Page 59 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART I Basic Bacteriology
48

Nonpathogenic
strain of E. coli

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mebooksfree.com mebooksfree.com (Enterotoxigenic PAI-A (Uropathic strain) PAI-B neonatal meningitis PAI-C mebooksfree.com mebooksfree.com
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E. coli causing
E. coli causing
E. coli causing
urinary tract
watery diarrhea
infection
(K-1 capsular
strain)
strain)
FIGURE 7–5
Pathogenicity islands encode virulence factors that determine the type of infection. The top of the figure depicts a non-
pathogenic strain of Escherichia coli that does not contain a pathogenicity island (PAI) in the genome DNA The black oval line within the E. coli
cell is the genome DNA PAIs can be transferred, by either conjugation or transduction, from another enteric gram-negative rod into the non-
pathogenic strain of E. coli. Acquisition of a PAI that encodes virulence factors endows the nonpathogenic E. coli with the ability to cause spe-
cific diseases. In this figure, PAI-A encodes an enterotoxin, PAI-B encodes the pili that bind to urinary tract epithelium, and PAI-C encodes the
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enzymes that synthesize the K-1 capsular polysaccharide. This results in three different strains of E. coli capable of causing three different
infections.
(4) The recovery period, also known as the convales-
cence period, during which the illness abates and the
FROM THE PATIENT ACTUALLY
patient returns to the healthy state. IgG and IgA antibodies
CAUSE THE DISEASE?
protect the recovered patient from reinfection by the same
organism.
permanent normal flora and as transient passengers, this
After the recovery period, some individuals become
can be an interesting and sometimes confounding ques-
chronic carriers of the organisms and may shed them
tion. The answer depends on the situation. One type of
while remaining clinically well. Others may develop a Because people harbor microorganisms as members of the
situation relates to the problems of a disease for which no
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latent infection, which can recur either in the same form
agent has been identified and a candidate organism has
as the primary infection or manifesting different signs and
been isolated. This is the problem that Robert Koch faced
symptoms. Although many infections cause symptoms,
many others are subclinical (i.e., the individual remains
cause of an infectious disease, namely, anthrax in cattle and
asymptomatic although infected with the organism). In
tuberculosis in humans. His approach led to the formula-
subclinical infections and after the recovery period is over,
tion of Koch’s postulates, which are criteria that he
the presence of antibodies is often used to determine that
an infection has occurred.
Time of Incubation Prodrome Specific Recovery Return to
period
period
period
infection
good health
disease period
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(Declining
(Antibody protects
(No symptoms)
(Typical
(Microbe
symptoms)
enters body)
against reinfection)
symptoms)
symptoms)
FIGURE 7–6
Typical stages of an infectious disease. After infection, the patient progresses through four main stages: incubation period,
prodrome period, specific disease period, and the recovery period. The patient then typically returns to good health and has antibody that pro-
tects against reinfection and disease.
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