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U N I T 4
Infection and Immunity
than 10 years to complete at a cost of more than $3
billon. Today newer sequencing methods, collectively
known as Next Generation Sequencing (NGS), have
allowed laboratories to sequence whole genomes of
bacteria, molds, mycobacteria, and even humans at
a fraction of the cost and within days rather than
decades. For infectious diseases, the applications of
sequencing are exciting and impactful. In epidemiol-
ogy, next-generation sequencing will likely replace
our current laboratory methods. For example, in
2011 a new strain of
E. coli
was detected in an out-
break of serious foodborne illness beginning in north-
ern Germany that ultimately affected nearly 4000
patients with 53 dead. Within 1 week of isolating
the new strain of
E. coli
, clinical microbiologists had
grown the organism, sequenced it, and identified the
source of the outbreak. With powerful laboratory
technologies like sequencing, epidemiologists will be
better equipped to identify outbreaks early or even
prevent them from occurring.
Treatment of Infectious
Diseases
The goal of treatment for an infectious disease is com-
plete removal of the pathogen from the host and the
restoration of normal physiologic function to damaged
tissues. Most infectious diseases of humans are self-lim-
iting in that they require little or no medical therapy for
a complete cure. When an infectious process gains the
upper hand and therapeutic intervention is essential, the
choice of treatment may be medicinal through the use of
antimicrobial agents or surgical by removing infected tis-
sues. The decision about which therapeutic modality or
combination of therapies to use is based on the extent,
urgency, and location of the disease process; the patho-
gen; and the availability of effective antimicrobial agents.
Antibacterial Agents
Antibacterial agents are generally called
antibiotics.
Most antibiotics are actually produced by other micro-
organisms, primarily bacteria and fungi, as by-products
of metabolism, and usually are effective only against
other prokaryotic organisms. An antibiotic is consid-
ered
bactericidal
if it causes irreversible and lethal dam-
age to the bacterial pathogen and
bacteriostatic
if its
inhibitory effects on bacterial growth are reversed when
the agent is eliminated. Antibiotics can be classified into
families of compounds with related chemical structure
and activity.
Not all antibiotics are effective against all patho-
genic bacteria. Some agents are effective only against
gram-negative bacteria, and others only gram-positive
bacteria. The so-called
broad-spectrum antibiotics
,
such as the newer cephalosporins, are active against a
wide variety of gram-positive and gram-negative bac-
teria. The four basic mechanisms of antibiotic action
C
A
G
T
A
C
C
G
T
T
A
A
A
A
A
A
A
C
C
C
C
C
C
C
C
C
G
G
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DENATURATION
ANNEALING
EXTENSION
PRODUCTS
Enzyme, dNTPs,
dye-labeled terminators
FIGURE 14-12.
Sanger sequencing. Sanger sequencing uses PCR to amplify DNA fragments and
labeled nucleotides as DNA elongation terminators.The DNA sample is divided into four separate
sequencing reactions; to each reaction one of the four dideoxynucleotide terminators (ddATP, ddGTP,
ddCTP, or ddTTP) are added to terminate DNA strand extension at random length, resulting in DNA
fragments of varying lengths.The newly labeled DNA fragments are denatured, and separated by size
(with a resolution of just one nucleotide) by gel or capillary electrophoresis. dNTPs, deoxynucleoside
triphosphates. (Copyright © 2014 LifeTechnologies Corporation;
Used
under permission.)
SUMMARY CONCEPTS
■■
The diagnosis of infectious disease relies
on the recovery of the probable pathogen or
evidence of its presence from infected sites in
the host and accurate documentation of signs
and symptoms compatible with an infectious
process.
■■
In the laboratory, the diagnosis of an infectious
agent is accomplished through the use of
culture, serology, and DNA/RNA sequencing
techniques.
