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Chapter 13
Microbe-Human
Interactions
Infection, Disease, and
Epidemiology
1
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We Are Not Alone
• The human body exists in a state of dynamic
equilibrium
• Many interactions between human body and
microorganisms involve the development of
biofilms
• Colonization of the body involves a constant
“give and take”
2
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Contact, Colonization, Infection, Disease
• Microbes that engage in mutual or commensal
associations – normal (resident) flora,
indigenous flora, microbiota
• Infection – a condition in which pathogenic
microbes penetrate host defenses, enter
tissues, and multiply
• Pathogen – infectious agent
• Infectious disease – an infection that causes
damage or disruption to tissues and organs
3
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Contact, Colonization, Infection, Disease
4
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Resident Flora
• Most areas of the body in contact with the outside
environment harbor resident microbes
• Internal organs, tissues, and fluids are microbe-free
• Transients – microbes that occupy the body for only
short periods
• Residents – microbes that become established
5
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Resident Flora
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• Bacterial flora benefit
host by preventing
overgrowth of harmful
microbes – microbial
antagonism
• Endogenous infections
– occur when normal
flora is introduced to a
site that was previously
sterile
6
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Initial Colonization of the Newborn
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• Uterus and contents are
normally sterile and
remain so until just
before birth
• Breaking of fetal
membrane exposes the
infant; all subsequent
handling and feeding
continue to introduce
what will be normal flora
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7
Indigenous Flora of Specific Region
8
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Flora of the Human Skin
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• Skin is the largest
and most accessible
organ
Hair shaft
Epidermis
Sweat pores
Sebaceous
(oil) gland
• Two cutaneous
populations
– Transients:
influenced by hygiene
– Residents:
stable, predictable,
less influenced by
hygiene
Dermis
Hair follicle
Hair root
Subcutaneous
tissue
Duct of
sudoriferous
(sweat) gland
(a)
(b)
9
Janice Carr/CDC
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Flora of the Gastrointestinal Tract
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• GI tract is a long hollow
tube, bounded by mucous
membranes
– Tube is exposed to the
environment
• Variations in flora
distribution due to shifting
conditions (pH, oxygen
tension, anatomy)
• Oral cavity, large
intestine, and rectum
harbor appreciable flora
Oral cavity
Pharynx
Esophagus
Stomach
Duodenum
Large
intestine
Small
intestine
Rectum
10
Anal canalEducation.
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Flora of the Mouth
• Most diverse and unique flora of the body
• Numerous adaptive niches
• Bacterial count of saliva (5 x 109 cells per
milliliter)
11
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Flora of the Large Intestine
• Has complex and profound interactions with
host
• 108-1011 microbes per gram of feces
• Intestinal environment favors anaerobic
bacteria
• Intestinal bacteria contribute to intestinal odor
12
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Flora of the Respiratory Tract
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• Oral streptococci, first
organisms to colonize
• Nasal entrance, nasal
vestibule, anterior
nasopharynx –
S. aureus
• Mucous membranes of
nasopharynx –
Neisseria
• Tonsils and lower
pharynx – Haemophilus
Sinuses
Nasal
vestibule
Nasal cavity
Nasal
entrance
Internal naris
Larynx
Nasopharynx
Soft palate
Epiglottis
Bronchus
Trachea
Bronchiole
Right lung
Left lung
13
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Flora of the Genitourinary Tract
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• Sites that harbor microflora
– Females – Vagina and
outer opening of urethra
– Males – Anterior
urethra
• Changes in physiology
influence the composition
of the normal flora
– Vagina (estrogen,
glycogen, pH)
Uterine tube
Ovary
Uterus
Rectum
Urinary
bladder
Vagina
Urethra
Anus
External
reproductive
organs
Urinary
bladder
Rectum
Penis
Urethra
Anus
Testis
14
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14
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
15
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Concept Check:
Normal resident microbes are absent from the
A. pharynx
B. lungs
C. intestine
D. hair follicles
E. mouth
16
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Concept Check:
Normal resident microbes are absent from the
A. pharynx
B. lungs
C. intestine
D. hair follicles
E. mouth
17
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Major Factors in the
Development of an Infection
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18
Major Factors in the
Development of an Infection
• True pathogens – capable of causing disease in healthy
persons with normal immune defenses
– Influenza virus, plague bacillus, malarial protozoan
• Opportunistic pathogens – cause disease when the
host’s defenses are compromised or when they grow in
part of the body that is not natural to them
– Pseudomonas sp & Candida albicans
• Severity of the disease depends on the virulence of the
pathogen; characteristic or structure that contributes to the
ability of a microbe to cause disease is a virulence factor.
19
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Becoming Established
Portals of entry – characteristic route a microbe follows to
enter the tissues of the body
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• Exogenous agents
originate from source
outside the body
Conjunctiva
Respiratory tract
Gastrointestinal
tract
• Endogenous agents
already exist on or in
the body (normal flora)
Skin
Pregnancy
and birth
Urogenital tract
20
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Portals of Entry
– Skin –
nicks, abrasions,
punctures, incisions
– Gastrointestinal tract –
food, drink, and other
ingested materials
– Respiratory tract –
oral and nasal cavities
– Urogenital tract –
sexual, displaced
organisms
– Transplacental
21
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Pathogens That Infect during Pregnancy
• STORCH - Syphilis, Toxoplasmosis, Other diseases
(hepatitis B, AIDS and chlamydia), Rubella,
Cytomegalovirus and Herpes simplex virus
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Placenta
Maternal blood pools
within intervillous space
Bacterial
cells
Umbilical cord
Umbilical
vein
Placenta
Umbilical
arteries (fetal blood)
Maternal
blood vessel
(a)
(b)
Umbilical
cord
22
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Requirement for an Infectious Dose (ID)
• Minimum number of microbes required for
infection to proceed
• Microbes with small IDs have greater virulence
• Lack of ID will not result in infection
23
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Attaching to the Host
• Adhesion – microbes gain a stable foothold at the portal of
entry; dependent on binding between specific molecules on
host and pathogen
–
–
–
–
–
–
–
Fimbrae
Flagella
Glycocalyx
Cilia
Suckers
Hooks
Barbs
24
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Adhesion Properties of Microbes
25
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Surviving Host Defenses
• Initial response of host defenses comes from phagocytes
• Antiphagocytic factors – used to avoid phagocytosis
• Species of Staphylococcus and Streptococcus produce
leukocidins, toxic to white blood cells
• Slime layer or capsule – makes phagocytosis difficult
• Ability to survive intracellular phagocytosis
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Capsule
Bacteria cannot
be engulfed
Blocked
Phagocyte
Continued growth
of microbes damages
host tissue
26
(c) Blocked phagocytic response
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Entering Host Tissues
• Some pathogens produce a secretion system to
insert specialized virulence proteins directly into
the host cells
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27
Causing Disease
• Virulence factors – traits used to invade and establish
themselves in the host, also determine the degree of
tissue damage that occurs – severity of disease
• Exoenzymes – dissolve extracellular barriers and
penetrate through or between cells
• Toxigenicity – capacity to produce toxins at the site of
multiplication
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Bacteria
Epithelial cell
Cell
cement
(a) Exoenzymes
Exotoxins
Bacteria
(b) Toxins
28
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Bacterial Toxins:
A Potent Source of Cellular Damage
• 2 Types of Bacterial Toxins:
– Endotoxin – toxin that is not secreted but is
released after the cell is damaged
• Composed of lipopolysaccharide (LPS), part of
the outer membrane of gram-negative cell walls
– Exotoxin – toxin molecule secreted by a living
bacterial cell into the infected tissue
• Strong specificity for a target cell
• Hemolysins
• A-B toxins (A-active, B-binding)
29
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Bacterial Toxins: Exotoxins and Endotoxins
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Exotoxins
(a)
Target organs are damaged;
heart, muscles, blood
cells, intestinal tract show
dysfunctions.
Endotoxins
(b)
General physiological effects–
fever , malaise, aches, shock
30
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Comparing Exotoxins & Endotoxins
31
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A-B Exotoxins
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Toxin precursor
(inactive)
A chain A
• The A (active)
component is
attached to the B
(binding) component
• The A component
inhibits a cellular
protein to cause the
damage
B chain B
B chain
attaches to
receptor
Binding
site
A
B
Host cell
membrane
with specific
receptor
Endocytosis
Vacuole
formation
A portion of
toxin is
released
from vacuole
A
B
B
A
Toxin acts on ribosomes
and blocks protein synthesis;
causes cell deat
32
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Concept Check:
Only Gram-positive bacteria can secrete exotoxins.
A. True
B. False
33
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Concept Check:
Only Gram-positive bacteria can secrete exotoxins.
A. True
B. False
34
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