Download Chapter 14a

Host-Microbe Interactions
Chapter 14
• Disease
• Etiology
• Pathogen
– Primary vs. opportunistic
• Virulence
• Contamination
• Infection vs. Infestation
• Pathogenesis
• Pathology
• Symptoms
• Signs
• Syndrome
• Sequelae
Anatomical Barriers as Ecosystem
• Skin and mucous
membranes are
physical barriers to
infection
– May supply foundation
for microbial ecosystem
• Humans are usually sterile in utero
• Soon after birth, microbial populations begin to
establish
• Normal microbiota
– Resident flora
– Transient flora
• Isolated colonies in
specific body regions
•Factors that influence
distribution:
•Nutrient availability,
salinity, oxygen
availability, host
defenses, mechanical
factors
•Dominant type of
organism may change
with age and situation
– Normal flora play a role in defenses
• protection from colonization by pathogens
• microbial antagonism
» Competitive exclusion
» Bacteriocins
• Symbiotic relationships form between microbes
and host
– Relationships may change depending on state of host
and attributes of microbes
– Mutualism (++)
– Intestinal bacteria
– Probiotics
– Commensalism (+ neutral)
– Flora on skin and conjunctiva
– Parasitism (+-)
– Pathogens
Pathogenicity
– State of host resistance usually determines
extent of infection
• primary infection
• secondary infection (typically opportunistic)
• Sub-clinical (inapparent) infection
Distribution of pathogens
– Local infection
– Systemic (generalized) infection
• Septicemia can result from:
– Bacteremia
– Toxemia
– Viremia
• Many people are carriers of pathogens
– Viruses; Neisseria; Salmonella; Streptococcus
• Why are they less affected or not at all?
• Predisposing factors:
– gender
– genetic background
– climate and weather
– inadequate nutrition
– age
– habits and lifestyle
– chemotherapy
– emotional disturbances
• Characteristics of infectious disease:
– communicable
• contagious
• Often reflects ID50
– non-communicable disease
Etiology of Infectious Diseases
• Robert Koch demonstrated that specific
microbes caused specific diseases
– experimented with grazing animals infected
with anthrax
• Koch’s postulates
– Same pathogen must be present in each case
of disease
– Pathogen is isolated from diseased host and
grown in pure culture
– Pure culture must cause disease when
inoculated into healthy animal
– Pathogen must be re-isolated from inoculated
animal
• Exceptions to Koch’s postulates
– some bacteria have unique culture requirements
– some diseases are caused by multiple
pathogens
• Polymicrobial diseases (mixed infections)
– Ethical considerations
Stages of an Infectious Disease
Duration of a Disease
– acute diseases
– chronic diseases
– latent disease
Establishment of Infection
• In order to cause disease pathogen must
follow a series of steps
– Gain entrance to host
– Adherence
– Colonization
– Avoid Host Defenses
– Cause host damage
Portals of entry
1. Mucus membranes
•
•
•
•
Respiratory tract
Gastrointestinal tract
Genitourinary tract
Placenta
2. Skin
3. Parenteral route
• Bite, puncture, injection,
wound
• Most microbes have a preferred portal of entry
– Streptococci when inhaled may cause pneumonia; when
ingested they do not
• A few microbes cause illness no matter how they enter
– May cause different illness based on portal
– Plague has 2 forms; bubonic and pneumonic
– Anthrax has three forms
• Adherence
– Critical Step
– Binding of adhesins to host receptors is highly specific
• Virulence factors
– Capsules
– Incomplete phagocytosis
– Fimbrae
– Components of cell wall
• Extra-cellular enzymes (exoenzymes)
– Coagulases & Kinases
• Form or dissolve blood clots
– Hyaluronidase & Collagenase
• Dissolve hyaluronic acid and collagen fibers
– IgA proteases & Leukocidins
• Destroy antibodies or WBCs
Damage to the Host
• Damage often facilitates dispersal of pathogen
– Diarrhea or coughing
• Main sources of damage:
–
–
–
–
Steal nutrients
Binding to and invading host cells
Induce hypersensitivity reactions (allergies)
Production of toxins (Toxigenicity)
• Exotoxins
– G+ bacteria
• Produced as part of their metabolism
• Secreted externally or following cell lysis
• Among most lethal substances
– Proteins
• Enzymatic nature
• Highly soluble
• Heat Liable
– Toxoids
• Inactivated exotoxins
• Induce antitoxins that provide immunity
– Antibodies against a specific toxin
• Passive immunity in form of antitoxin can be given
as treatment
– Grouped into functional categories
• Neurotoxins
• Enterotoxins
• Cytotoxins
• Staphylococcus aureus
– enterotoxin; exofoliatin toxin; Toxic Shock Syndrome toxin
• Vibrio cholera
– cholera enterotoxin
• Clostridium species
– botulinum neurotoxin; tetanus neurotoxin; gangrene toxin
• Endotoxins
– G- bacteria
– Released when cells die and their cell walls lyse
– Lipopolysaccharide (LPS) outer membrane
• The lipid portion (lipid A) is toxin
– Heat stable; not suitable for use as toxoids
• Do not cause formation of antitoxins
– All endotoxins produce the same symptoms
• Chills, fever, weakness, aches
• May activate blood clotting proteins
• May cause septic shock that can be fatal
– Salmonella, Proteus, Klebsiella and Neisseria
Portals of Exit
• Mucus membranes
– Respiratory & gastrointestinal
are most common
• Skin/wounds
• Biting insects
• Contaminated needles and
syringes