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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 association for the members of the symbiotic association.
Types of Symbiotic Associations
1. Mutualism. Both members of the association benefit.
2. Commensalism. There is no apparent benefit or harm to either member of the association.
3. Parasitism. One member of the association lives at the
expense
of the other member.
It is this type of a symbiotic association that draws our attention
in this course. For many parasites are or can become pathogens,
microorganisms
the cause disease.
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 is 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 are 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 might be referred to as an endogenous bacterial disease, referring to a disease brought on by bacteria 'from within'.
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 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 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.
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 immune 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.
The Immune System
The inducible defenses are so-called because they are induced upon primary exposure to 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.
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, perhaps 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.