Microbe-Human Interactions: Infection and Disease

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Chapter 13

• Microbe-Human Interactions Infection and
  Disease

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Contact-Infection-Disease The Host-Parasite
Relationship

• The human body is constantly in contact
  (contaminated) with microbes
• Some are pathogens that may cause an infection by
  circumventing the host defense system, entering
  normally sterile tissues, and multiplying there
• When infections lead to a disruption in tissues,
  infectious disease results
• The outcome is highly variable, but most contacts
  do not result in infection, and most infections
  do not lead to disease

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Associations between microbes and humans
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The Body as a Habitat

• The resident flora or microflora is a huge and
  rich mixed population of microorganisms residing
  on body surfaces exposed to the environment,
  including the skin, mucous membranes, parts of
  the gastrointestinal tract, urinary tract,
  reproductive tract, and upper respiratory tract

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The Body as a Habitat

• Anatomical sites lying within the body cavity
  (organs) and fluids (blood, urine) in those sites
  do not harbor flora
• Colonization begins just prior to birth and
  continues over an individuals life variations
  occur in response to individual differences in
  age, diet, hygiene, and health

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Normal Flora

• Bacteria may maintain a balance in the normal
  conditions
• Studies with axenic animals (free of any normal
  flora) show that flora contribute to the
  development of the immune and gastrointestinal
  systems and also to some diseases (dental caries)
• Normal flora are sometimes agents of infection

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Landscape of the skin

• Microbes live only in upper dead layers of
  epidermis glands, and follicles dermis and
  layers below are sterile

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Distribution of flora in the gastrointestinal
tract

• Oral cavity colonize the epidermal layer of
  cheeks, gingiva, pharynx surface of teeth found
  in saliva in huge numbers
• Large intestine and rectum have huge numbers of
  flora

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Colonized regions of the respiratory tract

• The moist mucous blanket of the nasopharynx has a
  well-entrenched flora.
• Lower regions of bronchi, bronchioles, and lungs
  are free of resident microbes.

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Flora of the reproductive tract

• In females, the external genitalia, vaginal, and
  cervical surfaces have flora
• In females, the first portion of the urethral
  mucosa has flora.
• In males, the entire reproductive and urinary
  tract is sterile, except for a short portion of
  the anterior urethra

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Factors Affecting the Course of Infection and
Disease
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Pathogenicity and Virulence

• Pathogenicity is the property of microorganisms
  to cause infection and disease
• Virulence is the precise factors used by the
  microbe to invade and damage host tissues it
  helps define the degree of pathogenicity
• Pathogenicity varies with a microbes ability to
  invade or harm host tissues and with the
  condition of host defenses
• A true pathogen produces virulence factors that
  allow it to readily evade host defenses and to
  harm host tissues
• True pathogens can infect normal, healthy hosts
  with intact defenses
• An opportunistic pathogen is not highly virulent
  but can cause disease in persons whose host
  defenses are compromised by predisposing
  conditions such as age, genetic defects, medical
  procedures, and underlying organic disease

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Mechanisms of Infection and Disease

• The portal of entry is the route by which
  microbes enter the tissues, primarily via skin,
  alimentary tract, respiratory tract (pneumonia),
  urogenital tract (sexually transmitted diseases),
  or placenta

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Transplacental infection of the fetus

• Microbes penetrate the maternal blood vessels and
  enter the blood pool of the placenta.
• They then invade the fetal circulation by way of
  the umbilical vein

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Mechanisms of Infection and Disease

• Pathogens that come from outside the body are
  exogenous those that originate from normal flora
  are endogenous
• The size of the infectious dose is of great
  importance

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Mechanisms of Infection and Disease

• In the process of adhesion, a microbe attaches to
  the host cell by means of fimbriae, flagella,
  capsules, or receptors that position it for
  invasion

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Virulence Factors

• Exoenzymes digest epithelial tissues, disrupt
  tissues, and permit invasion

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Virulence Factors

• Toxigenicity is a microbes capacity to produce
  toxins at site of multiplication which affect
  cellular targets
• Toxinoses are diseases caused by toxins that
  damage structure or function of host cells
• Toxemia refers to toxins absorbed into the blood
• Intoxication means ingestion of toxins

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Virulence Factors

• An exotoxin is a protein secreted by living
  bacteria with powerful effects on a specific
  organ
• Examples are hemolysins and tetanus and
  diphtheria toxins
• An endotoxin is the lipopolysaccharide portion of
  a gram-negative cell wall released when a
  bacterial cell dies causes generalized symptoms
  such as fever

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Virulence Factors

• Antiphagocytic factors include leukocidins (white
  blood cell poisons) and capsules

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Effects on Target Organ/Spread of Infection

• Patterns of Infection Stages in
  Infection/Disease
• Incubation period, the period from contact with
  infectious agent until appearance of first
  symptoms
• Prodromium, a short period of initial, vague
  symptoms
• Period of invasion, a variable period during
  which microbe multiplies in high numbers and
  causes severest symptoms
• Convalescent period, a period of recovery, with
  decline of symptoms

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Types of Infectious/Diseases

• Localized infection, microbe remains in isolated
  site
• Systemic infection, microbe is spread through the
  tissues by circulation
• Focal infection, microbe spreads from local site
  to entire body (systemic)
• Mixed infection, several microbes cause one type
  of infection simultaneously
• Primary infection, the initial infection in a
  series
• Secondary infection, a second infection that
  complicates a primary infection
• Septicemia and bacteremia refer to microbes in
  the blood
• Acute infection appears suddenly, has a short
  course, and is relatively severe
• Chronic infection persists over a long period of
  time
• Subacute infection has a pattern between acute
  and chronic

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Types of Infectious/Diseases
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Signs and Symptoms Manifestations of disease,
indicators of pathogenic effects on target organs

• A sign is objective, measurable evidence noted by
  an observer
• Examples include septicemia, change in number of
  white blood cells skin lesions inflammation
  necrosis, lysis or death of tissues
• A symptom is a subjective effect of disease as
  sensed by patient
• Examples are pain, fatigue, and nausea
• A syndrome is a disease that manifests as a
  predictable complex of symptoms infections that
  do not show symptoms are called asymptomatic,
  subclinical, or inapparent

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Major portals of exit of infectious diseases

• Through the portal of exit, microbe is released
  with bodily secretions and discharges to have
  access to new host portals include respiratory
  droplets from sneezing, coughing, saliva, skin,
  feces, urogenital tract (urine, mucus, semen) and
  blood
• A microbe may become dormant (latent) and cause
  recurrent infections
• Damaging effects that remain in organs and
  tissues after infection are sequelae

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Epidemiology

• Epidemiology is a science that determines the
  factors influencing causation, frequency, and
  distribution of disease in a community
• Epidemiologists are involved in surveillance of
  reportable diseases in populations and consider
  measures to protect the public health
• They are concerned with disease statistics such
  as prevalence (the total number of cases),
  incidence (the number of new cases), morbidity
  (general health of the population), and mortality
  (death)

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Graphical representation of epidemiological data
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Frequency of Disease

• Endemic, a disease constantly present in a
  certain geographic area
• Sporadic, a disease that occurs occasionally with
  no predictable patter
• Epidemic, sudden outbreak of disease in which
  numbers increase beyond expected trends
• Pandemic, worldwide epidemic

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Origin of Pathogens

• The reservoir is a place where the pathogen
  ultimately originates (its habitat)
• Source of infection refers to the immediate
  origin of an infectious agent
• Carrier is an individual that inconspicuously
  shelters a pathogen and spreads it to others

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Carriers in transmission of infectious agents

• Asymptomatic carrier is infected without symptoms
• Incubation carriers carry early in disease
• Convalescent carriers carry in last phases of
  recovery
• Chronic carriers carry for long periods after
  recovery
• Passive carriers are uninfected but convey
  infectious agents from infected persons to
  uninfected ones by hand and instrument contact

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Vectors/Zoonoses

• A vector is an animal that transmits pathogens
• A biological vector is an alternate animal host
  (mosquito, flea) that assists in completion of
  life cycle of microbe
• A mechanical vector is an animal that does not
  host microbial life cycle, but is a short-term
  transmitter (housefly)
• A zoonosis is an infection for which animals are
  the natural reservoir and host that can be
  transmitted to humans

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Acquisition of Infection

• Communicable infectious disease occurs when
  pathogen is transmitted from host to host
  directly or indirectly contagious diseases are
  readily transmissible through direct contact

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Acquisition of Infection

• Non-communicable diseases are not spread from
  host to host acquired from ones normal flora
  (pneumonia) or from a nonliving environmental
  reservoir (tetanus)

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Acquisition of Infection

• Direct transmission
• Infectious agent is spread through direct contact
  of portal of exit with portal of entry (STDs,
  herpes simplex)
• Indirect transmission
• A material (vehicle) contaminated with pathogens
  serves as intermediate source of infections
• A fomite is an inanimate object contaminated with
  pathogens (public facilities, personal items)
• Food serves as a vehicle
• Droplet nuclei are airborne dried particles
  containing infectious agents, formed by sneezing
  and coughing

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Acquisition of Infection

• Nosocomial infections are infectious diseases
  that originate in the hospital or clinical
  setting
• They commonly occur among surgical and
  chronically ill patients
• Hospitals monitor various asepsis procedures to
  help reduce the number of infections
• Isolation of patients and other universal
  precaution are necessary controls

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Kochs Postulates

• Kochs postulates defines a series of criteria
  that must be followed to determine the etiologic
  (causative) agent of disease