Microbe-Human Interactions
A Continuum of Interactions
Exists
• Routine Contact with Microorganisms
• Infection – a condition in which
pathogenic microbes penetrate host
defenses, enter tissues and multiply
• Disease – Deviation from Normal Health
The Normal State is that Humans
are Host to a Variety of
Microorganisms
Humans are Colonized from
the Moment of Birth
Babies Readily Acquire Flora
During Delivery and from the
External Environment
Regions that Host Flora
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Skin
Oral cavity
Lower GI tract
Upper Respiratory
Genitourinary Tract
Skin Flora
Staphylococcus, Micrococcus
Propionibacterium
Flora of the Mouth and
Pharynx
Streptococcus, Neisseria,
Staphylococcus, Lactobacillus,
Bacteroides
Colonized Regions of the
Respiratory Tract
Flora of the GI Tract
Bacterioides, Lactobacillus,
Enterococcus, Coliforms
Distribution of Flora
Flora of the Genitourinary
Tract
Lactobacillus, Streptococcus,
E. coli, Staphylococcus
Maintenance of the
Normal Resident Flora
• Normal flora is essential to the health
of humans
• Flora create an environment that may
prevent infections and can enhance
host defenses
• Antibiotics, dietary changes, and
disease may alter flora
• Probiotics – introducing known
microbes back into the body
The Process of Disease
Types of Pathogens
True vs. Opportunistic
Pathogens
True Pathogens
• Capable of causing disease in a healthy
person
• Generally associated with a
recognizable disease
• Can be weak to fatal in their effects
Opportunistic Pathogens
• Cause disease in weakened host
• Sometimes cause disease when they
colonize a different region of the body
• See Table 13.4 for pre-disposing
conditions
Pathogenicity vs. Virulence
• Pathogenicity = ability to cause disease
• Virulence = relative capacity of a
pathogen to invade and harm host cells
Gaining Access – Portal of
Entry
The Skin or Membrane Barrier
through which Pathogens Gain
Entry to the Body
Pathogens May be Exogenous or
Endogenous
Some Agents that Enter the Skin
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Staphylococcus aureus
Streptococcus pyogenes
Clostridium sp.
Insect-borne diseases
Hypodermic needle contaminants
GI Tract Entrants
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Salmonella sp.
Vibrio sp.
E. coli
Shigella sp.
Entamoeba sp.
Giardia sp.
Respiratory Entrants
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This is the most frequent route of entry
Streptococcus (Strep throat)
Influenza viruses
C. diptheriae
B. pertussis
Many others
Urogenital Entry Agents
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Syphilis
Gonorrhea
Human Papilloma Virus
HIV
Chlamydia
Hepatitis B
Placental & Neonatal Agents
STORCH = Syphilis, toxoplasmosis,
others (hepatitis), rubella,
cytomegalovirus, herpes simplex
Infectious Dose
Minimum number of organisms
required for an infection to be
successful; lack of ID will not result
in infection
To be Successful, a Pathogen
Must
• Adhere to the host
• Penetrate host barriers
• Establish a colony in host tissue
Some Methods of Adhesion
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Fimbriae
Flagella
Adhesive Capsules
Binding to Receptors (Viruses)
Virulence factors
Traits used by pathogens to invade
and establish themselves in the
host; also determine the degree of
tissue damage that occurs
Some Virulence Factors
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Extracellular Enzymes
Exotoxins
Endotoxins
Anti-phagocytic Factors
Exotoxins – Secreted by
Living Cells
Examples: Botulin Toxin,
Hemolysins (Strep and Staph)
Endotoxins are Released by
Dead Cells
Example: Lipopolysaccharides
from Gram- cells
Antiphagocytic Factors
Ex.: Leukocidins, Slime Layers
The Process of Infection and
Disease
• Distinct stages of clinical infections:
– incubation period - time from initial contact with
the infectious agent to the appearance of first
symptoms; agent is multiplying but damage is
insufficient to cause symptoms; several hours to
several years
– prodromal stage – vague feelings of discomfort;
nonspecific complaints
– period of invasion – multiplies at high levels,
becomes well established; more specific signs
and symptoms
– convalescent period – as person begins to
respond to the infection, symptoms decline
Patterns of Infection
• Localized - Contained
• Systemic – Agent Circulates Freely
• Focal – Localized infection breaks
loose or attacks a distant target with
toxin
• Mixed – more than one agent invades
Temporal Patterns of Infection
• Acute = short-lived but severe
• Chronic = long-lived, persistent
Signs of Disease vs Symptoms
• Signs = objective evidence of disease
• Symptoms = subjective evidence
sensed by patient (discomfort)
Some Signs of Infection
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Inflammation
Skin lesions
Elevated WBC count
Bacteria or virus in blood
Portals of Exit for Pathogens
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Respiratory/ Salivary
Skin
GI tract
Urogenital
Blood
Persistence of Infection
• Latent infections & recurrent disease
• Sequelae – long term damage to
organs/tissues
Epidemiology
The Study of Disease in
Populations
Classifications of Diseases
• Endemic – exhibits a relatively stable
frequency in a particular location over time
• Sporadic – occasional cases at irregular
intervals
• Epidemic – increase beyond what might be
expected in a given population
• Pandemic – spread of epidemics across
continents
Patterns of Diseases
Pathogen Reservoirs vs Sources
• Reservoir = habitat where the organism
occurs
• Source = Actual contact which provides
the infection
Disease Carriers
Shelter and Spread a Pathogen
Inconspicuously
Carrier States
• Asymptomatic – No signs of infection
• Incubation – Spreads infection while it
is incubating (still no symptoms)
• Convalescent – Sheds microbes while
recuperating
• Chronic – Latent infections can be
sheltered after apparent recovery
• Passive – Mechanically picks up and
transfers microbes
Disease Vectors
Animals that transport an
infection from one host to
another
Biological Vectors
• Actively participate in pathogen’s life
cycle
• Can inject infected saliva (mosquito)
• May defecate around bite wound (flea)
• Regurgitate blood into a wound (tsetse
fly)
Mechanical Vectors
Move the Pathogen from one
Host to Another Without being
Infected
Zoonosis
An infection naturally found in
animals, but transmissible to
humans (See Table 13.10)
Communicable & Contagious
Diseases
• Communicable diseases can be
transferred from one host to another
and infection established
• Contagious diseases are highly
transmissible and move readily from
host to host
• Non-communicable diseases are not
transmitted from one host to another
Mechanisms of Transmission
• Direct – Portal of Exit from one
Individual contacts Portal of Entry of
another
• Indirect – Pathogen is carried by
intermediary vehicle from one host to
another
Fomites
Inanimate objects that harbor
and transmit pathogens
Airborne Spread
• Droplet nuclei from sneezes and
coughs
• Aerosols
Nosocomial Infections
• Diseases that are acquired during a
hospital stay
• Most commonly involve urinary tract,
respiratory tract, & surgical incisions
• Most common organisms involved are
Gram-negative intestinal flora, E. coli,
Pseudomonas, Staphylococcus
Koch’s Postulates
• Find evidence of a particular microbe in all
cases of a disease
• Isolate the suspect microbe from an infected
individual and culture it in the lab
• Inoculate a healthy individual with the pure
lab culture and observe the resulting disease
• Isolate the microbe from the test individual