Microbe-Human
Interactions:
Infection and
Disease
Chapter 13
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13.1 The Human Host
• Pathology – science of study of disease
• Body surfaces are constantly exposed to microbes
• Leads 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
Resident Biota
• Normal (resident) biota
• Cell for cell, microbes on the human body
outnumber human cells at least ten to one
• Ex: S. epidermidis on skin, E. coli in intestine
• Commensalism – one benefits, other unaffected;
can become opportunistic (cause disease when
given opportunity)
• Mutualism – both benefit (ex. E. coli produces vit
K, we provide the habitat)
Acquiring the Normal Microbiota
• Birth canal
• Food
• Breathing
• Natural succession
• Lactobacilli
• Coliforms
• Anaerobes
Impact of the Normal Flora
• Biological success (their “perspective”)
• Exclude potential pathogens
• Provide some nutrition to the host
• Can improve host nutrition
• Immune stimulation
• Also have potential negative impacts
Sites Thought to be Sterile
• Sterilization
• Removes all
viable
microorganisms
including viruses
and bacterial
endospores
• Material is said to
be sterile
13.2 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
Factors that Increase Susceptibility
Definition of a Terms
• Pathogen: A microbe that can cause disease is a
susceptible individual
• Virulence: The ability of a microbe to cause disease in
another organism, how quickly they infecti
• Infection: The colonization of an organism by a
microorganism - with or without disease
• Communicable: The ability of a disease-causing
organism to be spread between individuals
• Virulence Factors: factors that cause disease or aid in
spread of disease quickly in host or other hosts
Pathogenicity
• Pathogenicity: an organism’s potential to
cause infection or disease
• True pathogens
• Opportunistic pathogens
Virulence
• 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
• Pathogen: actual agent of disease, MRSA –
S. aureus
• BACTERIAL, VIRAL, FUNGAL, HELMINTH
• Carrier: Infected healthy individual, no
symptoms (asymptomatic), or very mild form of
disease, yet they both can spread disease to
others – many bacterial and viral pathogens
• Classic case was typhoid Mary (look it up)
Microbiology – Chapter 13
Reservoir: Where pathogen is maintained , can be
in an organism (animal), in the environment
(stagnant water - Legionella), or even in soil
(Clostridia)
Vector: Agent that spreads pathogens from host to
host
1. Arthropod: flea; mosquito, tick
2. Inanimate: things, toys, dirty hands,
needles, (sometimes called
“fomites”)
• Mechanical vectors
• Passive carriers
• Pathogens external
• Biological vector
• Vector is infected
• Pathogens internal
Stages in Infections
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Finding a Portal
of Entry
Attaching Firmly
Surviving Host
Defenses
Causing Damage
(Disease)
Skin
Fimbriae
Directdamage
GI tract
Capsules
Avoiding
phagocytosis
Respiratory tract
Surface proteins
Urogenital tract
Viral spikes
Endogenous biota
Avoiding death
Inside phagocyte
Absence of specific
immunity
Toxins and/or
enzymes
Indirect damage
Inducing
inappropriate,
excessive host
response
Exiting Host
Portals of exit
Respiratory tract,
salivary glands
Skin cells
Fecal matter
Urogenital tract
Blood
Becoming Established: Step OnePortals of Entry
• Microbe enters the tissues of the body by a
portal of entry
• Usually a cutaneous or membranous
boundary
• Normally the same anatomical regions that
support normal biota
Infectious Agents that Enter the Skin
• Nicks, abrasions, and punctures
• Intact skin is very tough- few microbes can
penetrate
• Some create their own passageways using
digestive enzymes or bites
• Examples
• Staphylococcus aureus
• Streptococcus pyogenes, Haemophilus aegyptius
• Chalmydia trachomatis
• Neisseria gonorrhoeae
The Gastrointestinal Tract as Portal
• Pathogens contained in food, drink, and other
ingested substances
• Adapted to survive digestive enzymes and pH
changes
• Examples
• Salmonella, Shigella, Vibrio, Certain strains of
Escherichia coli, Poliovirus, Hepatitis A virus,
Echovirus, Rotavirus, Entamoeba histolytica,
Giardia lamblia
The Respiratory Portal of Entry
• The portal of entry for the greatest number of
pathogens
• Respiratory droplets: cough, sneeze, air-born
droplet
• Examples
• Flu, colds, strept throat, staph infections of
wounds
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
• Genital warts
• Chlamydia
• Herpes
Sexually Transmitted Diseases
Pathogens that Infect During Pregnancy
and Birth
• Horizontal: Mother to infant
• Prenatal - can cross the placenta (ex. the syphilis
spirochete, HIV)
• Perinatal - when the child is contaminated by the
birth canal at birth
• STDs like gonorrhea and syphilis,chlamydia
blindness
Becoming Established: Step TwoAttaching to the Host (Adhesion)
Figure 13.4
Becoming Established: Step ThreeSurviving Host Defenses
• Phagocytes -WBCs 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
Causing Disease: How Virulence Factors
Contribute to Tissue Damage
Figure 13.5
Extracellular Enzymes
• Break down and inflict damage on tissues or dissolve
the host’s defense barriers
• Examples
• Mucinase
• Collagenase – brks down collagen to allow spread
• Hyaluronidase – brk down hyluranic acid, the sugar
that holds some cells together
• Some react with components of the blood (coagulase –
cause blood clots, and hemolysin – lyses RBCs)
• Enzymes: Collagenase, Hyaluronidase - Causes
necrosis and blackening of tissue (inches of
progression in hours)
Bacterial Toxins
• Specific chemical product that diffuse into tissue
causing damage/ enhance invasion/ avoid defenses
• Exotoxin – excreted outside of cell, Gram +/bacteria
• Ex: C. botulimun – powerful neurotoxin,
Necrotoxin of flesh eating Strep
• Endotoxin – released by Gram (-) bacteria when
cells lyse
• Ex. Lipid A – Shigella can cause high fevers
and even shock
Endotoxin - Lipid A – raises fever, and shock in Gram (-)
pathogens
Endotoxins and Exotoxins
Figure 13.6
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
Establishment and Location of Infection
Figure 13.7
Stages in Infections
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Finding a Portal
of Entry
Attaching Firmly
Surviving Host
Defenses
Causing Damage
(Disease)
Skin
Fimbriae
Directdamage
GI tract
Capsules
Avoiding
phagocytosis
Respiratory tract
Surface proteins
Urogenital tract
Viral spikes
Endogenous biota
Avoiding death
Inside phagocyte
Absence of specific
immunity
Toxins and/or
enzymes
Indirect damage
Inducing
inappropriate,
excessive host
response
Exiting Host
Portals of exit
Respiratory tract,
salivary glands
Skin cells
Fecal matter
Urogenital tract
Blood
The Portal of Exit: Vacating the Host
Figure 13.9
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, Hepatitis B, AIDS, EpsteinBarr
• Sequelae: long-term or permanent damage to
tissues or organs
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
Patterns of Transmission in
Communicable Diseases
Figure 13.12
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
Direct Contact
• Touching
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• Kissing
Umbilical cord
• Sexual Intercourse
• Vertical transmission
• prenatal -- across
placenta
(a)
• perinatal -- during
or shortly after birth
Placenta
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
•Asepsis: any practice that prevents
the entry of infectious agents into
sterile tissues
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