Diagnosis of Bacterial Infection

Question 1

What factors impact the choice of diagnostic methods?

Answer 1

nature of the infectious agent suspected, host species, availability of tests

Question 2

Detection of Infecting Agent

Answer 2

1. Direct detection of agent
2. Isolation and identification
3. direct detection of antigens, toxins
4. Nucleic acid detection

Question 3

Detection of Host Immune Response

Answer 3

Humoral response detects antibodies to an infectious agent

Cell mediated immune response detects cellular response to an infectious agent

Question 4

Specimen Collection Guidelines

Answer 4

1. collect appropriate sample
2. submit adequate quantity
3. minimize contamination
4. collect early in disease process
5. submit in transport media
6. collect before initiation of antimicrobial therapy
7. multiple specimens for multiple lesions

Question 5

Biosafety Levels

Answer 5

based on risk assessment. combination of lab practices and techniques, safety equipment and lab facilities

Question 6

What is the single most important and cost effective lab procedure?

Answer 6

direct microscopic examination

Question 7

What info does direct microscopic examination provide?

Answer 7

number, morphologic characteristics, sometimes host cellular response, likelihood of an infection, likely pathogens, predominant organisms

Question 8

Limitations of direct microscopic examination

Answer 8

low sensetivity, low specificity, some bacteria do not stain well

Question 9

Gram-Negative Stain

Answer 9

Pink. picks up safranin because of the thin peptidoglycan layer that allows the passage of large crystal violet and grams iodine complexes

Question 10

Gram-Positive Stain

Answer 10

Blue. Retains crystal violet because the large crystal violet and Grams iodine complexes cannot escape the thick peptidoglycan layer

Question 11

Acid-Fast Positve

Answer 11

Pink. retains the color of Carbol-Fuchsin because of the presence of mycolic acid

Question 12

Acid-Fast Negative

Answer 12

Blue. decolorized and retain the color of the counterstain because of the absence of mycolic acid

Question 13

Routine culture procedures

Answer 13

aerobic culture, anaerobic culture, microaerophilic culture, fungal culture

Question 14

Special Culture Procedures

Answer 14

mycoplasma, salmonella, listeria, mycobacterium, campylobacter

Question 15

PCR

Answer 15

amplification and detection of DNA

1. Primers (forward and reverse)
2. deoxynucleotide phosphates
3. enzyme (taq polymerase)
4. template DNA

Question 16

Real-Time PCR

Answer 16

1. primers
2. labelled probes/fluorescent dyes
3. DNTP
4. taq polymerase
5. template DNA

Question 17

Antigen detection tests

Answer 17

utilizes specific antibody reagents to detect pathogens in clinical samples

Question 18

Detection of humoral immune response

Answer 18

agglutination, precipitation, ELISA

Question 19

Detection of cell mediated immune response

Answer 19

tuberculin reaction, interferon gamma test

Question 20

Seroconversion

Answer 20

antibody development after exposure to a pathogen or an antigen

Question 21

Antibody Titer

Answer 21

Measurement of serum antibody level against an infectious agent.

Question 22

Paired Titers

Answer 22

Titers determined at 2-4 week interval during the course of infection. Four fold increase in paired titers is suggestive of an active infection

Question 23

How do you interpret positive culture results?

Answer 23

Heavy pure culture of a bacteria indicated infection. Three or more types of organism indicative of contamination.

Question 24

What are the limitations of PCR?

Answer 24

1. sensitivity and specificity issues related to individual PCR
2. PCR is target specific and you can only look for the specific organism/organisms you are looking for
3. limits the number of targets you can detect

Question 25

How does the antibody response work in infectious disease?

Answer 25

An antigen on bacteria bind to lymphocyte receptor on B cell. This produces helper T cells, which produce antibodies and plasma cells.