ML: MALDI-TOF MS Flashcards
(9 cards)
True or False: The College of American Pathologists (CAP) Microbiology checklist includes a section on MALDI TOF-MS.
TRUE
Feedback: TRUE. MALDI-TOF MS is now recognized as an accepted method in clinical microbiology for identification. As such, the CAP Microbiology checklist includes a section on MALDI TOF-MS.
What are the different sample spotting methods for MALDI-TOF MS?
a. Direct application with a toothpick
b. Adding a drop of supernatant from a full extraction tube
c. Mixing the specimen with formic acid
d. All the above
d. All the above
Feedback: Each of the described methods is acceptable for spotting organisms prior to adding matrix and performing MALDI-TOF MS. A variety of methods can be used to prepare a microorganism for MALDI-TOF MS analysis. These include:
Direct microorganism spotting on the target.
Direct formic acid overlay on the organism on the target.
Full chemical protein extraction.
Direct spotting fails to yield acceptable identification. What would be the next step to perform to attempt identification?
a. Use a different method of application; swab instead of toothpick.
b. Add formic acid to the spotted organism before applying matrix.
c. Transfer the organism to a liquid broth and test the next day.
d. Reincubate the culture plate and perform direct spotting the next day.
b. Add formic acid to the spotted organism before applying matrix.
Feedback: The addition of formic acid to the spot has been shown to improve the quality of spectra and result in better identification. If this method should fail, then a full extraction procedure can be performed.
Using a different application such as a swab is still considered a direct spotting method. This will not fix the issue.
Organisms should be transferred from liquid broth to solid media prior to performing direct spotting, not the reverse.
Reincubating the culture plate and trying again the next day will not fix the issue. Another extraction technique is needed to proceed forward.
True or False: Several studies have examined the effect of medium type on MALDI-TOF MS analysis and determined that all selective media affect the microorganism identification accuracy.
FALSE.
Several studies have examined the effect of medium type on MALDI-TOF MS analysis and determined that medium type does not affect the microorganism identification accuracy. However, analysis of isolates cultivated using selective media may negatively affect the spectral quality, possibly by interfering with protein ionization. This may lead to identification failures or low confidence, although it should not lead to erroneous identifications.
What common gram positive organism may be difficult to differentiate from closely related species by MALDI-TOF MS?
a. Staphylococcus aureus
b. Corynebacterium jeikeium
c. Streptococcus agalactiae
d. Streptococcus pneumoniae
d. Streptococcus pneumoniae
Streptococcus pneumoniae may require confirmatory testing (bile solubility or optochin disk) to be differentiated from closely related viridans streptococci like Streptococcus mitis.
True or False: The two types of target plates primarily used for microorganism identification are reusable and disposable.
TRUE
The two types of target plates primarily used for microorganism identification are reusable and disposable.
In addition to trifluoroacetic acid, which solvent is used for matrix preparation?
a. methanol
b. acetic acid
c. Acetonitrile (ACN)
d. water
c. Acetonitrile (ACN)
The matrix is solubilized in a mixture of trifluoroacetic acid and acetonitrile (ACN).
True or False: Both solid and liquid media can be used for bacterial identification by MALDI-TOF MS.
TRUE.
Numerous studies have compared the use of solid and liquid media for bacterial identification by MALDI-TOF MS. While these studies showed that visible differences in mass spectra were observed, the identification and accuracy were not affected. Since culture purity is essential, isolates should be grown on solid media for routine applications. Using solid media, purity can be visually assessed, and individual colonies can be selected for identification. An approximate biomass of 105 cfu/mL is required for identification.
What is the most commonly used matrix in whole-cell microorganism identification for MALDI-TOF MS?
a. 2,5-dihydroybenzoic acid
b. α-Cyano-4-hydroxycinnamic acid (CHCA)
c. Formic acid
d. Acetic acid
b. α-Cyano-4-hydroxycinnamic acid (CHCA)
CHCA solubilized in a mixture of trifluoroacetic acid and acetonitrile is the most commonly used matrix in whole-cell microorganism identification for MALDI-TOF MS.
