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The Nature of Bacterial Host-Parasite Relationships in Humans (page 1)
(This chapter has 2 pages)
© Kenneth Todar, PhD
Bacteria are consistently associated with the body surfaces of animals.
There are many more bacterial cells on the surface of a human
the gastrointestinal tract) than there are human cells that make up the
animal. The bacteria and other microbes that are consistently
associated with an animal are
called the normal flora, or more properly the "indigenous microbiota", of the
animal. These bacteria have a full range of symbiotic
interactions with their animal hosts.
In biology, symbiosis is
defined as "life together", i.e., that two organisms
live in an association with one another. Thus, there are at least
three types of relationships based on the quality of the relationship
for each member of the symbiotic association.
Types of Symbiotic Associations
1. Mutualism. Both members of the association benefit. For
humans, one classic mutualistic association is that of the the lactic
acid bacteria that live on the vaginal epithelium of a woman. The
bacteria are provided habitat with a constant temperature and supply of
nutrients (glycogen) in exchange for the production of lactic acid,
which protects the vagina from colonization and disease caused by yeast
and other potentially harmful microbes.
in association with a vaginal epithelial cell (CDC).
2. Commensalism. There is no apparent benefit or harm to
member of the association. A problem with commensal relationships is
that if you look at one long enough and hard enough, you often discover
that at least one member is being helped or harmed during the
association. Consider our relationship with Staphylococcus epidermidis, a
consistent inhabitant of the skin of humans. Probably, the bacterium
produces lactic acid that protects the skin from colonization by
harmful microbes that are less acid tolerant. But it has been suggested
that other metabolites that are produced by the bacteria are an
important cause of body odors (good or bad, depending on your personal
point of view) and possibly associated with certain skin cancers.
"Commensalism" best works when the relationship between two organisms
is unknown and not obvious.
3. Parasitism. In biology, the term parasite refers to an organism that
grows, feeds and is sheltered on or in a different organism while
contributing nothing to the survival of its host. In microbiology, the
mode of existence of a parasite implies that the parasite is capable of
causing damage to the host. This type of a symbiotic association draws
attention because a parasite may become pathogenic if the damage to the
host results in disease. Some parasitic bacteria live as normal flora
of humans while waiting for an opportunity to cause disease. Other
nonindigenous parasites generally always cause disease if they
associate with a nonimmune host.
Parasitology, actually a
branch of microbiology, refers to the scientific study of parasitism
but somehow it developed into a discipline that deals with eucaryotic
A pathogen is a microorganism (or virus) that is able to
disease. Pathogenicity is the ability of a microorganism to
disease in another organism, namely the host for the pathogen.
implied above, pathogenicity may be a manifestation of a host-parasite
In humans, some of the normal bacterial flora (e.g. Staphylococcus
aureus, Streptococcus pneumoniae, Haemophilus
are potential pathogens that live in a commensal or parasitic
without producing disease. They do not cause disease in their host
they have an opportunity brought on by some compromise or weakness in
host's anatomical barriers, tissue resistance or immunity. Furthermore,
the bacteria are in a position to be transmitted from one host to
giving them additional opportunities to colonize or infect.
There are some pathogens that do not associate with their host
in the case of disease. These bacteria may be thought of as obligate
though some may rarely occur as normal flora, in asymptomatic or
carriers, or in some form where they cannot be eliminated by the host.
Bacteria which cause a disease in a compromised host which typically
would not occur in a healthy (noncompromised) host are acting as opportunistic
pathogens. A member of the normal flora can such as Staphylococcus
aureus or E. coli can cause an opportunistic infection,
but so can an environmental organism such as Pseudomonas aeruginosa.
When a member of the normal flora causes an infectious disease, it
sometimes referred to as an endogenous bacterial disease,
a disease brought on by bacteria 'from within'. Classic opportunistic
infections in humans are dental caries and periodontal disease caused
by normal flora of the oral cavity.
A photomicrograph of
aeruginosa, one of the most
common opportunistic pathogens of
humans. The bacterium causes urinary tract infections, respiratory
system infections, dermatitis, soft tissue infections, bacteremia and a
variety of systemic infections, particularly in cancer and AIDS
patients who are immunosuppressed. CDC.
The normal flora, as well as any "contaminating" bacteria from the
are all found on the body surfaces of the animal; the blood and
tissues are sterile. If a bacterium, whether or not a component of the
normal flora, breaches one of these surfaces, an infection is
to have occurred. Infection does not necessarily lead to infectious
In fact, infection probably rarely leads to infectious disease. Some
rarely cause disease if they do infect; some bacteria will usually
disease if they infect. But other factors, such as the route of entry,
the number of infectious bacteria, and (most importantly) the status of
the host defenses,
a role in determining the outcome of infection.
Determinants of Virulence
Pathogenic bacteria are able to produce disease because they possess
certain structural or biochemical or genetic
that render them pathogenic or virulent. (The term virulence
is best interpreted as referring to the degree of pathogenicity.)
The sum of the characteristics that allow a given bacterium to produce
are the pathogen's determinants of virulence.
Some pathogens may rely on a single determinant of virulence, such
toxin production, to cause damage to their host. Thus, bacteria such as
tetani and Corynebacterium diphtheriae, which have hardly
invasive characteristics, are able to produce disease, the symptoms of
which depend on a single genetic trait in the bacteria: the ability to
produce a toxin. Other pathogens, such as Staphylococcus aureus,
pyogenes and Pseudomonas aeruginosa, maintain a large
of virulence determinants and consequently are able to produce a more
range of diseases that affect different tissues in their host.
photomicrograph of Corynebacterium
diphtheriae bacteria using a
Gram stain technique.
Corynebacterium diphtheriae causes
diphtheria that affects the upper respiratory tract, where an
inflammatory exudate causes severe obstruction to the breathing
airways, and sometimes suffocation. CDC.