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. Innate 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. Innate defenses include anatomical and structural barriers, inflammation, phagocytosis and the presence of a normal bacterial flora. The innate defenses have also been referred to as "natural" or "consitutive" 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 innate defenses, they are not immediately ready to come into play until after the host is appropriately exposed to the parasite. The inducible defenses are synonymous with acquired or adaptive immunity and involve the immunological responses to a pathogen causing an infection.

Adaptive immunity is generally quite specifically directed against an invading pathogen. The innate defenses are not so specific, and are directed toward general strategic defense. Innate defenses, by themselves, may not be sufficient to protect a host against pathogens. Such pathogens that evade or overcome the relatively nonspecific innate defenses are usually susceptible to the more specific inducible defenses, once they have developed.

Special note. Most immunologists have subverted some of the "innate" 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 innate defenses 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 or Adaptive 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.