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Chapter 13
Microbe-Human Interactions: Infection and Disease
Chapter Outline
13.1. The Human Host
A. Contact, infection, disease—A continuum
1. Microbes invade the tissues leading to infection
2. Pathologic state that results in damage or disruption of organs or tissues is known
as a disease
3. Infectious disease: disruption of a tissue or organ caused by microbes and their
products
B. Resident biota: The human as a habitat
1. Normal resident biota or indigenous biota
2. Acquiring resident biota
a. Human body has wide variety of environmental niches that vary in:
i. Temperature
ii. pH
iii. Nutrients
iv. Oxygen tension
b. Most areas of the body in contact with the outside environment harbor biota
c. Internal organs and tissue, including fluids, are generally microbe-free
d. Bacterial biota can benefit the human host by preventing overgrowth of
harmful microbes:
i. Fermentation of glycogen by lactobacilli keeps pH in vagina very acidic
ii. Prevents the overgrowth of the yeast Candida albicans
e. Immunosuppression such as AIDS
i. Normal biota can lead to infection in these patients
ii. Streptococcus pneumoniae and pneumococcal pneumonia
f. Endogenous infections can occur when normal biota is introduced into a
previously sterile site
3. Initial colonization of the newborn
a. First exposure is the breaking of the fetal membranes, when vaginal microbes
can enter the womb
b. Comprehensive exposure during passage through birth canal
c. Large intestine microbiota:
i. Bottle-fed infants have coliforms, lactobacilli, enteric streptococci, and
staphylococci
ii. Breast-fed infants have Bifidobacterium
C. Indigenous biota of specific regions: Table 13.3
1. Biota of human skin
2. Biota of the gastrointestinal tract
a. Biota of the mouth
b. Biota of the large intestine
i. Bacteroides, Bifidobacterium, Fusobacterium, Clostridium
ii. Small amounts of E. coli, other coliforms
3. Biota of the respiratory tract
4. Biota of the genitourinary tract
13.2. The Progress of an Infection
A. Pathogenicity
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1. True or opportunistic pathogens
2. Predisposing factors
3. Virulence and virulence factors
B. Becoming established: Step one—portals of entry
1. Infectious agents that enter the skin
2. The gastrointestinal tract as portal
3. The respiratory portal of entry
4. Urogenital portals of entry
a. STD (sexually transmitted disease)
5. Pathogens that infect during pregnancy and birth
a. TORCH (toxoplasmosis, other such as hepatitis, rubella, cytomegalovirus, and
herpes simplex viruses) are the common infections of fetus and neonate
C. The size of the inoculum
1. Infectious dose
D. Becoming established: Step two—Attaching to the host
1. How pathogens attach (adhesion) (Table 13.6)
E. Becoming established: Step three—Surviving host defenses
1. How microbes escape phagocytosis
a. Leukocidins
b. Slime layer or capsule formation
c. Survival inside phagocytes after ingestion
F. Causing disease
1. How virulence factors contribute to tissue damage
a. Extracellular enzymes (exoenzymes)
i. Mucinase
ii. Keratinase
iii. Collagenase
iv. Hyaluronidase
v. Streptokinase; Staphylokinase
b. Bacterial toxins: A potent source of cellular damage
i. Toxigenicity
ii. Toxinoses
iii. Toxemia
iv. Intoxication
v. Exotoxin
1. Hemolysins
vi. Endotoxin
c. Inducing an injurious host response
G. The process of infection and disease
1. Establishment, spread, and pathological effects
a. Necrosis
b. Cytopathic effects of viral infections
2. Patterns of infection
a. Localized infection
b. Systemic infection
c. Focal infection
d. Mixed infection (polymicrobial disease)
e. Primary and secondary infection
f. Acute and chronic infections
H. Signs and symptoms: Warning signals of disease
1. Sign
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2. Symptom
3. Syndrome
4. Signs and symptoms of inflammation
a. Edema
b. Granulomas and abscesses
c. Lymphadenitis
d. Rash
e. Lesion
5. Signs of infection in the blood
a. Leukocytosis
b. Leukopenia
c. Septicemia, bacteremia, viremia
6. Infections that go unnoticed
a. Asymptomatic
b. Subclinical or inapparent
I. The portal of exit: Vacating the host
1. Respiratory and salivary portals
2. Skin scales
3. Fecal exit
4. Urogenital tract
5. Removal of blood or bleeding
J. The persistence of microbes and pathologic conditions
1. Chronic infection and latency
2. Sequelae
K. Reservoirs: Where pathogens persist
1. Reservoir versus source
2. Living reservoirs
a. Carriers
i. Asymptomatic carrier
ii. Incubation carrier
iii. Convalescent carrier
iv. Chronic carrier
v. Passive carrier
b. Animals as reservoirs and sources
i. Biological vectors
ii. Mechanical vectors
iii. Zoonosis
3. Nonliving reservoirs
L. The acquisition and transmission of infectious agents
1. Communicable disease
2. Contagious
3. Noncommunicable disease
4. Patterns of transmission in communicable diseases
a. Modes of contact transmission
b. Routes of indirect transmission
c. Indirect spread by vehicles: Contaminated materials
i. Vehicle
ii. Fomites
d. Indirect spread by vehicles: Air as a vehicle
i. Droplet nuclei
ii. Aerosols
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M. Nosocomial infections: The hospital as a source of disease (fig. 13.13)
N. Universal blood and body fluid precautions
O. Which agent is the cause? Using Koch's Postulates to determine etiology
1. Identify microbe by evidence in patient
2. Isolate and cultivate microbes
3. Inoculate a suitable test subject
4. Reisolate the agent in test subject
13.3. Epidemiology: The Study of Disease in Populations
A. Who, when, and where? Tracking disease in the population
1. Morbidity and Mortality Report (CDC)
2. Epidemiological statistics: Frequency of cases
a. Prevalence
b. Incidence
c. Mortality and morbidity rate
d. Endemic
e. Sporadic
f. Epidemic
g. Pandemic
Key Terms and Phrases
Resident biota
Germ-free
Axenic
True pathogens
Virulence
Opportunistic pathogens
Predispose
Portal of entry
Exogenous
Endogenous
Sexually transmitted disease
STD
Inoculum
Infectious dose
Adhesion
Exoenzymes
Toxin
Toxigenicity
Toxinoses
Toxemia
Intoxication
Hemolysins
Hemolyze
Leukocidins
Incubation period
Period of invasion
Systemic
Focal
Mixed infection
Primary infection
Secondary infection
Acute
Chronic
Symptom
Inflammation
Edema
Lesion
Leukocytosis
Leukopenia
Sequelae
Epidemiology
Morbidity
Septicemia
Bacteremia
Viremia
Subclinical
Asymptomatic
Inapparent
Portal of exit
Latency
Prevalence
Incidence
Pandemic
Asymptomatic
Topics for Discussion
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Asymptomatic carrier
Incubation carrier
Convalescent carrier
Chronic carrier
Passive carrier
Biological vector
Mechanical vector
Zoonosis
Communicable
Noncommunicable
Fomite
Droplet nuclei
Nosocomial
Asepsis
Isolation
Etiologic
Koch's postulates
Prodromal stage
Systemic infection
Necrosis
Convalescent period
Granuloma
Abscess
Virulence factor
Lymphadenitis
Microbial antagonism
Student discussion should be directed toward the principles of epidemiology and the importance of this
science to society. This provides a good opportunity to present case studies for students. There are a
number of excellent video documentaries that illustrate the epidemiological process. Students can also
discuss the criteria for designating diseases as reportable to the CDC.
This chapter lends itself to laboratory study. The students can isolate normal biota from their skin,
throat, feces, and/or urine.
The concept of virulence is very important to this chapter. The students should discuss the factors
that allow a microorganism to enter the body and produce disease in a healthy host versus what is needed
in order for a microbe to cause disease in an immune-compromised host, such as an AIDS patient.
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