Page 76 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 9 Laboratory Diagnosis
pathogens in the genital tract is Neisseria gonorrhoeae. The
whereas human bites primarily involve the mouth
laboratory diagnosis of gonorrhea is made by microscopic
anaerobes.
examination of a gram-stained smear and by culture of the
organism.
Because anaerobes are frequently involved in these types
of infection, it is important to place the specimen in anaer-
Specimens are obtained by swabbing the urethral canal
(for men), the cervix (for women), or the anal canal (for
ratory. Because many of these infections are due to multiple
men and women). A urethral discharge from the penis is
frequently used. Because N. gonorrhoeae is very delicate,
organisms, including mixtures of anaerobes and nonanaer-
obes, it is important to culture the specimen on several
the specimen should be inoculated directly onto a Thayer- obic collection tubes and transport it promptly to the labo-
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Martin chocolate agar plate or onto a special transport
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different media under different atmospheric conditions.
The Gram stain can provide valuable information regard-
medium (e.g., Trans-grow).
Gram-negative diplococci found intracellularly within
ing the range of organisms under consideration.
neutrophils on a smear of a urethral discharge from a man
have over 90% probability of being N. gonorrhoeae. Because
smears are less reliable when made from swabs of the endo-
cervix and anal canal, cultures are necessary. The finding of
These methods are described in more detail in Chapter 64.
only extracellular diplococci suggests that these neisseriae
However, it is of interest here to present information on
may be members of the normal flora and that the patient
how serologic reactions aid the microbiologic diagnosis.
may have nongonococcal urethritis.
Nongonococcal urethritis and cervicitis are also
known antibody to identify the microorganism, and
extremely common infections. The most frequent cause is
(2) using known antigens to detect antibodies in the
Chlamydia trachomatis, which cannot grow on artificial There are essentially two basic approaches: (1) using
patient’s serum.
medium but must be grown in living cells. For this purpose,
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cultures of human cells or the yolk sacs of embryonated
Identification of an Organism with
eggs are used. The finding of typical intracytoplasmic
inclusions when using Giemsa stain or fluorescent anti-
Capsular Swelling (Quellung) Reaction
body is diagnostic. Because of the difficulty of culturing
C. trachomatis, nonbacteriologic methods, such as enzyme-
Several bacteria can be identified directly in clinical speci-
linked immunosorbent assay (ELISA) to detect chlamydial
mens by this reaction, which is based on the microscopic
antigens in exudates or urine or DNA probe assays to
observation that the capsule swells in the presence of
detect chlamydial nucleic acids (nucleic acid amplification
homologous antiserum. Antisera against the following
test, NAAT), are now often used to diagnose sexually trans-
organisms are available: all serotypes of S. pneumoniae
mitted diseases caused by this organism.
Because Treponema pallidum, the agent of syphilis, can-
groups A and C.
not be cultured, diagnosis is made by microscopy and
serology. The presence of motile spirochetes with typical (Omniserum), H. influenzae type b, and N. meningitidis
Slide Agglutination Test
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morphologic features seen by darkfield microscopy of the
Antisera can be used to identify Salmonella and Shigella by
fluid from a painless genital lesion is sufficient for the diag-
causing agglutination (clumping) of the unknown organ-
nosis. The serologic tests fall into two groups: (1) the non-
treponemal antibody tests such as the Venereal Disease
Salmonella and Shigella are commonly used in hospital
Research Laboratory (VDRL) or rapid plasma reagin (RPR)
laboratories. Antisera against the flagellar H antigens and
test and (2) the treponemal antibody tests such as the fluo-
the capsular Vi antigen of Salmonella are used in public
rescent treponemal antibody-absorption (FTA-ABS) test.
health laboratories for epidemiologic purposes.
These tests are described on pages 66 and 202.
Latex Agglutination Test
Wound & Abscess Cultures
in the presence of the homologous bacteria or antigen. This
A great variety of organisms are involved in wound and
test is used to determine the presence of the capsular anti-
abscess infections. The bacteria most frequently isolated Latex beads coated with specific antibody are agglutinated
gen of H. influenzae, N. meningitidis, several species of
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differ according to anatomic site and predisposing factors.
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streptococci, and the yeast C. neoformans.
Abscesses of the brain, lungs, and abdomen are frequently
caused by anaerobes such as Bacteroides fragilis and gram-
positive cocci such as S. aureus and S. pyogenes. Traumatic
In this test, the unknown bacterial antigen and a known
open-wound infections are caused primarily by members
specific antibody move toward each other in an electrical
of the soil flora such as Clostridium perfringens; surgical-
wound infections are usually due to S. aureus. Infections of
field. If they are homologous, a precipitate forms within the
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