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The page below is a sample from the LabCE course Microbial Identification Using MALDI-TOF MS. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Selective versus Nonselective Culture Media

Several studies have examined the effect of medium type on MALDI-TOF MS analysis and determined that the type of non-selective media used does not affect microorganism identification accuracy. However, analysis of isolates cultivated using selective media may negatively affect the spectral quality, possibly by interfering with protein ionization due to pigment acquisition and other non-descript environmental factors. This may lead to identification failures or low-confidence, although it generally does not lead to erroneous identifications. The effect is most apparent when microorganisms are directly transferred to the target slide or plate without an intervening extraction step. Thus, spectral quality can often be improved by performing an extraction method from selective agar. Alternatively, isolates cultivated using selective media can be sub-cultured to a nonselective medium before analysis to obtain optimal spectra.
The reader is referred to the manufacturer’s recommendations for a list of media that have been demonstrated to be compatible with the MALDI-TOF MS. The use of any other media must be validated to determine any potential influence on the spectral quality produced, the confidence scores (if applicable), and/or the accuracy of microorganism identifications.
Acceptable media types can include, but not limited to:
  • Trypticase soy agar (TSA) with 5% sheep blood
  • Chocolate agar
  • MacConkey agar
  • Columbia CNA agar with 5% sheep blood
  • Brucella agar with 5% sheep blood
  • CDC anaerobic agar with 5% sheep blood
  • CDC anaerobic phenylethyl alcohol agar with 5% sheep blood
  • Sabouraud-dextrose agar
  • Campylobacter agar with five antimicrobials and 10% sheep blood
  • Bacteroides bile esculin agar