Host Microbe Interactions

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Host Microbe Interactions

• Kathy Huschle
• Northland Community and Technical College

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Host Microbe Interactions

• daily we
• ingest thousands of microorganisms on the food we
  eat
• inhale hundreds of thousands of microorganisms in
  the air we breath
• have microorganisms stick to us wherever we go
• most of these invaders have no ill effect on us
  as we slough, cough, gag, urinate and defecate
  them away
• we are also protected by the friendly resident
  microorganisms found throughout our body

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Host Microbe Interactions

• microorganisms very easily colonize the warm,
  moist, nutrient rich environment we call the
  human body
• usually they live as normal flora
• in some cases, they are able to overcome the
  bodies defenses, and cause disease
• organisms that can cause any noticeable damage,
  invade tissue, or produce toxins are called
  pathogens
• please review the terms used for the study of
  infectious disease found in Table 19.1 on page
  460 in your text

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Anatomical Barriers

• in addition to providing barriers to the
  microbial world, skin and mucous membranes also
  create an environment for interacting
  microorganisms and the human body
• these interactions are referred to as symbiosis,
  which means living together
• the players in symbiosis are referred to as
  symbionts

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Anatomical Barriers

• symbiotic relationships between microorganisms
  and a host include
• mutualism
• commensalism
• parasitism

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Mutualism

• in this type of relationship, both partners
  benefit
• E. coli synthesizes vitamin K in the intestine
• in exchange the large intestine provides
  nutrients necessary for survival of the
  microorganisms

E. coli
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Commensalism

• one organism is benefited and the other is
  unaffected by this type of relationship
• many of the microorganisms that make up our
  normal flora inhabit places like the eyes, ears,
  and external genitalia
• these bacteria live on secretions and sloughed
  off cells
• they bring no benefit to the host and yet the
  microorganisms benefit greatly from the
  environment they inhabit

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Parasitism

• one organism benefits at the expense of the
  other
• all pathogens are parasites

parasitic microorganisms
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Normal Flora

• microorganisms that colonize a host without
  causing disease
• two types of normal flora exist
• resident flora are microorganisms that inhabit
  sites on the body for extended periods
• transient flora are microorganisms that are
  temporary

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Normal Flora

• the presence of normal flora
• cover potential adherence sites for invading
  microorganisms
• consume the available nutrients
• produce compounds toxic to other microorganisms

bacteria found on skin
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Normal Flora

• when the balance between normal flora and
  pathogens is upset, disease can result
• the normal bacterial microorganisms of the adult
  human vagina maintain the pH at about 3.4 4.5
• the presence of this normal flora inhibits the
  overgrowth of Candida albicans, yeast

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Normal Flora

• if the presence of the normal flora is eliminated
  by antibiotics, or excessive douching, the pH of
  the vagina becomes nearly neutral, creating an
  environment very conducive to the growth of C.
  albicans

C. albicans
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Principles of Infectious Disease

• a parasitic relationship between a microorganism
  and a host is called an infection
• infections can be subclinical or inapparent
  meaning no symptoms or the symptoms are so mild
  as to be noticed
• infection that causes impairment of body function
  is called disease

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Principles of Infectious Disease

• pathogenicity is the ability of a microorganism
  to cause disease by overcoming the host defenses
• this can be accomplished with
• a primary pathogen microorganism capable of
  causing disease in a healthy host
• opportunistic pathogen is only capable of
  causing disease when the immune system is
  overcome, or the organism is introduced to an
  unusual location
• opportunistic organisms can be part of the normal
  flora or found in the environment

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Principles of Infectious Disease

• the more virulent a pathogen is the more disease
  promoting attributes it possesses
• virulence factors are substances or features of a
  microorganism that help it infect and cause
  disease
• they may include
• ability to adhere
• ability to overcome host defense
• ability to evade host defense

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Terminology of Infectious Diseases

• communicable
• disease that spreads from one host to another,
  either through direct or indirect contact
• infectious dose
• of microbes needed to establish infection
• some microorganisms are less contagious than
  others and as a result require a larger number of
  pathogens present to establish disease

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Terminology of Infectious Diseases

• sign
• objective changes that are observable and
  measurable
• examples of signs include swelling, fever,
  paralysis
• symptoms
• subjective effects experienced by patient
• examples of symptoms include pain or nausea

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Terminology of Infectious Diseases

• disease stages
• incubation
• the time between the initial infection and the
  first appearance of any signs or symptoms
• this time can vary depending on the pathogen and
  the condition of the host
• illness
• signs and symptoms of the disease are experienced
• if the disease is not successfully overcome or
  treated, the patient dies during this period
• convalescence
• person regains strength and the body returns to
  its pre-diseased state

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Terminology of Infectious Diseases

• types of infectious diseases
• acute disease
• rapid onset, short duration
• influenza is an acute disease
• chronic disease
• develop slowly, last longer
• the bodys reaction may be less severe
• hepatitis B is a chronic disuse
• latent disease
• causative agent is never completely eliminated
• remains inactive, but can become reactivated and
  symptomatic if immune response is diminished
• shingles is a latent disease

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Terminology of Infectious Diseases

• localized infections
• invading microorganisms are limited to a small
  area
• boils and abscesses are local infections
• systemic infections
• infectious agent spread throughout body by blood
  or lymph
• measles is a systemic infection

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Establishing the Cause of Infectious Disease

• Kochs Postulates
• criteria for establishing the fact that specific
  microbes cause specific diseases
• determined by Robert Koch in 1877 while looking
  for the causative agent for anthrax

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Establishing the Cause of Infectious Disease

• Kochs Postulates
• in order to determine that a microbe causes a
  particular disease, the following postulates must
  be met
• The microorganism 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.

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Establishing the Cause of Infectious Disease

• exceptions to Kochs Postulates include
• some organisms cannot be cultured in a lab
• some pathogens can cause several disease
  conditions such as S. pyogens which can cause
  sore throat, scarlet fever, skin infections and
  other diseases
• there may be ethical reasons that does not allow
  testing

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Mechanisms of Pathogenesis

• pathogenesis is the manner in which a disease
  develops
• patterns that disease-causing microorganisms may
  follow include
• production of ingested toxins
• foodborne intoxication
• the causative agent must produce toxins
• few organisms are capable of causing disease this
  way, the few that can include Clostridium
  botulinum or Staphylococcus aureus

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Mechanisms of Pathogenesis

• colonization of host surface, then toxin
  production
• invading pathogen is able to grow to high numbers
  on host surfaces such as the respiratory and
  intestinal tract
• they then produce a toxin that is damaging to the
  cells
• organisms that use this mechanism include Vibrio
  cholerae, which causes cholera or Corynebacterium
  diphtheriae, which causes diphtheria

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Mechanisms of Pathogenesis

• invasion of host tissue
• breaching bodys barriers then multiplies in the
  bodys tissues
• these organisms have mechanisms that allow them
  to avoid macrophage destruction
• some are also capable of avoiding detection by
  antibodies
• organisms that use this mechanism include
  Mycobacterium tuberculosis, causative agent for
  tuberculosis, and Yersinia pestis, causative
  agent for plaque

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Mechanisms of Pathogenesis

• invasion of tissue, then toxin production
• breach the bodys barriers, then make toxins
• in addition to invasion, these organisms also
  make toxins
• organisms that use this mechanism include
  Shigella dysenteriae and Streptococcus pyogenes

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Mechanisms of Pathogenesis

• in order to cause disease microorganisms need to
  be able to
• adhere and colonize host tissue
• avoid the innate defenses
• avoid the adapted defenses
• cause damage related to the disease

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Adherence

• to establish disease the causative agent needs to
• adhere
• difficult to overcome our first-line defenses so
  adherence is imperative
• many bacteria have adhesions, generally found on
  the pili

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Colonization

• causative agent needs to
• multiply in order to colonize
• to multiply, they must compete successfully with
  the normal flora for space and nutrients
• toxins that may be produced by the normal flora
  must be overcome

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Avoiding Innate System

• while some bacteria are able to cause disease
  while remaining on the surface of the skin or
  mucosa, many need to penetrate that barrier
• once this is done, those pathogens have it on
  easy street, exclusive rights to rich nutrition
  and multiplying without any competition
• penetrating the skin is extremely difficult
• bacteria take advantage of trauma to provide a
  break in the skin
• West Nile Virus is transmitted to the host
  through a mosquito bite, a penetration of the
  skin

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Avoiding Innate System

• mucous membranes penetration is the most common
  entry for most microorganisms
• one method that is used is referred to as
  ruffling
• once the microorganism attaches to the membrane,
  it can direct the that cell to engulf the
  bacterium this is referred to as ruffling

ruffling on the surface of mucous membrane
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Avoiding Innate and Adaptive System

• several mechanisms can be used by microorganisms
  to avoid the potentially lethal effects of our
  immune system
• hide inside a host cell
• phagocytes, complement and antibodies cant find
  them remember self and non-self!
• interfere with the activation of complement
  (which attracts phagocytes)

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Avoiding Innate and Adaptive System

• avoid destruction by phagocytes by simply
  preventing encounters with phagocytes
• C5a peptides are an enzyme that is made by some
  bacteria
• C5a peptide destroys the complement component
• if the complement is not activated, neither are
  the phagocytes
• some bacteria produce membrane-damaging toxins
  that kill phagocytes

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Avoiding Innate and Adaptive System

• avoid recognition and attachment to phagocytes by
• producing capsules to prevent phagocytosis
• Streprococcus pneumoniae procduces capsules
• survive in the phagocyte
• they dont worry about being engulfed, simply
  enjoy the free ride
• some microorganisms can escape from the phagosome
  before being fused with the enzyme lysosome

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Avoiding Innate and Adaptive System

• survive in the phagocyte
• some microorganisms can block the fusion of the
  phagosome and lysosome
• a few organisms can actually survive the lysosome
  environment

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Avoiding the Adaptive System

• avoiding antibodies which integral to the
  adaptive system this can be accomplished several
  ways including
• IgA protease
• cleaves IgA class of antibodies found in mucus
  and other secretions
• antigenic variation
• alter structure of antigens
• stay ahead of antibody production and destruction
  by altering the structure the antibodies are
  searching for
• mimic host molecules
• some microorganisms have the ability to cover
  themselves with molecules similar to self

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Host Damage

• in order for disease to happen damage of some
  sort must happen to the host
• in most cases damage to the host facilitates
  dispersal of the pathogen
• damage to the host can occur either
• directly
• indirectly

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Direct Host Damage

• toxins produced by the invading pathogen cause
  direct damage to the host which results in
  disease
• toxins capable of causing damage include
• exotoxins
• a protein toxin released from a living cell
• mostly found in Gram cells

Bacillus anthraxis produces an exotoxin
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Exotoxoins

• exotoxins are secreted by the bacteria or
  released following lysis
• exotoxins are soluble in body fluids which makes
  them easily diffused into blood and then are
  rapidly transported throughout the body
• exotoxins work by destroying particular parts of
  the host cells or by inhibiting certain metabolic
  functions

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Exotoxins

• exotoxins are highly specific
• exotoxins are among the most lethal substances
  known to man
• 1 gram of the exotoxin produced from Clostridium
  botulinum is capable of killing the entire
  population of the United States, close to 300
  million people
• the danger with exotoxins is not the ingestion of
  the bacterium, but the ingestion of the toxin

Clostricium botulinum
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Exotoxins

• most exotoxins are grouped according to the
  tissues they adversely impact
• neurotoxins damage the nervous system
• entereotoxins upset the intestinal system
• cytotoxins afflict their damage on many different
  types of cells by disrupting cellular function of
  by lysing the cell
• please look at the list of exotoxins in Table
  19.2 on page 47e in your textbook

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Endotoxins

• endotoxins are lioopolysaccharides (LPS) found in
  the lipid portion of the outer wall of Gram
  bacteria
• endotoxins are released when Gram bacteria die
  and the cell wall undergoes lysis
• antibiotics that are used to treat Gram
  diseases can lyse the bacterial cells, releasing
  the endotoxin
• this can lead to an immediate worsening of the
  symptoms
• these symptoms usually improve as the endotoxins
  break down

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Endotoxins

• endotoxins can also activate blood-clotting
  proteins, causing the formation of small blood
  clots
• blood clots obstruct capillaries, resulting in
  decreased blood supply, which can lead to tissue
  death
• this is referred to as disseminated intravascular
  coagultaion
• endotoxins also cause fever (pyrogenic response)
  and rapid blood pressure decrease

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Immune Response Damage

• inflammatory response can destroy tissue
• antibody-antigen complexes formed during the
  immune response settle in kidneys and joints
• activates complement, which produces damaging
  inflammation

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Viral Pathogenesis Mechanisms

• viral pathogenesis is very dependent on
• gaining access to the host
• evading the hosts defenses
• causing damage to or death of the host cell while
  continuing to reproduce themselves
• viral access to the host was discussed in the
  virus lecture remember viral attraction is
  specific to the host
• viruses bind more successfully to organisms found
  in mucous membranes

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Viral Evasion of Host

• interferons play a role in limiting the ability
  of viruses moving from neighbor cell to neighbor
  cell
• once infected cells are capable of producing a
  protein that can regulate and limit viral
  replication
• some viruses are able to encode proteins to shut
  down this cellular protective device

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Viral Evasion of Host

• though limited in the ability to control viruses,
  those few antibodies that are used can be
  circumvented by viruses that have developed
  methods to transfer directly from one cell to its
  immediate neighbor
• since antibodies control viruses by neutralizing
  extracellular viral particles, the above renders
  this useless

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Virus and Host Damage

• some viruses take-over and destroy the cell
• virus causes inflammatory response more damage
  more activation of inflammatory response
• often times, in particular with the case of the
  common cold, the inflammatory response initiated
  by the virus causes much less effect than the
  domino effect of the inflammatory response that
  follows

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Mechanisms of Eukaryotic Pathogenesis

• these mechanisms are not clearly understood,
  though the mechanisms include colonization of the
  host, evasion of the host defenses and damage to
  the host
• fungi
• these organisms are generally opportunistic,
  taking advantage of a weakening or change in our
  immune system
• excessive growth of Candida albicans is often a
  result in immunocompormised hosts
• C. albicans is the causative agent of thrush, a
  common occurrence in AIDS patients

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Mechanisms of Eukaryotic Pathogenesis

• eukaroytic parasites
• are generally found in the intestinal tract or
  have gained access through an insect bite
• attach with specific receptors
• are capable of hiding within the host cell
• the damage they can inflict varies
• some cause malnutrition by competing for
  nutrients
• some can cause direct damage by the enzymes they
  produce