface
receptors on the host cell and causes the transport of both parts across the
plasma membrane into the cell.
c. The two parts separate
d. Polypeptide A inhibits protein
synthesis inside the cell.
2. Erythrogenic toxin—Streptococcus pyogenes bacteria can carry a prophage which causes
them produce three types of cytotoxins, A, B, and C, which are also called
erythrogenic toxins because they damage capillaries under the skin and produce
a red rash. The disease is called scarlet fever because of this rash. These act
as superantigens. Without the prophage, Streptococcus
pyogenes produces what we call strep throat, but not scarlet fever.
3.
Botulinum toxin—produced by Clostridium
botulinum, this is a powerful A—B neurotoxin which is ingested with food
where the organisms have grown. It prevents the transmission of nerve impulses
from nerve to muscle by binding to the nerve call and inhibiting the release of
acetylcholine. This causes flaccid (limp) paralysis and can be fatal. The
organisms produce endospores which can survive some home canning methods.
4. Tetanus toxin (tetanospasmin)---Clostridium tetani produces a different
A—B neurotoxin. Instead of paralysis, this one causes characteristic
convulsions known as tetanic spasms. These organisms enter with a deep puncture
wound.
5. Vibrio enterotoxin---Vibrio cholerae produces cholera toxin. In consists of two
polypeptides, A (active) and B (binding). The B polypeptide binds to cells
lining the small intestine. The A component then induces the formation of
cylcic AMP from ATP in the cytoplasm. This causes the lining cells to release
large quantities of fluids and electrolytes into the intestine, along with
muscle contractions that contribute to severe diarrhea and vomiting. Without
treatment, cholera can be fatal in a matter of hours. Some strains of Escherichia coli produce a toxin called
heat-labile enterotoxin that has a similar action, but not so extreme.
6. Staphylococcal enterotoxin---Staphylococcus aureus produces a toxin
that also affects the intestines in a way similar to cholera toxin and causes a
form of food poisoning. Some strains of Staphylococcus
aureus produce a similar toxin that causes toxic shock syndrome.
Endotoxins
are a part of the outer membrane of gram-negative bacteria. This is a membrane
outside the peptidoglycan layer and consists of lipoproteins, phospholipids,
and lipopolysaccharides. The lipid portion of the lipopolysaccharide, called
lipid A, is the endotoxin. Endotoxins are lipopolysaccharides instead of
proteins. They produce their effects after the gram-negative bacteria die and
their cell walls undergo lysis. Some antibiotics used against gram-negative
bacteria cause lysis of the cell wall and may temporarily make symptoms worse.
All endotoxins produce about the same signs and symptoms, although these may be
more severe with some organisms than with others. They include chills, fever,
weakness, aching, and in extreme cases shock and even death. They do this by
stimulating macrophages to release excess amounts of cytokines. Endotoxins can
contribute to miscarriages.
Endotoxins
can activate blood-clotting proteins, causing the formation of many small blood
clots that block capillaries. Tissues thus deprived of their blood supply die.
This is called disseminated intravascular clotting (DIC).
Fever
is caused in the following way:
1. Gram-negative bacteria are phagocytized
2. As they are broken down inside the
phagocytic cell, the lipopolysaccharides of the outer membrane are released.
This causes the macrophages to produce interleukin-1.
3. Interleukin-1 is carried to the
hypothalamus of the brain, where the temperature control mechanism for the body
is located.
4. IL-1 causes the hypothalamus to release
prostaglandins which “reset” the body’s thermostat to a higher temperature, so
we begin to run fever.
5. Aspirin, Tylenol, etc. inhibit the
release of the prostaglandins, which reduces the fever.
Shock
in general is a severe drop in blood pressure. Shock caused by bacteria is
called septic shock. If the bacteria are gram-negative, it can also be called
endotoxic shock. Like fever, it is related to a substance produced by macrophages.
Following phagocytosis and lysis of the gram-negative bacteria, the phagocytic
cell secretes a polypeptide called tumor necrosis factor (TNF) or cachectin.
This substance binds to many body tissues and alters their metabolism. One
effect is damage to capillaries that increases their permeability and causes
them to leak fluid, thus lowering blood pressure and leading to shock. The
lowered blood pressure is also harmful to kidneys, lungs, and the digestive
tract.
Certain
gram negative bacteria such as Hemophilus
influenzae type b in cerebrospinal fluid cause the release of both IL-1 and
TNF, which weaken the protective blood-brain barrier and allow bacteria to
enter the CNS. Septic shock is very dangerous—up to 50% of cases may be fatal.
We
can produce antibodies against endotoxins, but they do not protect us against
the effects and seem to even enhance the effects in some cases.
Organisms
that produce endotoxins include:
1. Salmonella
typhi (typhoid fever)
2. Proteus
(frequent cause of urinary tract infections)
3. Neisseria
meningitidis (meningococcal meningitis)
If
bacteria have grown and produced endotoxins in material that is later
sterilized, the endotoxins retain their potency even though no living bacteria
are still present. A test called the Limulus
amoebocyte lysate (LAL) can be used to detect even traces of endotoxin.
Plasmids
are small circular pieces of DNA that are not part of the main bacterial
chromosome and contain genes not found on the main chromosome. They are
replicated and passed on to daughter cells during cell division. One group of
plasmids are known as R plasmids or R factors, because their genes make
bacteria resistant to antibiotics. Other plasmids may contribute to bacterial
pathogenicity, often by carrying genes for making toxins. Bacterial strains
that lack the plasmid may be harmless or only cause certain symptoms. With the
plasmid, the bacteria cause additional harm to the host.
Some
bacteriophages can incorporate their DNA into the bacterial chromosome,
becoming a prophage. This state is called lysogeny, and cells containing the
prophage are called lysogenic cells. Genes carried on the phage DNA may give
lysogenic cells new characteristics. This is called lysogenic conversion. The
bacterial cell will now be immune to infection by the same type of phage, and
also may develop new pathogenic properties. Toxins produced due to genes of
prophages:
Diphtheria toxin
Erythrogenic toxin
Staphylococcal enterotoxin
Pyrogenic toxin
Botulinum neurotoxin
Capsule of Streptococcus pneumoniae (not exactly a toxin but contributes to
virulence)
Vibrio toxin
PATHOGENIC PROPERTIES OF VIRUSES
Viruses
must gain access to the host, evade the host’s defenses, and cause damage or
death to the host cell while reproducing themselves.
1. Viral mechanisms for evading host
defenses—viruses have attachment sites for receptors on host cells. This allows
the virus to bind to and penetrate the cell. Once it is inside, it is protected
from host defenses. Some viruses manage to enter cells by mimicking a substance
useful to the cell and using the receptor for that substance. For example, the
rabies virus mimics acetylcholine and enters cells that way. The AIDS virus has
attachment sites complementary to the CD4 protein found on human cells, mainly
on T lymphocytes. The virus can attach to these receptors and enter cells.
Antibodies are produced but are not very effective because the antibodies cannot
reach the attachment sites.
2. Cytopathic effects of viruses—infection
of a host cell by an animal virus usually results in death of the cell, due to
accumulation of large numbers of viruses, effects of the virus that change the
permeability of the host cell plasma membrane, or by inhibition of synthesis of
the host cell’s DNA, RNA, and proteins.
Visible effects that can lead to death or damage of a host cell are
called cytopathic effects (CPEs). CPEs vary with the virus and often can be
used to diagnose viral infections. Viruses can produce one or more of these
cytopathic effects:
1) At some stage, cytocidal viruses stop
synthesis of host cell macromolecules or stop mitosis.
2) The virus may cause the host cell’s
lysosomes to release their enzymes.
3) Inclusion bodies may be formed. These
are granules found in the cytoplasm or the nucleus of the host cell. These
granules may be viral parts such as nucleic acids or viral proteins in the
process of being assembled into new virions. The inclusion bodies may appear at
various stages of the infection and stain in different ways. These features are
used in diagnosis. Diagnostic inclusion bodies are formed in these viral
infections:
Rabies (Negri bodies)
Measles
Chickenpox
Smallpox
Herpesvirus
Adenoviruses
4) Some viruses may cause adjacent cells
to fuse together, forming a very large cell called a syncitium. Paramyxoviruses
are known for this.
5) Some viral infections result in
changes in function but no visible changes in appearance.
6) Some virus-infected cells produce
interferons. Infection by the virus causes this, but the genes for the
interferons are located in the host cell DNA, and the infection causes those
genes to be expressed. Interferons protect nearby uninfected cells from
invasion by the virus.
7) Viral infections may cause changes in
the antigens on the surface of infected cells. The immune system of the host
can then recognize that the cells are abnormal and target them for destruction.
8) Some viruses can cause changes in the
chromosomes of the host cells. Chromosomes may be damaged or broken. Oncogenes
(genes that may cause cancer) may be
brought in by a virus, or oncogenes present in the cell’s DNA may be activated
by the virus.
9) Most normal cells growing in tissue
cultures stop growing when they are in contact with other cells on all sides.
This is known as contact inhibition. Certain viruses can transform cells so
that their appearance changes and they no longer recognize contact inhibition.
PATHOGENIC PROPERTIES OF FUNGI, PROTOZOA,
HELMINTHS, AND
ALGAE
1.
Fungi---some fungi can cause disease, but as a group they do not have a well-defined
set of virulence factors.
a. Some fungi do produce toxic chemicals.
Trichothecenes are toxins that inhibit protein synthesis in eukaryotic cells. Stachybotrys
is an example.
b. Two fungi that cause skin infections, Candida albicans and Trichophyton secrete proteases, which
may modify host cell membranes to allow attachment of the fungi.
c.
The body may produce an allergic response to the fungus.
d. At least one fungus organism, Cryptococcus neoformans, produces a
capsule that helps it resist phagocytosis.
One
example of a fungus disease caused by a toxin is ergotism. This was common in
Europe in the Middle Ages and is believed by some to have been the cause of the
phenomena blamed on witchcraft early in the history of this country. The
fungus, Claviceps purpurea, grows on
grains and the toxin is ergot, which is chemically related to LSD. It can cause
hallucinations and constriction of capillaries which can lead to gangrene of
limbs.
Peanuts,
peanut butter, etc. may contain traces of aflatoxin, a toxin formed by the mold
Aspergillus flavus. This toxin can
possible act as a carcinogen.
Some
mushrooms produce a strong toxin that destroys the liver. These are called
mycotoxins and include phalloidin and amantin. One mushroom, Amanita phalloides, is called the death
angel.
2. Protozoa---this group causes disease by
various means:
a. Plasmodium
(causative agent of malaria) invades host cells (RBC) and causes them to
rupture. Waste products are also toxic to the host.
b. Toxoplasma—attaches
to macrophages and enters by phagocytosis. The parasite prevents normal
digestion by lysosome enzymes and multiplies within the phagocytic vesicle.
c. Giardia
lamblia---attaches to host cells and digests the host cells and tissue fluids.
Some
protozoa are able to alter their antigenic properties, making it difficult for
the immune system of the host to produce effective antibodies. Giardia and Trypanosoma have this characteristic and can cause long-lasting
infections.
3. Helminths---these organisms can use host
tissue for their own growth or compete with the host for nutrients. Large
masses of parasites may interfere with normal function. Waste products of
metabolism may contribute to disease.
Waste products of the parasite may also be harmful.
4. Algae---only a few species of algae are
harmful to humans. Some dinoflagellates produce a neurotoxin, saxitoxin, which is ingested by mollusks.
Although the mollusk is not harmed, humans eating the toxin develop symptoms
similar to botulism (paralytic shellfish poisoning).
Microbes
leave the body by specific routes called portals of exit. These are usually
elated to the part of the body that had been infected. Often the portal of exit
is the same as the portal of entry.
1.
Respiratory tract—from mouth & nose—coughing, sneezing, drippy nose
2.
Gastrointestinal tract—saliva & feces
3.
Genitourinary tract—STD's, typhoid fever, brucellosis
4.
Drainage from wounds
5. Blood