MicrobeHuman Interactions Infection and Disease

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Microbe-Human Interactions- Infection and
Disease
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The Human Host

• Constantly exposure to microbes
• Humans excellent hosts for microorganisms
• May lead to infection pathogenic microorganisms
  penetrate the host defenses, enter the tissues,
  and multiply
• Infectious disease the disruption of a tissue
  or organ caused by microbes or their products

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Symbiotic Relationships

• Symbiosis means to live together
• Describes the relationship between microorganisms
  and their human hosts
• Three types
• Mutualism
• Commenalism
• Parasitism

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The Three Types of Symbiotic Relationships
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Resident Biota

• Resident Biota The Human as a Habitat
• Cell for cell, microbes on the human body
  outnumber human cells at least ten to one
• Human Microbiome Project

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Resident Microbiota
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Resident Microbiota
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Axenic (A-Zen-ik) or sterile sites in the human
host
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Biota

• Microbial antagonism- resident biota may benefit
  the human host by preventing the overgrowth of
  harmful microorganisms
• Hosts with compromised immune systems could be
  infected by their own biota
• Endogenous infections caused by biota that are
  already present in the body

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Acquisition of Normal Microbiota

• Development in the womb is generally free of
  microorganisms
• Microbiota begins to develop during the birthing
  process
• Much of ones resident microbiota established
  during the first months of life

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Initial Colonization of the Newborn
Figure 13.1
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The Progress of an Infection

• Pathogen a microbe whose relationship with its
  host is parasitic and results in infection and
  disease
• Type and severity of infection depend on
  pathogenicity of the organism and the condition
  of its host

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Pathogenicity

• Pathogenicity an organisms potential to cause
  infection or disease
• True pathogens
• Opportunistic pathogens
• -Normal microbiota or other normally harmless
  microbes that can cause disease under certain
  circumstances

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Conditions that provide opportunities for
pathogens
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Virulence

• Virulence is the degree of pathogenicity
• Determined by its ability to
• Establish itself in the host
• Cause damage
• Virulence factor any characteristic or
  structure of the microbe that contributes to its
  virulence
• Different healthy individuals have widely varying
  responses to the same microorganism hosts evolve

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Becoming Established Step One- Portals of Entry

• Microbe enters the tissues of the body by a
  portal of entry
• Major portals
• Skin
• Gastrointestinal tract
• Respiratory tract
• Urogenital tract
• Placenta
• Perinatal route
• Source of infectious agent
• Exogenous (external)
• Endogenous (internal)

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Infectious Agents that Enter the Skin

• Outer layer of packed, dead, skin cells usually
  acts as a barrier to pathogens- few microbes can
  penetrate
• Nicks, abrasions, punctures and insect bites
  -possible entry points
• Some create their own passageways using digestive
  enzymes or bites
• Examples
• Staphylococcus aureus (boils)
• Streptococcus pyogenes (impetigo)
• Haemophilus aegyptius (pink eye)
• Chalmydia trachomatis (trachoma)
• Neisseria gonorrhoeae (gonorrhoea)

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The Gastrointestinal Tract as Portal

• Pathogens contained in food, drink, and other
  ingested substances
• Adapted to survive digestive enzymes and pH
  changes
• Examples of enteric pathogens
• Gram-negative rods- Salmonella, Shigella, Vibrio,
  Certain strains of Escherichia coli,
• Viruses- Poliovirus, Hepatitis A virus,
  Echovirus, Rotavirus,
• Protozoa- Entamoeba hitolytica, Giardia lamblia

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The Respiratory Portal of Entry

• The portal of entry for the greatest number of
  pathogens
• Examples
• Streptococcal sore throat, Meningitis,
  Diphtheria, Whooping cough, Influenza, Measles,
  Mumps, Rubella, Chickenpox, Common cold, Bacteria
  and fungi causing pneumonia

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Urogenital Portals of Entry

• Sexually transmitted diseases (STDs)
• Enter skin or mucosa of penis, external
  genitalia, vagina, cervix, and urethra
• Some can penetrate an unbroken surface
• Examples
• Syphilis
• Gonorrhea
• Genital warts
• Chlamydia
• Herpes

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Pathogens that Infect During Pregnancy and Birth

• Some microbes can cross the placenta (ex. the
  syphilis spirochete)
• Other infections occur perinatally when the child
  is contaminated by the birth canal
• TORCH (toxoplasmosis, other diseases, rubella,
  cytomegalovirus, and herpes simplex)

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Some Pathogens that Cross the Placenta
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Figure 13.3
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The Size of the Inoculum

• The quantity of microbes in the inoculating dose
• For most agents, infection only proceeds if the
  infectious dose (ID) is present
• Microorganisms with smaller IDs have greater
  virulence

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Becoming Established Step Two- Attaching to the
Host (Adhesion)
Adhesion- attachment to host Mediated by
- adhesion factors - specialized
structures (e.x. adhesion discs
on some protozoa) - adhesion proteins
- match receptor molecules on
host (e.x. adhesins found of
fimbrae)
Figure 13.4
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Adhesion and invasion of enteropathogenic
bacteria into human epithelial cells.
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Becoming Established Step Three- Surviving Host
Defenses

• Phagocytes
• White blood cells that engulf and destroy
  pathogens
• Antiphagocytic factors used by some pathogens
  to avoid phagocytes
• Leukocidins toxic to white blood cells,
  produced by Streptococcus and Staphylococcus
• Extracellular surface layer makes it difficult
  for the phagocyte to engulf them, for example-
  Streptococcus pneumonia, Salmonella typhi,
  Neisseria meningitides, and Cryptococcus
  neoformans
• Some can survive inside phagocytes after
  ingestion Legionella, Mycobacterium, and many
  rickettsias

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Virulence Factors of Infectious Disease

• Pathogenicity ability of a microorganism to
  cause disease
• Virulence degree of pathogenicity
• Virulence factors contribute to an organisms
  virulence
• Adhesion factors
• Biofilms
• Extracellular enzymes
• Toxins
• Antiphagocytic factors

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Causing Disease How Virulence Factors
Contribute to Tissue Damage

• Exoenzymes
• Enzymes secreted by the pathogen
• Dissolve structural chemicals in the body
• Help pathogen maintain infection, invade further,
  
• and avoid body defenses (ex. Mucinase,
  collegenase,
• coagulase, kinase)

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Bacterial Toxins
Toxins - Chemicals that harm tissues or trigger
host immune responses that cause damage Toxemia
refers to toxins in the bloodstream that are
carried beyond the site of infection Two
types Exotoxins (secreted from living bacterial
cells e.x. hemolysins) Endotoxins (not secreted-
part of outer membrane released from cell)
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Figure 13.6
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Comparison of endo- and exotoxins
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The Stages of Infectious Disease

• Following infection, sequence of events called
  the disease process occurs
• Many infectious diseases have five stages
  following infection
• Incubation period
• Prodromal period
• Illness
• Decline
• Convalescence

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The Stages of Infectious Disease
Figure 14.10
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Incubation periods for selected infectious
diseases
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The Process of Infection and Disease

• Establishment, Spread, and Pathologic Effects
• Microbes eventually settle in a particular target
  organ and continue to cause damage at the site
• Frequently weakens host tissues
• Necrosis accumulated damage leads to cell and
  tissue death
• Patterns of Infection

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Figure 13.7
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Signs and Symptoms Warning Signals of Disease

• Sign any objective evidence of disease as noted
  by an observer
• Symptom the subjective evidence of disease as
  sensed by the patient
• Syndrome when a disease can be identified or
  defined by a certain complex of signs and
  symptoms
• Asymptomatic, or subclinical, infections lack
  symptoms but may still have signs of infection

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Signs and Symptoms of Inflammation

• Fever, pain, soreness, swelling
• Edema- accumulation of fluid
• Granulomas and abscesses- walled-off collections
  of inflammatory cells and microorganisms
• Lymphadenitis- swollen lymph nodes
• Lesion the site of infection or disease

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Signs of Infection in the Blood

• Changes in the number of circulating white blood
  cells
• Leukocytosis- increase in level of white blood
  cells
• Leukopenia - decrease in level of white blood
  cells
• Septicemia general state in which
  microorganisms are multiplying in the blood and
  are present in large numbers
• Bacteremia or viremia microbes are present in
  the blood but are not necessarily multiplying

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Movement of Pathogen Out of Host

• Pathogens leave host through portals of exit

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Exit Portals

• Respiratory and Salivary Portals
• Coughing and sneezing
• Talking and laughing
• Skin Scales
• Fecal Exit
• Urogenital Tract
• Removal of Blood or Bleeding

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The Persistence of Microbes and Pathologic
Conditions

• Latency a dormant state
• The microbe can periodically become active and
  produce a recurrent disease
• Examples
• Herpes simplex
• Herpes zoster
• Hepatitis B
• AIDS
• Epstein-Barr
• Sequelae long-term or permanent damage to
  tissues or organs

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Reservoirs Where Pathogens Persist

• Reservoir the primary habitat in the natural
  world from which a pathogen originates
• Source the individual or object from which an
  infection is actually acquired
• Living Reservoirs
• Carrier an individual who inconspicuously
  shelters a pathogen and spreads it to others
  without any notice
• Asymptomatic carriers
• Incubation carriers (HIV)
• Convalescent carriers (Diptheria)
• Chronic carrier (long-term carriage e.x. herpes,
  tuberculosis)
• Passive carrier (e.x. transfer of nosocomial
  infections by care-givers)

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Figure 13.9
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Animals as Reservoirs and Sources

• Vector a live animal that transmits an
  infectious agent from one host to another
• Majority are arthropods
• Larger animals can also be vectors
• Biological vector actively participates in a
  pathogens life cycle
• Mechanical vectors transport the infectious
  agent without being infected

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Figure 13.10
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Zoonosis

• Zoonosis an infection indigenous to animals but
  naturally transmissible to humans
• Human does not contribute to the persistence of
  the microbe
• Can have multi-host inovlement
• At least 150 worldwide
• Example SWINE-FLU!

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Nonliving Reservoirs

• Human hosts in regular contact with environmental
  sources
• Soil
• Water

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The Acquisition and Transmission of Infectious
Agents

• Communicable disease when an infected host can
  transmit the infectious agent to another host and
  establish infection in that host
• Transmission can be direct or indirect
• Contagious agent highly communicable
• Noncommunicable disease does not arise through
  transmission of the infectious agent from host to
  host
• Acquired through some other, special circumstance
• Compromised person invaded by his or her own
  microbiota
• Individual has accidental contact with a microbe
  in a nonliving reservoir

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Patterns of Transmission in Communicable Diseases
Figure 13.11
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Transmission

• Contact transmission
• Indirect transmission
• Vehicle any inanimate material commonly used by
  humans that can transmit infectious agents (food,
  water, biological products, fomites)
• Contaminated objects (doorknobs, telephones,
  etc.)
• Food poisoning
• Oral-fecal route
• Air as a vehicle
• Indoor air
• Droplet nuclei
• Aerosols

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Figure 13.12
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Nosocomial Infections The Hospital as a Source
of Disease

• Nosocomial infections infectious diseases that
  are acquired or develop during a hospital stay
• 2-4 million cases a year
• The importance of medical asepsis

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Figure 13.13
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Universal Blood and Body Fluid Precautions

• Universal precautions (UPs) guidelines from the
  Centers for Disease Control and Prevention
• Assume that all patient specimens could harbor
  infectious agents
• Include body substance isolation (BSI)techniques
  to be used in known cases of infection

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Which Agent is the Cause? Using Kochs
Postulates to Determine Etiology

• Etiology study of the cause of the disease
• Etiologic agent the causative agent
• Robert Koch developed a standard for
  determining causation that would stand the test
  of scientific scrutiny

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Figure 13.14
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Kochs Postulates

• Find evidence of a particular microbe in every
  case of a disease
• Isolate that microbe from an infected subject and
  cultivate it in pure culture in the laboratory
• Inoculate a susceptible healthy subject with the
  laboratory isolate and observe the same resultant
  disease
• Reisolate the agent from this subject

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Epidemiology The Study of Disease in Populations

• Epidemiology the study of the frequency and
  distribution of disease and other health-related
  factors in defined human populations
• Involves not only microbiology but also anatomy,
  physiology, immunology, medicine, psychology,
  sociology, ecology, and statistics

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Who, When, and Where? Tracking Disease in the
Population

• Epidemiologists concerned with virulence, portals
  of entry and exit, and the course of the disease
• Also interested in surveillance collecting,
  analyzing, and reporting data on the rates of
  occurrence, mortality, morbidity, and
  transmission of infections
• Reportable diseases by law, must be reported to
  authorities
• Centers for Disease Control and Prevention (CDC)
  in Atlanta, Georgia
• Weekly notice the Morbidity and Mortality
  Report
• http//www.cdc.gov/mmwr/
• Shares statistics with the World Health
  Organization (WHO)

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Epidemiological Statistics Frequency of Cases

• Prevalence the total number of existing cases
  with respect to the entire population
• Prevalence (total number of cases in population
  / total number of persons in population) x 100
  
• Incidence the number of new cases over a
  certain time period
• Incidence number of new cases / total number of
  susceptible persons
• Mortality rate the total number of deaths in a
  population due to a certain disease
• Morbidity rate the number of persons afflicted
  with infectious diseases

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Figure 13.15
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Figure 13.16
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Current epidemiological data on H1N1

• As of October 3, 2009, 99 of circulating
  influenza viruses in the United States were 2009
  H1N1 influenza.
• Among people who become infected with 2009 H1N1,
  certain groups appear to be at increased risk of
  complications and may benefit most from early
  treatment with antiviral medications.
• Based on currently available data, approximately
  70 of persons hospitalized with 2009 H1N1
  influenza have had a recognized high risk
  condition. These groups are similar to those who
  are at increased risk for seasonal
  influenza-related complications
• Children younger than 2 years old
• Adults 65 years of age or older
• Pregnant women and women up to 2 weeks postpartum
  (including following pregnancy loss)
• Persons with the following conditions
• Chronic pulmonary (including asthma),
  cardiovascular (except hypertension), renal,
  hepatic, hematological (including sickle cell
  disease), or metabolic disorders (including
  diabetes mellitus)
• Disorders that that can compromise respiratory
  function or the handling of respiratory
  secretions or that can increase the risk for
  aspiration (e.g., cognitive dysfunction, spinal
  cord injuries, seizure disorders, or other
  neuromuscular disorders)
• Immunosuppression, including that caused by
  medications or by HIV