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Microbiology
20 cards from 2 lessons in Microbiology, each linking to the passage it came from.
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Urinary Tract Bacteriology and Identification Methods
How do you convert a urine culture colony count to CFU/mL?
Colonies counted ÷ inoculated volume in mL.
The calibrated loop supplies the volume represented by the colony count.
Read the passage: Quantitative culture and colony-count interpretation
What factor converts a colony count from a 0.001-mL calibrated loop to CFU/mL?
Multiply the colony count by 1,000.
A 0.01-mL loop delivers one hundredth of a milliliter, so its factor is 100.
Read the passage: Quantitative culture and colony-count interpretation
Why does a single urine colony-count cutoff not apply to every specimen?
The collection method, the symptoms, and the organism pattern change what a count means.
A low count can be significant from a suprapubic aspirate and reflect contamination in a voided specimen.
Read the passage: Quantitative culture and colony-count interpretation
Where is a small, fresh urine culture specimen collected from a catheter that has a needleless sampling port?
From the disinfected sampling port, with sterile aspiration equipment.
Urine from the drainage bag is unsuitable for culture.
What microbial material produces the fingerprint used in routine MALDI-TOF identification?
Abundant proteins, especially ribosomal proteins.
The measured mass spectrum is compared with a reference library.
What reference information is matched to the measured spectrum in MALDI-TOF identification?
A database of organism-associated mass spectra.
The organism whose library spectrum best matches the measured spectrum gives the identification.
Why does a positive NAAT result not by itself prove infection?
The target may be present during colonization or asymptomatic carriage.
For example, CDC describes this limitation for *C. difficile* molecular testing.
Read the passage: Molecular detection: target selection and amplification
What do you select for MALDI-TOF identification from culture?
An isolated colony from a pure culture.
Mixed growth can prevent a reliable identification of the intended isolate.
Antimicrobial Susceptibility Testing and Resistance Mechanisms
What is the minimum inhibitory concentration (MIC)?
The lowest antimicrobial concentration that prevents visible growth under the specified test conditions.
It measures growth inhibition only. A susceptible, intermediate, or resistant category comes from comparing it with the breakpoint for that organism and drug.
Read the passage: Interpretation: MIC, breakpoints, and categories
What measurement is read from a disk-diffusion susceptibility test?
The diameter of the growth-inhibition zone, in millimeters.
Compare the diameter with the interpretive criteria for that organism, drug, and method.
How does broth microdilution expose an isolate to different antimicrobial concentrations?
It uses a series of drug dilutions in small broth wells.
The lowest concentration with no visible growth is the MIC.
Why does an MIC alone not provide a clinical susceptibility category?
The category comes from comparing the MIC with the breakpoint for that organism and drug.
Breakpoints also depend on the dose given and the infection site, such as meningitis.
Read the passage: Interpretation: MIC, breakpoints, and categories
How does the CLSI intermediate category help when measurements fall near a breakpoint?
It acts as a buffer between the susceptible and resistant categories where measurement is uncertain.
This reduces the risk of calling a resistant isolate susceptible, or the reverse.
Read the passage: Interpretation: MIC, breakpoints, and categories
What does the CLSI susceptible-dose dependent category communicate?
Susceptibility depends on a dosing regimen higher than the standard one.
The laboratory reports the category with the exposure its breakpoint requires, and the clinical team uses that report to select the regimen.
Read the passage: Interpretation: MIC, breakpoints, and categories
What does *mecA* encode in methicillin-resistant staphylococci?
The altered penicillin-binding protein PBP2a.
Its low affinity for many β-lactams underlies the resistance.
Why is cefoxitin useful for phenotypic detection of methicillin resistance in *S. aureus*?
It is a strong inducer of *mecA* expression and gives clear endpoints.
Read the result against the cefoxitin criteria for that staphylococcal species and method.
Why does a negative *mecA* PCR result not rule out methicillin resistance?
A targeted assay misses mechanisms outside its target, such as *mecC*.
A cefoxitin phenotype or PBP2a test resolves the disagreement, and an unresolved conflict in *S. aureus* is reported as resistant.
What kind of susceptibility result is needed for vancomycin against *S. aureus*?
A true MIC value in µg/mL.
Vancomycin disk diffusion misses vancomycin-intermediate *S. aureus* (MIC 4 to 8 µg/mL).
Read the passage: Vancomycin-intermediate and -resistant S. aureus
What do you check when an *S. aureus* isolate shows unexpected reduced vancomycin susceptibility?
Culture purity, the organism identification, and a repeat MIC.
A mixed culture can create a misleading resistance result.
Read the passage: Vancomycin-intermediate and -resistant S. aureus
What do antimicrobial susceptibility testing (AST) control results have to show before affected patient results are reported?
Each control strain's result must fall inside its published QC range for that drug.
Investigate an out-of-range result and document corrective action before reporting resumes.
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