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Bacteriology at UW-Madison |
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.
Lactobacilli
in association with a vaginal epithelial cell (CDC).
2. Commensalism. There is no apparent benefit or harm to
either
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.
Staphylococcus
epidermidis
(CDC).
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
our
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
parasites exclusively.
Bacterial Pathogenesis
A pathogen is a microorganism (or virus) that is able to produce disease. Pathogenicity is the ability of a microorganism to cause disease in another organism, namely the host for the pathogen. As implied above, pathogenicity may be a manifestation of a host-parasite interaction.
In humans, some of the normal bacterial flora (e.g. Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae) are potential pathogens that live in a commensal or parasitic relationship without producing disease. They do not cause disease in their host unless they have an opportunity brought on by some compromise or weakness in the host's anatomical barriers, tissue resistance or immunity. Furthermore, the bacteria are in a position to be transmitted from one host to another, giving them additional opportunities to colonize or infect.
There are some pathogens that do not associate with their host except in the case of disease. These bacteria may be thought of as obligate pathogens, even though some may rarely occur as normal flora, in asymptomatic or recovered carriers, or in some form where they cannot be eliminated by the host.
Opportunistic Pathogens
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,
referring to
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
Pseudomonas
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.
Infection
The normal flora, as well as any "contaminating" bacteria from the environment, are all found on the body surfaces of the animal; the blood and internal tissues are sterile. If a bacterium, whether or not a component of the normal flora, breaches one of these surfaces, an infection is said to have occurred. Infection does not necessarily lead to infectious disease. In fact, infection probably rarely leads to infectious disease. Some bacteria rarely cause disease if they do infect; some bacteria will usually cause 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, play 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 traits 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 disease are the pathogen's determinants of virulence.
Some pathogens may rely on a single determinant of virulence, such
as
toxin production, to cause damage to their host. Thus, bacteria such as
Clostridium
tetani and Corynebacterium diphtheriae, which have hardly
any
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,
Streptococcus
pyogenes and Pseudomonas aeruginosa, maintain a large
repertoire
of virulence determinants and consequently are able to produce a more
complete
range of diseases that affect different tissues in their host.
A
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.
Properties of the Host
The host in a host-parasite interaction is the animal that maintains the parasite. The host and parasite are in a dynamic interaction, the outcome of which depends upon the properties of the parasite and of the host. The bacterial parasite has its determinants of virulence that allow it to invade and damage the host and to resist the defenses of the host. The host has various degrees of resistance to the parasite in the form of the host defenses.
Host Defenses
A healthy animal can defend itself against pathogens at different stages in the infectious disease process. The host defenses may be of such a degree that infection can be prevented entirely. Or, if infection does occur, the defenses may stop the process before disease is apparent. At other times, the defenses that are necessary to defeat a pathogen may not be effective until infectious disease is well into progress.
Typically the host defense mechanisms are divided into two groups:
1. Constitutive Defenses. Defenses common to all healthy animals. These defenses provide general protection against invasion by normal flora, or colonization, infection, and infectious disease caused by pathogens. The constitutive defenses have also been referred to as "natural" or "innate" resistance, since they are inherent to the host.
2. Inducible Defenses. Defense mechanisms that must be induced or turned on by host exposure to a pathogen (as during an infection). Unlike the constitutive defenses, they are not immediately ready to come into play until after the host is appropriately exposed to the parasite. The inducible defenses involve the immunological responses to a pathogen causing an infection.
The inducible defenses are generally quite specifically directed
against
an invading pathogen. The constitutive defenses are not so specific,
and
are directed toward general strategic defense. The constitutive
defenses,
by themselves, may not be sufficient to protect the host against
pathogens.
Such pathogens that evade or overcome the relatively nonspecific
constitutive
defenses are usually susceptible to the more specific inducible
defenses, once they have developed.
Special note. Most
immunologists have subverted some of the "constitutive" defenses and
moved them to the "inducible" category, although these defenses are not
usually thought of as part of the immunological system. This refers to
complement activation, the inflammatory response and the phagocytic
response. Their reasoning is that these responses are, in fact,
elicited or turned on by some chemical, physical or biological
stimulation. However, the components or cells involved are constitutive
components of the host. Nonetheless, these innate responses to
pathogens may initiate, participate with, or otherwise affect an
immunological response.
The Immune System
The inducible defenses are so-called because they are induced upon
primary
exposure to a pathogen or one of its products. The inducible
defenses
are a function of the immunological system and the immune
responses.
The constitutive defenses are innate and immediately available for host
defense. The inducible defenses must be triggered in a host and
initially
take time to develop. The type of resistance thus developed in the host
is called acquired immunity. The term immune usually
means
the ability to resist infectious disease. Immunity refers to
the
relative state of resistance of the host to a specific pathogen brought
on by the activities of the immunological system.
Acquired immunity, itself, is sometimes divided into two types, based on how it is acquired by the host.
In active immunity, the host undergoes an immunological response and produces the cells and factors responsible for the immunity, i.e., the host produces its own antibodies and/or immuno-reactive lymphocytes. Active immunity can persist a long time in the host, up to many years in humans.
In passive immunity there is acquisition by a host of immune factors which were produced in another animal, i.e., the host receives antibodies and/or immuno-reactive lymphocytes originally produced in another animal. Passive immunity is typically short-lived and usually persists only a few weeks or months.
Antigens
Antigens are chemical substances of relatively high molecular weight, that stimulate the immune response in animals. Bacteria are composed of various macromolecular components that are antigens or " antigenic" in their host and bacterial antigens interact with the host immunological system in a variety of ways.
Natural Antibodies
Studies on germ-free animals have confirmed that a normal bacterial flora in the gastrointestinal tract are necessary for full development of immunological (lymphatic) tissues in the intestine. Furthermore, the interaction between these immune tissues and intestinal bacteria results in the production of serum and secretory antibodies that are directed against bacterial antigens. These antibodies probably help protect the host from invasion by its own normal flora, and they can cross react with antgenically-related pathogens. For example, antibodies against normal E. coli could react with closely-related pathogenic Shigella dysenteriae. These type of antibodies are sometimes called natural or cross-reactive antibodies.
Bacterial Antigens made into Vaccines
In another way, bacterial antigens that are the components or products of pathogens are the substances that induce the immune defenses of the host to defend against, and to eliminate, the pathogen or disease. In the laboratory, these bacterial antigens can be manipulated or changed so that they will stimulate the immune response in the absence of infection or pathology. These isolated or modified antigens are the basis for active immunization (vaccination) against bacterial disease. Thus, a modified form of the tetanus toxin (tetanus toxoid), which has lost its toxicity but retains its antigenicity, is used to immunize against tetanus. Or, antigenic parts of the whooping cough bacterium, Bordetella pertussis, can be used to induce active formation of antibodies that will react with the living organism and thereby prevent infection.
Antimicrobial Agents
One line of defense against bacterial infection is chemotherapy with antimicrobial agents such as antibiotics. The ecological relationships between animals and bacteria in the modern world are mediated by the omnipresence of antibiotics. Antibiotics are defined as substances produced by a microorganism that kill or inhibit other microorganisms. Originally, a group of soil bacteria, the Streptomyces, were the most innovative producers of antibiotics for clinical usage. They were the source of streptomycin, tetracycline, erythromycin and chloramphenicol, to name just a few antibiotics. Because bacteria evolve rapidly toward resistance, because bacteria can exchange genes for antibiotic resistance, and because we have overused and misused antibiotics, many pathogens are emerging as resistant to antibiotics. There have already been reported infections by Enterococcus, Staphylococcus aureus and Pseudomonas aeruginosa that are refractory to all known antibiotics. Bacterial resistance to antimicrobial agents has become part of a pathogen's determinants of virulence. These are examples of genetic means by which bacteria exert their virulence.
The usage of antibiotics to control the growth of parasites is an artificial way to intervene in the natural process of the host-parasite interaction. But, of course, it is done for the obvious purpose of curing the disease. The body heals itself: most antibiotics just stop bacterial growth, and the host must rely entirely on its native defenses to accomplish the neutralization of bacterial toxins or the elimination of bacterial cells. The judicious use of antibiotics in the past five decades has saved millions of lives from infections caused by bacteria.