Diagnosis of Bacterial Infection Flashcards
What factors impact the choice of diagnostic methods?
nature of the infectious agent suspected, host species, availability of tests
Detection of Infecting Agent
- Direct detection of agent
- Isolation and identification
- direct detection of antigens, toxins
- Nucleic acid detection
Detection of Host Immune Response
Humoral response detects antibodies to an infectious agent
Cell mediated immune response detects cellular response to an infectious agent
Specimen Collection Guidelines
- collect appropriate sample
- submit adequate quantity
- minimize contamination
- collect early in disease process
- submit in transport media
- collect before initiation of antimicrobial therapy
- multiple specimens for multiple lesions
Biosafety Levels
based on risk assessment. combination of lab practices and techniques, safety equipment and lab facilities
What is the single most important and cost effective lab procedure?
direct microscopic examination
What info does direct microscopic examination provide?
number, morphologic characteristics, sometimes host cellular response, likelihood of an infection, likely pathogens, predominant organisms
Limitations of direct microscopic examination
low sensetivity, low specificity, some bacteria do not stain well
Gram-Negative Stain
Pink. picks up safranin because of the thin peptidoglycan layer that allows the passage of large crystal violet and grams iodine complexes
Gram-Positive Stain
Blue. Retains crystal violet because the large crystal violet and Grams iodine complexes cannot escape the thick peptidoglycan layer
Acid-Fast Positve
Pink. retains the color of Carbol-Fuchsin because of the presence of mycolic acid
Acid-Fast Negative
Blue. decolorized and retain the color of the counterstain because of the absence of mycolic acid
Routine culture procedures
aerobic culture, anaerobic culture, microaerophilic culture, fungal culture
Special Culture Procedures
mycoplasma, salmonella, listeria, mycobacterium, campylobacter
PCR
amplification and detection of DNA
- Primers (forward and reverse)
- deoxynucleotide phosphates
- enzyme (taq polymerase)
- template DNA
Real-Time PCR
- primers
- labelled probes/fluorescent dyes
- DNTP
- taq polymerase
- template DNA
Antigen detection tests
utilizes specific antibody reagents to detect pathogens in clinical samples
Detection of humoral immune response
agglutination, precipitation, ELISA
Detection of cell mediated immune response
tuberculin reaction, interferon gamma test
Seroconversion
antibody development after exposure to a pathogen or an antigen
Antibody Titer
Measurement of serum antibody level against an infectious agent.
Paired Titers
Titers determined at 2-4 week interval during the course of infection. Four fold increase in paired titers is suggestive of an active infection
How do you interpret positive culture results?
Heavy pure culture of a bacteria indicated infection. Three or more types of organism indicative of contamination.
What are the limitations of PCR?
- sensitivity and specificity issues related to individual PCR
- PCR is target specific and you can only look for the specific organism/organisms you are looking for
- limits the number of targets you can detect
How does the antibody response work in infectious disease?
An antigen on bacteria bind to lymphocyte receptor on B cell. This produces helper T cells, which produce antibodies and plasma cells.
