Host pathogen interactions

Host-Pathogen
Interactions
Submitted by PALANI ANANTH.S
II M.Sc. Microbiology

Introduction
 Microbes are found everywhere. On a daily basis we ingest, inhale, and transport thousands of
organisms (i.e., bacterial, viral, protozoal, or parasitic). Most have no ill effects due to protective
mechanisms in our body (i.e., coughing, urinating, sneezing, and defecating). Humans and animals have
“friendly” organisms throughout their bodies that survive as normal flora and colonize a host but do not
cause disease. How- ever, if these organisms begin to cause any obvious damage to a host by invading
tissues and/or producing toxins, they then become pathogens
significance of host-pathogen interactions
 Host-pathogen interactions provide information that can help scientists and researchers understand disease
pathogenesis, the biology of one or many pathogens, as well as the biology of the host. It is through these
interactions that basic research discoveries are made

Koch’s Postulates: Experimental Steps to Determine Disease
Causation
Over a century ago, Robert Koch established that infectious diseases were caused by microbes. He was
looking for the causative agent for anthrax. Koch’s postulates are experimental criteria that are used to
determine if a microbe caused a specific disease. The criteria include:
 The microbe must be present in every case of the disease.
 The organism must be grown in a pure culture from diseased hosts.
 The same disease must be produced when a pure culture of the organism is introduced into a susceptible
host.
 The organism must be recovered from the experimentally infected hosts.
However, there are some exceptions to these criteria. These include:
 Some organisms cannot be cultured in a lab and grown on artificial media.
 Some pathogens can cause several disease conditions such as M. tuberculosis, which can cause lung disease
and other diseases of the skin, bone, and internal organs.
 There may be ethical reasons that do not allow testing, (i.e., human diseases with no animal mod- el –
smallpox, rubella).

Host-Pathogen Interactions
 An individual’s or animal’s skin and mucous membranes generate an
environment for microorganisms to interact with the body. This interaction
between the host and the organism is referred to as symbiosis. There are three
forms of symbiotic relationships that can occur at an anatomical level. In order
for this to take place the following components will need to occur:
 Mutualism: In mutualism, both the microorganism and the body work together.
An example of this relationship would be cows and the bacteria in their rumen.
Bacterial cellulose facilitates digestion in the animal, while the bacteria benefit
from nutrients in the rumen.
 Commensalism: In commensalism, either the body or the microorganism
benefits, while the other is not affected by the interaction. Examples of this
include microorganisms that make up the normal flora that inhabit the eyes.
These organisms thrive on secretions and dead cells, but do not affect the host.
 Parasitism: In parasitism, one organism benefits at the expense of the other. For
example, parasites use the gastrointestinal tract of a human or animal as an
environment in which to reproduce.
Components for the host pathogen
interaction.

The Body’s Normal Flora
 The body contains two types of normal flora: 1) resident flora (survive for extended periods), and 2) transient flora
(temporary). Normal flora help to provide defenses against invading pathogens by covering adherence sites,
producing compounds toxic to other organisms, and preventing pathogens from consuming available nutrients. For
disease to occur there must be a change in the body’s environment, which, in turn, allows the pathogen to
overcome the normal flora. This can occur through a change in the pH of the body or elimination of normal flora
due to antibiotics.
Host Defenses
 A microorganism will not be able to invade unless it overcomes an animal’s or individual’s host defenses. Specific
host defenses may include:
 skin and mucosal secretions
 Non-specific local responses (e.g., pH)
 Non-specific inflammatory responses
 Specific immune responses (e.g., lymphocytes)
 The ability for a pathogen to overcome host defenses can be accomplished by two distinct components: a primary
pathogen (causes disease in a healthy host) or opportunistic pathogen (causes disease if host is
immunocompromised).

Pathogen Defenses
 Pathogens contain virulence factors that promote disease formation and provide the opportunity
for a microbe to infect and cause disease. The greater the virulence, the more likely disease will
occur. Such factors include:
 Ability of a pathogen to adhere to a host
 Ability of a pathogen to colonize (overcome) a host
 Ability of a pathogen to evade host defenses

Mechanisms of pathogenesis.
 Pathogenesis is the method by which a disease can develop. This can
occur through foodborne intoxication where the causative agent
produces toxins in the body (e.g., botulism). Another route is the
colonization of an invading pathogen on the host surface, which
allows the pathogen to increase in numbers and produce toxins that
are damaging to the host’s cells (e.g., Vibrio and Corynebacterium).
 Pathogenesis can also occur by pathogens invading and breaching
the body’s barrier in order to multiply. These organisms have
mechanisms that will not allow macrophages (the body’s defense
against pathogens) to destroy them. They can also evade antibody
detection (e.g., tuberculosis and plague). Finally, organisms can
invade tissues within the body and produce toxins (e.g., Shigella).
 The relationship between a host and pathogen is dynamic.
Production of disease occurs through a process of steps. The first five
mechanisms make up a pathogen’s invasiveness (i.e., ability to invade
tissues).

Transmission
 In order to begin infection and eventually cause disease, pathogens must find a transmission route. Trans-
mission of an infectious agent can occur in many ways, but it is typically through exposed skin (e.g., a cut,
abrasion, puncture, or wound) or mucous membranes (e.g., gastrointestinal tract, respiratory tract, or
urogenital tract)
Adherence
 Once the pathogen has gained access to the body, it must have some means of attaching itself to the host’s
tissues. This attachment is called adherence and is a necessary step in pathogenicity. Microbes contain
ligands, which are projections that attach host receptors or surface proteins. Pathogens may have specific
adherence mechanisms to attach to cells or tissue surfaces. Examples of this include: 1) tissue tropism (i.e.,
pathogens that prefer specific tissues over others), 2) species specificity (i.e., pathogens that only infect
certain species), and 3) genetic specificity (i.e., surface mutations that occur so previous antibodies do not
recognize the invading pathogen). If a microorganism cannot adhere to a host cell membrane, disease will
not occur.

Invasion
 At this point, microbes begin to invade the host and produce a bacteremia (i.e., presence of bacteria in the
bloodstream) or viremia (presence of a virus in the bloodstream). Microorganisms are exposed to many
barriers after introduction into the host. Some bacteria are able to cause disease while remaining on the
epithelial barriers, while many need to penetrate that barrier. Once this barrier has been penetrated, these
pathogens can multiply without competition.
Colonization
 Colonization is the multiplication of pathogenic organisms where toxins are produced and the normal flora
are overcome. During this stage, pathogens compete with normal flora for space and nutrients. Pathogens
usually colonize host tissues that are in contact with the external environment. During colonization, the host
begins to show signs of septicemia (i.e., blood infection where bacteria are reproducing). For infection to
proceed an infectious dose should be determined. This is the minimal number of microbes necessary to
establish infection. Certain pathogens are less contagious and therefore require larger numbers of pathogens
to cause disease (i.e., 10-100 for Shigella and 1,000,000 for Salmonella).

Evasion of Host Defenses
 After colonization, pathogens circumvent the host’s innate and adapted defenses by phagocytosis. Multiple
mechanisms are used by pathogens to evade a host’s immune system. For the innate system this includes:
 Intracellular pathogens that live inside a host cell
 Avoid phagocyte recognition by producing capsules prevents phagocytosis
 Producing membrane damaging toxins which can kill phagocytes (e.g., leukocidins)
 Interfere with complement activation
 Survive in the phagocyte
 Pathogens must also avoid adapted defenses. Pathogens can produce proteases (i.e., allow each pathogen to
avoid antibodies) or catalases (i.e., prevents the digestion of an engulfed pathogen). They can also uti- lize
antigenic variation to alter the antigen structure. In addition, pathogens can mimic host molecules, which can
cause disease-related damage.

Cause Damage or Disease to Host While Avoiding Host Defenses
 Damage can occur through direct or indirect path- ways. Direct methods produce toxins, which are poisonous
substances that produce toxemia within a host.
Three types of toxins are produced to cause damage:
 Exotoxins: Proteins secreted by pathogens that cause damage to the host (botulinum toxin, teta-
nus toxin, hemolysin (ruptures red blood cells).
 Endotoxins: Toxic substances that are released when a cell is killed (Lipolysaccharides).
 Exoenzymes: Enzymes that function outside the host cells or tissues. These include coagulase
(forms a fibrin clot that “hides” the microbe from phagocytosis), hyaluronidase (breaks connective
tissues down), or fibrinase (breaks down blood clots to allow pathogens to continue spreading).

Exiting the Host
A pathogen must exit the body. This occurs through various routes. Examples include
sneezing, coughing, diarrhea, coitus, pus, blood, or insect bites.
Survival Outside the Host
Finally, a pathogen must be able to survive in the environment long enough to be
transmitted to another host. Some are hardy and can survive for several weeks be- fore a new
host is found. There are others that survive in animal reservoirs or require direct contact because
they are fragile.

Infectious Diseases
 An infectious disease is a clinically evident deviation from health. It occurs when there is a
parasitic relationship between a host and a microorganism. Several different factors influence a
microorganism’s relationship to its host and level of severity. These include:
 Pathogenicity: The ability to produce disease in a host organism.
 Virulence: The degree of pathogenicity of a microorganism. Determinants of virulence for a
pathogen include a pathogen’s genetic, biochemical, or structural features. For example, one
strain of influenza may only cause a fever and sore throat, while another may cause pneumonia
or other serious respiratory condition.
 Infectivity: The level at which a microorganism is able to infect or invade a host.
 Transmissibility: The measure of a microorganism’s ability to spread from one host to the next.
This can include both distance and number of affected individuals.

Infectious Diseases
Disease Stages
 Infections can be either subclinical or in apparent. If clinical signs are seen, disease occurs. The incubation
period is the time between infection and first appearance of signs or symptoms. This period depends on the
dose level of microbes, microbe type, virulence, and host health.
 The prodromal period of 1 to 2 days follows incubation in some diseases. This includes early, mild signs/
symptoms such as, fatigue, muscle aches, and/or headache. Recovery occurs when signs/symptoms are
subsiding but the host may remain more susceptible to secondary, opportunistic infections. During the
convalescent period, the body returns to a pre-diseased state.

Host pathogen interactions

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