Host
- Microbe Interactions
I.
The term symbiosis
refers to two organisms of different speices living together. This close
interaction between organisms can be of benefit to both organisms, benefit one
organism with no negative affect on the other, or can benefit one and adversely
affect the other.
A.
Mutualism refers to
those interactions that benefit both organisms. The normal flora of our gut live in symbiosis with us.
We benefit form their production of certain vitamins while they benefit
from our provision of food and shelter.
B.
Commensalism refers
to relationships that benefit one organism with no good or bad effect for the
other.
C. Parasitism
refers to relationships in which one organism benefits at the other expense.
Humans are stricken with a range of parasitic worms.
Viruses and pathogens are usually considered parasitic organisms.
II.
The body harbors large numbers of bacteria that exist in a mutualistic or
commensulistic relationship with the host.
These organisms are referred to as the resident
flora, indigenous flora or, more commonly, the normal
flora. When a baby is born it lacks any normal flora.
Passing through the birth canal and during the first weeks of life it is
exposed to bacteria and fungi that will invade and colonize most exposed
surfaces of the skin, the mucus membranes of the GI tract and portions of
urinary tract and respiratory tract.
A.
The epidermis harbors extensive numbers of bacteria belonging to the
genera of Staphylococcus, Micrococcus and
Corynebacterium and Propionibacterium.
These organisms invade the upper layers of the stratum corneum (cornified
layer of the epidermis) and the glands of the skin (sweat glands and sebaceous
glands), they do not colonize the underlying dermal layers.
Contamination of
the outer most layers of the skin regularly occurs.
Contaminating organisms can include viruses, bacteria and fungi but they
usually do not colonize and are usually easily removed.
These are referred to as the transient
populations.
B.
The alimentary canal harbors large numbers of bacteria.
The oral cavity has specific types of organisms that are most commonly
found in the mouth. Once again only the epithelial layer is colonized.
Underlying connective tissues remain sterile.
1. The bacteria that inhabit the colon survive upon the
undigested material that passes through the small intestine. These bacteria metabolize these “undigested wastes”.
The action of these organism results in the production of the chemicals
that give feces its distinctive odor and produce the gas that results in
flatulence.
2. Production of certain vitamins by the bacteria that the host
can not synthesize is a great benefit to the host.
3. Disruption of the normal flora of the gut by the use of broad
spectrum antibiotics can result in the overgrowth of certain species normally
found in low numbers in the gut. This
can result in problems such as diarrhea or inflammation of the gut mucosa.
C.
The outer portion of the male and female urethra is colonized by
several types of organisms. Spread
of these organisms beyond the distal portion of the urethra is not normal and
often is the precursor to disease.
D. Resident
populations growing on our skin or mucus membranes use the available nutrients,
produce metabolites and sometimes produce substances that have a direct
antibiotic affect. Collectively this inhibits other organisms (which
might be disease causing) from establishing populations on these sites of our
body. When one organism’s growth interferes with the colonization and growth
of another organism it is referred to as microbial
antagonism.
II. Most of the bacteria found in or on the human body cause no disease if they are in
the proper place and in the proper number. But certain microorganisms do cause disease when
introduced into an otherwise healthy individual. These organisms are referred to as the true
pathogens. By overcoming the defenses of a normal healthy host, true pathogens
colonize and cause disease within that host. Many organisms cause disease only in a
weakened host, a host with disrupted normal flora or if they have been allowed entry to a
site where that bacteria is not usually found. This type of pathogen is referred to as an opportunistic
pathogen. If the pathogen responsible for a disease can be spread from one host to another, the
disease is said to be communicable. Most of the diseases that we will talk
about in this course are communicable. Noncommunicable diseases are either
inherited, the result of ingestion of toxins or caused by normal flora introduced into the
wrong environment. (ex. Puncture of the bowel introduces E. coli into the
peritoneal cavity.) Communicable diseases that are easily spread are said to be contagious. The combination of common symptoms and signs that are strongly related to a
particular disease are referred to as a syndrome.
A. The biochemical characteristics that influence the severity of disease that a
particular organism causes are referred to as virulence factors. Often times
virulence factors involve production of proteins that allow for adherence to certain
surfaces, aid in evading or destroying cells of the body's defenses or are secreted
toxins.
B. For an organism to cause disease it must gain entry to particular sites. The route
by which an organism gains entry is referred to as the portal of entry.
If the organism is acquired from the environment or from contact with another host, the
spread is said to be exogenous. Transfer of normal flora from its
appropriate site to a site where it will cause disease is referred to as endogenous
spread.
C. Though certain organisms can colonize the glands of the skin resulting in
inflammation and eventually disease, in most cases, introduction of organisms onto a
healthy skin surface results in little or no pathology. If the skin has been cut, abraded,
burned or otherwise disrupted, it is susceptible to infection.
D. The gastrointestinal (GI) tract is particularly susceptible to colonization by a
wide range of pathogens which usually gain entry to the gut when food or water
contaminated with feces is ingested. The pathogens will be shed in the feces. This is
referred to as the oral-fecal route of transmission. Often these pathogens
produce toxins that result in diarrhea or bleeding into the gut. Recently the role of
bacteria in certain types of ulcers (gastric and duodenal ulcers) has been demonstrated.
Often inanimate objects contaminated with microscopic amounts of feces can serve as a vehicle for the
transmission of a pathogen. Such inanimate objects are referred to as
fomites (fomites can play a role in the transmission of
respiratory and sexually transmitted pathogens as well).
E. Colonization of the pharynx, trachea, bronchi and pleura is not uncommon. In
many, if not most, cases the pathogens are inhaled. Once infection is established, the
organisms leave the host when secretions of the respiratory tract are aerosolized
during
sneezing or coughing. The droplets that are produced carry the organism. In some cases the
pathogen can survive the dehydration of this droplet. The remaining dried mucus is
referred to as a droplet nuclei. Organisms that can be transmitted via
droplet nuclei usually are more easily spread. This route of transmission is often
referred to as aerosol transmission.
1. Pneumonia refers to inflammation of the lungs which results in the
collection of fluid in the air spaces. Often this inflammation is caused by a pathogen
(bacterial, viral or fungal).
2. If the agent invades the lining of the lung (the pleura) it results in pleuritis.
Often initial infection of the respiratory tract with viral agents can result in an
opportunistic infection by bacteria that are part of the normal flora of the upper
respiratory tract.
F. Infection of the reproductive tracts and the male urinary tract often is the result
of sexual activity. Sexually transmitted diseases (STD's) involve pathogens
that colonize the reproductive organs and/or shed in the body fluids secreted during
intercourse. Condoms can reduce the risk of transmission of most STD's but they do not
eliminate the risk.
G. Infection of the fetus by transplacental transmission of pathogens is rare. Most
pathogens lack the ability to cross the placenta and thus do not invade the fetus. Those
organisms that can cross the placenta will do considerable damage to the fetus due to the
lack of a fully functioning immune system in the fetus. Cytomegalovirus which causes
little or no disease in the healthy adult does considerable damage to the fetus.
Rupture of the fetal membranes can lead to infection of the fetus by not only
normal flora of the vagina but also any pathogens that are present in the vagina or it's
secretions.
1. As the fetus passes through the birth canal, it will be inoculated with those
organisms present on the mucous membrane of the vagina and those organisms present in the
secretions of the vaginal. Women infected with gonococcus, herpes simplex
virus and other STD's will infected to the neonate as it is delivered.
2. Because of the immature state of the immune system in the newborn and the lack of
resident populations of bacteria, it is susceptible to many opportunistic infections.
H. Many pathogenic organisms require transfer of body fluids. This may involve sexual
contact, sharing contaminated needles or surgical equipment, blood transfusions or bug
bites.
1. The pathogens that cause sexual transmitted diseases (STD's) are found on the
mucous membrane of the reproductive tract or shed in the semen or vagina secretions.
These organisms usually are transmitted during the sex act as body fluids are
exchanged.
2. Parenteral transmission involves introduction of the
pathogen into the tissues, blood stream or internal body cavities usually by way of
injection. Sharing of needles often leads to parenteral transmission.
I. Disease agents are maintained in the environment in what is referred to as reservoirs.
1. Some pathogens continue their existence by constantly moving from one infected host
to another. Often this results in endemic occurrence of the disease in a population. An
example of this is the common cold. Other pathogens establish carrier states
in which the host shows no apparent signs of illness but continues to shed the organism
and infect those around them. Typhoid fever is a good example of this.
2. Certain diseases are maintained in animals and only occasionally infect humans. Such
diseases are referred to as zoonotic diseases. Rabies virus is a good
example of an agent disease that infects humans but usually only infects animals (skunks
in this part of the country).
3. Non-living reservoirs can include soil, water and food. In the case of water,
contamination by sewage or animal wastes can introduce parasitic worms or bacteria that
infect the gut. When contaminated water is used for cleaning, processing, or growing food
the contaminating organism can be transferred.
III. Factors that influence virulence include the volume of the
inoculum, the proteins
that the pathogen displays for adherence and the proteins that the pathogen releases into
the host.
A. For most pathogens the infectious dose (ID) is the minimum number of
organisms or virus particles needed to cause disease. For some diseases as few as one
agent is all that is needed but for most pathogens 100-10,000 microbes must be introduced
into the body for disease to be probable.
B. Proteins that allow the bacteria to adhere to the surface of certain
tissues also play a significant role in the development of disease. Lack of these proteins
precludes colonization and thus disease.
1. The capsule and fimbriae are most important in attachment. It should be noted that
the capsule also plays a role in the evasion of phagocytic cells such as macrophages.
2. A viruses ability to spread from the initial site of infection to other sites within
the body is limited by that viruses ability to attach to possible target cells.
Those viruses with the capacity to infect a wide range of cells will often create a
much more severe infection.
C. Release of proteins into the surrounding environment can cause several things to
occur.
1. Exoenzymes often destroy proteins or other chemicals designed to kill
or limit the spread of the pathogen.
a. Mucinase is release by several pathogens that cause intestinal
diseases. This protein destroys the protective layer of mucus on the intestine wall and
thus allows access to the underlying epithelial tissue.
b. Keratinase destroys the protein keratin which is found within cornified
epidermal cells. By destroying keratin in epidermal cells, the pathogen can gain entry
into the deeper layers of the epidermis.
c. Collagenase destroys collagen. This is the main protein of most
connective tissues. Once the collagen has been digested the pathogen can spread much more
readily.
d. Hyaluronidase dissolves the protein that cements cells together in many
tissues. Organisms that secrete this protein can then bypass the barrier normally
presented by these tissues. This allows the bacteria to more rapidly gain access to the
circulatory system.
2. Endotoxins are a component of the gram negative cell wall. They are
released when a cell dies and begins to undergo autolysis. Endotoxins are the lipopolysaccharide
found in the outer membrane of gram negative cell walls.
3. Toxins which are produced and then released from the bacterial cell are referred to
as exotoxins. Bacteria that can synthesize and release a toxin are said to
be toxogenic. Often bacteria infections will be accompanied by the release
of toxins into the blood stream; this is referred to as a toxemia. If the
bacteria produced the toxin outside the body (while growing in food) and then the toxin
was ingested the condition is referred to as an intoxication. In most cases
exotoxins are proteins.
a. Exotoxins that affect the kidneys are referred to as nephrotoxins.
Those that exert their effect on the mucous membrane of the digestive tract are referred
to as enterotoxins.
b. Those that result in the rupture of white blood cells and red blood cells are referred
to as leukotoxins and hemotoxins (or hemolysins),
respectively.
D. Many of the proteins mentioned above assist the bacteria in evading the body's
defenses. Along with these proteins the presence of a capsule plays a big role in
determining the pathogenicity of a microbe. The capsule appears to allow for adherence to
certain sites and to make the bacteria a poor target for phagocytosis. Phagocytosis by a
neutrophil or macrophage is a crucial first step in the recruitment of other white blood
cells and in the development of immunity.
IV. Within a single host a disease will "run a particular course". This is
reflected in the severity and duration of clinical signs and symptoms and involves several
distinct phases of infection:
A. Upon initial exposure to a pathogen there is a period in which no signs or symptoms
are present. This is referred to as the incubation period. During this
period the pathogen is increasing in numbers within the body and often spreading to other
sites in body. The prodomal period follows in which the patient begins to
feel sick but signs and symptoms are mild. The period of invasion or acute
phase of infection is the time when pathogens are at their highest level within
the body and signs and symptoms are most acute. If the patient is to recover, a convalescent
period follows. Depending on the pathogen, the patient will be able to transmit
the pathogen to others at certain stages of the disease's progression. Some pathogens are
released in high numbers only during the prodomal period; most are released in greatest
numbers during the period of invasion. Some are released at all stages and long after the
disease state has resolved.
1. Acute diseases usually develop quickly and are resolved quickly.
2. Diseases that do not resolve, resolve incompletely or reoccur are said to be chronic.
Usually these diseases to not develop quickly.
3. Subacute diseases never cause severe clinical signs. If they
produce so little pathology that the host is unaware of infection, they are said to
be subclinical.
4. Latent infections may result from a clinical or subclinical infection.
The disease agent is carried by the host for long periods of time without the host showing
any clinical signs. Depending on the pathogen, the host may be infectious during this long
period or not. In some cases, the infectivity of the host waxes and wanes.
5. Some
pathogens will create pathological changes of sites other than those sites
initially infected long after the resolution of the initial infection.
Such a pathological change is referred to as a sequela of
infection.
B. Symptoms are those effects of a disease that the patient describes.
Those changes in body function that can be observed or measured are referred to as clinical
signs. Consequently, a pediatrician working with a 1 year old, technically is
working only with signs and has no symptoms.
C. Within the host, the etiologic agent of a disease can demonstrate several different
types of distribution and different extents of pathology.
1. When the infection is limited in its spread throughout the body the infection is
said to be local. Puncture wounds and lacerations often introduce local
infections. Skin lesions such as athlete's foot or boils are also examples of local
infections.
2. Systemic infections result from the spread of the agent throughout the
body. Often this involves movement of organisms or toxins by way of the blood or lymph. If
a toxin produced by a local infection is spread by the blood it is referred to as toxemia.
When organisms are distributed by the blood it is referred to as a bacteremia.
If the organisms actually multiply in the blood it is a septicemia.
3. Infections caused by opportunistic pathogens in patients with immune systems that are
not fully functional due to an acute primary infection result in what
is referred to as a secondary infection. In the case of elderly or patients
with poorly functioning immune systems secondary infections create considerable problems.
Want to know more? Here are some good links!
Medical Microbiology
Textbook Normal Flora http://gsbs.utmb.edu/microbook/ch006.htm
Medical Microbiology Textbook University of Texas
University of Connecticut Fundamentals of Microbiology,
Mechanisms of
Pathogenesis
http://www.sp.uconn.edu/~terry/229sp98/pathogens.html
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