Page 42 - Review of Medical Microbiology and Immunology ( PDFDrive )

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P
A
T

Pathogenesis

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CHAPTER C ONTENT S

Principles of Pathogenesis
Different Strains of the Same Bacteria Can Produce
Why Do People Get Infectious Diseases?
Different Diseases
Types of Bacterial Infections
Typical Stages of an Infectious Disease
Stages of Bacterial Pathogenesis
Did the Organism Isolated from the Patient Actually
Determinants of Bacterial Pathogenesis
1. Transmission
Self-Assessment Questions
2. Adherence to Cell Surfaces
Practice Questions: USMLE & Course Examinations
3. Invasion, Inflammation, & Intracellular Survival Cause the Disease?
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4. Toxin Production
5. Immunopathogenesis

PRINCIPLES OF PATHOGENESIS
to adhere well to mucous membranes, whether they pro-
A microorganism is a pathogen if it is capable of causing
duce exotoxins or endotoxins, whether they possess a cap-
disease; however, some organisms are highly pathogenic
sule to protect them from phagocytosis, and whether they
(i.e., they often cause disease), whereas others cause disease
rarely. Opportunistic pathogens are those that rarely, if
in the stomach).
ever, cause disease in immunocompetent people but can
There are two uses of the word parasite. Within the
cause serious infection in immunocompromised patients. can survive various nonspecific host defenses such as acid
context of this chapter, the term refers to the parasitic rela-
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These opportunists are frequent members of the body’s
tionship of the bacteria to the host cells (i.e., the presence
normal flora. The origin of the term opportunistic refers to
of the bacteria is detrimental to the host cells). Bacteria
the ability of the organism to take the opportunity offered
by reduced host defenses to cause disease.
parasites. Some bacterial pathogens are obligate intracel-
Virulence is a quantitative measure of pathogenicity
lular parasites (e.g., Chlamydia and Rickettsia), because
and is measured by the number of organisms required to
they can grow only within host cells. Many bacteria are
cause disease. The 50% lethal dose (LD ) is the number of
facultative parasites because they can grow within cells,
50
organisms needed to kill half the hosts, and the 50% infec-
outside cells, or on bacteriologic media. The other use of
tious dose (ID ) is the number needed to cause infection
the term parasite refers to the protozoa and the helminths,
50
in half the hosts. Organisms with a lower LD (or ID ) are
said to be more virulent than those with a higher LD (or
ID ) because fewer organisms are needed to cause death or
50
WHY DO PEOPLE GET INFECTIOUS
disease. 50 50 50 which are discussed in Part VI of this book.
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The infectious dose of an organism required to cause
DISEASES?
disease varies greatly among the pathogenic bacteria. For
example, Shigella and Salmonella both cause diarrhea by
People get infectious diseases when microorganisms over-
infecting the gastrointestinal tract, but the infectious dose
the organism and the host shifts in favor of the organism).
of Shigella is less than 100 organisms, whereas the infec-
The organism or its products are then present in sufficient
tious dose of Salmonella is on the order of 100,000 organ-
isms. The infectious dose of bacteria depends primarily on
amount to induce various symptoms, such as fever and
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