Page 77 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com The two most widely used treponemal tests are the FTA- mebooksfree.com
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PART I Basic Bacteriology
66
(2) The treponemal tests use T. pallidum as the antigen.
agar matrix. Because antibodies are positively charged at
the pH of the test, only negatively charged antigens, usually
ABS and the MHA-TP (microhemagglutination–Treponema
capsular polysaccharides, can be assayed. The test can be
used to detect the presence in the spinal fluid of the capsu-
pallidum) tests. In the FTA-ABS test, the patient’s serum
lar antigens of H. influenzae, N. meningitidis, S. pneumoniae,
sample, which has been absorbed with treponemes other
and group B streptococci.
reacted with nonviable T. pallidum on a slide. Fluorescein-
Enzyme-Linked Immunosorbent Assay
labeled antibody against human immunoglobulin G (IgG)
is then used to determine whether IgG antibody against
In this test, a specific antibody to which an easily assayed than T. pallidum to remove nonspecific antibodies, is
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T. pallidum is bound to the organism. In the MHA-TP test,
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enzyme has been linked is used to detect the presence of
the patient’s serum sample is reacted with sheep erythro-
the homologous antigen. Because several techniques have
cytes coated with antigens of T. pallidum. If antibodies are
been devised to implement this principle, the specific steps
used cannot be detailed here (see Chapter 64). This test is
useful in detecting a wide variety of bacterial, viral, and
fungal infections.
Cold Agglutinin Test
Patients with Mycoplasma pneumoniae infections develop
Fluorescent Antibody Tests
autoimmune antibodies that agglutinate human red blood
A variety of bacteria can be identified by exposure to
cells in the cold (4°C) but not at 37°C. These antibodies
known antibody labeled with fluorescent dye, which is
detected visually in the ultraviolet microscope. Various
thus false-positive results can occur.
methods can be used, such as the direct and indirect tech-
niques (see Chapter 64). occur in certain diseases other than Mycoplasma infections;
NUCLEIC ACID–BASED METHODS
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Identification of Serum Antibodies
There are three types of nucleic acid–based tests used in the
with Known Antigens
tests, nucleic acid probes, and nucleic acid sequence analy-
Slide or Tube Agglutination Test
sis. Nucleic acid–based tests are highly specific, quite sensi-
In this test, serial twofold dilutions of a sample of the
tive (especially the amplification tests), and much faster
patient’s serum are mixed with standard bacterial suspen-
than culturing the organism. These tests are especially use-
sions. The highest dilution of serum capable of agglutinat-
ful for those bacteria that are difficult to culture, such as
ing the bacteria is the titer of the antibody. As with most
Chlamydia and Mycobacterium species.
tests of a patient’s antibody, at least a fourfold rise in titer
Nucleic acid amplification tests (NAAT) utilize the poly-
between the early and late samples must be demonstrated
for a diagnosis to be made. This test is used primarily to aid
increase the number of bacteria-specific DNA or RNA
in the diagnosis of typhoid fever, brucellosis, tularemia, merase chain reaction (PCR) or other amplifying process to
molecules so the sensitivity of the test is significantly higher
plague, leptospirosis, and rickettsial diseases.
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mebooksfree.com mebooksfree.com mebooksfree.com detecting C. trachomatis and N. gonorrhoeae in urine sam- mebooksfree.com
than that of unamplified tests. Many bacteria can be identi-
fied using these tests, but they are especially useful in
Serologic Tests for Syphilis
The detection of antibody in the patient’s serum is fre-
ples in sexually transmitted disease (STD) clinics. These
quently used to diagnose syphilis, because T. pallidum does
tests are also used to identify M. tuberculosis in sputum
not grow on laboratory media. There are two kinds of tests.
samples.
Tests that use nucleic acid probes are designed to detect
(1) The nontreponemal tests use a cardiolipin–lecithin–
bacterial DNA or RNA directly (without amplification)
cholesterol mixture as the antigen, not an antigen of the
organism. Cardiolipin (diphosphatidylglycerol) is a lipid
using a labeled DNA or RNA probe that will hybridize spe-
extracted from normal beef heart. Flocculation (clumping)
of the cardiolipin occurs in the presence of antibody to
to perform than the amplification tests but are less sensitive.
T. pallidum. The VDRL and RPR tests are nontreponemal
Nucleic acid sequence analysis is used to identify bacte-
tests commonly used as screening procedures. They are not cifically to the bacterial nucleic acid. These tests are simpler
ria based on the base sequence of the organism’s ribosomal
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RNA. An organism that has never been cultured, Tropheryma
specific for syphilis but are inexpensive and easy to
perform.
whipplei, was identified using this approach.
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