Review of Medical Microbiology and Immunology, Seventeenth Edition
Concise, easy to grasp immunology overview. Information needed to understand the module present in high level English grammar . Contains method of telling which does not require a prior reading of chapter from book. Well understood by first-time readers or for pre-exam revision. Well detailed summa...
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LEVINSON MICROBIOLOGY
CHAPTER 7
PATHOGENESIS
PRINCIPLES OF PATHOGENESIS:
Microorganism: is a pathogen if it is capable of causing disease
Some organisms are highly pathogenic, others rarely cause disease.
Opportunistic pathogens: rarely cause disease in immunocompetent people, but can cause
serious infection in patients with reduced host defenses (immunocompromised).
Virulence: a quantitative measure of pathogenicity.
Measured by the number of organisms required to cause disease.
LD50: 50% lethal dose
Is the number of organisms needed to kill half of the hosts that are exposed to the pathogen
ID50: 50% infectious dose
Is the number of organisms needed to cause infection in half of the exposed hosts.
Organisms with a lower LD50 (or ID50) are said to be more virulent than those with a higher
LD50 (or ID50) because fewer organisms are needed to cause death or disease.
,The infectious dose of an organism required to cause disease varies greatly among the
pathogenic bacteria. For example, Shigella and Salmonella both cause diarrhea by infecting the
gastrointestinal tract, but the infectious dose of Shigella is less than 100 organisms, whereas the
infectious dose of Salmonella is on the order of 100,000 organisms.
ID50 depends on: their virulence factors.
1. whether their pili allow them to adhere well to mucous membranes
2. If they produce exotoxins or endotoxins
3. If they possess a capsule to protect them from phagocytosis
4. if they can survive various nonspecific host defenses such as acid in the stomach
Parasite refers to the protozoa and the helminths.
Some pathogenic bacteria are also referred to as parasites because their presence is detrimental to
the host. For example, Chlamydia and Rickettsia are called obligate intracellular parasites
because they can grow only within host cells.
WHY DO PEOPLE GET INFECTIOUS DISEASES?
1. When microorganisms overpower our host defenses
2. Organisms or its toxins are present in sufficient amounts to induce various symptoms,
such as fever and inflammation, which we interpret as those of an infectious disease.
Two critical determinants in overpowering the host are:
1. the number of organisms to which a person is exposed
2. virulence of these organisms.
Clearly, the greater the number of organisms, the greater is the likelihood of infection.
,A small number of highly virulent organisms can cause disease just as a large number of
less virulent organisms can.
Virulence of an organism is determined by its ability to produce various virulence factors,
several of which were described previously.
The production of specific virulence factors also determines what disease the bacteria cause.
For example, a strain of Escherichia coli that produces one type of exotoxin causes watery
(nonbloody) diarrhea, whereas a different strain of E. coli that produces another type of exotoxin
causes bloody diarrhea.
This chapter describes several important examples of specific diseases related to the production
of various virulence factors. From the host’s perspective, the two main arms of our host defenses
are innate immunity and acquired immunity, the latter of which includes both antibody-mediated
and cell-mediated immunity. A reduction in the functioning of any component of our host
defenses shifts the balance in favor of the organism and increases the chance that an infectious
disease will occur.
Some important causes of a reduction in our host defenses include:
1. genetic immunodeficiencies such as agammaglobulinemia
2. acquired immunodeficiencies such as acquired immunodeficiency syndrome (AIDS)
3. drug-induced immunosuppression in patients with organ transplants
4. cancer patients who are receiving chemotherapy.
5. Patients with diabetes and autoimmune diseases also may have reduced host defenses.
Asymptomatic Infections: when a person can acquire an organism without subsequent
development of disease because the host defenses were present.
, Very common and are typically recognized by detecting antibody against the organism in the
patient’s serum.
TYPES OF BACTERIAL INFECTIONS:
Infection? an organism has infected the person (i.e., it has entered the body of that person).
A person can be infected with an organism of low pathogenicity and not develop symptoms of
disease.
Disease? Is different.
Bacteria cause disease by two major mechanisms:
(1) toxin production
(2) invasion and inflammation.
(3) immunopathogenesis
Both exotoxins and endotoxins by themselves can cause symptoms; the presence of the
bacteria in the host is not required.
Invasive bacteria, grow to large numbers locally and induce an inflammatory response
consisting of erythema, edema, warmth, and pain.
Communicable or contagious (i.e. Infections that can spread from host to host). For example,
tuberculosis
Epidemics: are infections that occur much more frequently than usual.
Pandemics: If there is worldwide distribution of an epidemic.
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