5.58

Question 1

For ruling out syphilis (2)

Answer 1

•Rapid plasma reagin (RPR) test
•Venereal Diseases Reference
Laboratory(VDRL) test

Question 2

•Venereal Diseases Reference
Laboratory(VDRL) test
•These tests have a low

Answer 2

false
negative rate (very sensitive)

Question 3

Follow up a positive (2) test
to confirm syphilis (need a very
— test)

Answer 3

RPR or VDRL

specific

Question 4

Diagnosing infections by microscopy

Answer 4

• Direct microscopic examination of clinical specimens

Question 5

• Direct microscopic examination of clinical specimens

Syphilis -

Answer 5

spirochetes observed in scrapings of lesions

Question 6

Diagnosing infections by microscopy

• Stains

Answer 6

Gram stain and acid-fast stain

Question 7

Gram stain and acid-fast stain (3)

Answer 7

1. Bacteria in normally sterile body fluid (CSF, pleural fluid, urine)
2. Staining properties part of larger effort at diagnosis
3. Actual diagnosis (sometimes)
   e.g. Gram-negative diplococci in leukocytes of CSF
   = meningococci (agent of meningitis)
   e.g. acid-fast bacteria in smears of sputum
   = Mycobacterium tuberculosis

Question 8

Gram stain

4

Answer 8

•Apply the primary stain (crystal violet) to a
heat-fixed smear of bacteria.
•Add a trapping agent (or mordant) (Gram
s
iodine).
•Decolorize with acetone or alcohol. Thick
peptidoglycan layer of Gram positives keeps
crystal violet-iodine complex trapped within
cells. This step must be done quickly
(seconds); if too long the crystal violet will be
washed from both Gram-negatives and Gram
positives).
•Counterstain with safranin to stain
decolorized cells.

Question 9

acid-fast stain (Ziehl-Neelsen method)

3

Answer 9

•Apply the primary stain (fuschin) + mordant
(carbolic acid) (combination is called
carbolfuchsin) to a heat-fixed smear of
bacteria. Place piece of absorbant paper
soaked with carbolfuchsin over the smear and
heat the slide to drive the stain+mordant into
cells.
•Decolorize with dilute acid in alcohol. The
carbolfuchsin will wash out of most cells, but
not those with high levels of mycolic acid in
their membranes (e.g. mycobacteria)
•Counterstain with methylene blue to stain
decolorized cells.

Question 10

Gram-negative diplococci in urethral pus =

Answer 10

gonococci (gonorrhea) (only see counterstain)

Question 11

• Antibody-based identification -

Answer 11

Direct Fluorescent Antibody (DFA) test

Question 12

Diagnosing infections by culture

most bacteria:

Answer 12

culturing involves use of numerous kinds of growth media
can provide preliminary information about biochemical nature of bacterium
additional biochemical tests used following isolation

Question 13

Diagnosing infections by culture

some bacteria are not routinely cultured

Answer 13

rickettsias, chlamydiae, and mycoplasmas

identified with special stains, immunologic tests, or molecular methods

Question 14

Open-ended sampling versus looking for a particular pathogen

Answer 14

Pus specimen from brain abscess

Throat culture examined for
Streptococcus pyogenes

Question 15

selective media

Answer 15

prepared by the addition of specific substances to a culture medium that will permit growth of one group of bacteria while inhibiting growth of some other groups

ex. SS

Question 16

differential media

Answer 16

incorporation of certain chemicals into a medium may result in diagnostically useful growth or visible change in the medium after incubation

ex. EMB

Question 17

enrichment media

Answer 17

the addition of blood, serum will support the growth of many fastidious bacteria. these media are used primarily to isolate bacteria from CSF, pleural fluid, suputum, and wound abscesses

Question 18

characteristic media

Answer 18

characteristic media are used to test bacteria for particular metabolic activities, products or requirements

ex. urea broth is used to detect the enzyme urease. some enteric bacteria are able to break down urea, using urease, into ammonia and co2

Question 19

Bacteriophage Typing
• only done by
• based on specificity of

Answer 19

CDC and certain labs
phage surface molecules for host
cell surface molecules

Question 20

Bacteriophage Typing

• phagovar

Answer 20

– bacterium with sensitivity to certain collection of phage types

Question 21

Measuring the antibody response to infection

Answer 21

patient serum incubated with antigen on solid support
unbound ab washed away
enzyme labeled anti immunglobilin binds to patient ab bound to the antigen on the solid support
amount of enzyme activity is measured

Question 22

Diagnosing infection by detecting microbial macromolecules (4)

Answer 22

1. Detecting microbial antigens
2. Nucleic acid amplification (polymerase chain reaction (PCR))
3. Microarrays
4. Next generation sequencing

Question 23

Tests for SARS-CoV-2 Virus (COVID-19)

Answer 23

Real-time reverse transcription-polymerase chain reaction (rRT-PCR) or
sequencing of reverse transcribed SARS-CoV-2 RNA genome regions
Use nasal swab or saliva
Sensitive and specific

Question 24

Diagnosing HIV infection

Standard tests detect (2)

Answer 24

HIV antigen and anti-HIV antibodies

Question 25

Tests for Zika Virus

For symptomatic persons suspected for Zika virus infection

Answer 25

1. Real-time reverse transcription-polymerase chain reaction (rRT-PCR)
   Use serum or urine collected within 2 weeks after symptom onset.
   A positive rRT-PCR result confirms Zika virus infection and no
   additional testing is indicated.
   Sensitive and specific but subject to limited time window

Question 26

Tests for Zika Virus

For symptomatic persons suspected for Zika virus infection

Answer 26

2. A negative rRT-PCR result does not exclude Zika virus infection and
   serum should be analyzed by IgM antibody (serological) testing.
   Sensitive with longer time window but not as specific
   cross-reactivity with other flaviviruses