07 Lab Identification Flashcards
What are the direct detection methods in primary specimens?
Microscopy. Antigen detection. Molecular methods (DNA/RNA probes, PCR). Toxin detection
What is Microscopy?
Direct observation of organisms with or without staining. Gram stain
What is Antigen detection?
Antigens of organisms are released into various body sites and can be detected even when the organism is not present is sufficient quantity to culture. Antigen may be the organism itself or a toxin
What are some common bacteria that antigen detection is used for?
Group A Strep, Chlamydia, Cryptococcus, N. meningitidis, H. influenzae. Think of the ones that vaccines are made for
What are the 2 methods of Nucleic Acid Detection?
1) Patient sample (fluid or tissue) mixed directly with DNA probe. 2) Patient sample undergoes PCR (DNA amplification) then DNA probe will search for specific sequence of genes
What is a DNA probe?
A single stranded DNA molecule used in laboratory experiments to detect the presence of a complementary sequence among a mixture of other single-stranded DNA molecules
What is the incubation time like for bacteria?
2-5 days
What is the incubation time like for Mycobacteria?
42 days
What is the incubation time like for fungi?
28 days
What is the incubation time like for Viruses?
21 days
What can be used with Agar-based medium?
Bacteria, Mycobacteria, Fungi
What can be used with Broth-based medium?
Bacteria, Mycobacteria
What is tissue culture used for?
Viruses
What is Step 1 in identifying bacteria grown in culture?
Organisms inoculated into a wide variety of media types containing biochemical reagents and nutritional supplements
What is Step 2 in identifying bacteria grown in culture?
Incubation or growth phase
What is Step 3 in identifying bacteria grown in culture?
Interpretation of results: growth vs. no growth, colony type, color change, oxidase, catalase, fermenter
What is an alternate process to Step 3?
Automated processors (Vitek, Microscan, Pheonix). Add bacteria from positive culture to automated “cards”. Specific cards for Gram positive and Gram negative pathogens with different antibiotics. Identification and sensitivity results in 4 hours
What are the indications for antimicrobial susceptibility testing?
Potentially pathogenic bacteria based on source of specimen, species isolated, numbers of colonies present, normal flora, underlying host factors (S. viridans in sputum - normal flora –> not tested). Bacteria with unpredictable susceptibility to the drugs of choice that would be used in treating an infection caused by the bacterium (S. pyogenes in wound - always susceptible to PCN –> not tested)
What are the routine antibacterial susceptibility tests performed in most clinical microbiology laboratories?
Disk Diffusion (Kirby Bauer), E-test. Broth microdilution. Automated tests (e.g. MicroScan, Vitek). Beta-lactamase test
What does the Beta-Lactamase test do?
Measures the presence or absence of B-lactamase for Enterococcus, Hemophilus, Moraxella sp., Neisseria gonorrheae
What is Disk Diffusion?
A suspension of the bacterial strain is adjusted to a standard density and the suspension is swabbed evenly on a Mueller-Hinton agar plate. ABX disks are applied to the inoculated surface of the agar plate. ABX is diffused out from the disk onto the agar surface as the bacteria multiply. NO zone of inhibition if bacterial cells are not inhibited by the ABX. The more susceptible, the larger the zone of inhibition. The diameter of the zone of inhibition is indirectly proportional to the minimal inhibitory concentration (MIC)
What is an E-Test?
Antibiotic is impregnated onto the strip in a gradient of doubling concentrations. MIC is where the zone of inhibition and bacteria meet the strip
What is Broth Microdilution?
Each well contains broth media to support growth of organisms. ABX is added at varying concentrations (2-fold dilution) to each well. Standard inoculum of organisms is added to each well; plus positive and negative controls
What is the Minimum Bactericidal Concentration (MBC)?
Measures the lowest concentration of abx that kills a bacterial isolate