The Nature of Bacterial Host-Parasite Relationships in Humans (page 2)
(This chapter has 2 pages)
© Kenneth Todar, PhD
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.
END OF CHAPTER
Previous Page