MALDI-TOF MS
One-Sentence Definition
Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry identifies microorganisms by matching the mass spectrum of their abundant cellular proteins against a reference spectral library.
Simple Explanation
Put a colony on a plate, fire a laser, weigh the proteins that fly off, and match the pattern to a database. Minutes instead of a day.
Principle
- A colony is spotted onto a target plate and overlaid with a chemical matrix
- A laser pulse desorbs and ionizes the sample
- Ions accelerate through a flight tube; lighter ions arrive first
- Time of flight → mass-to-charge ratio → a spectrum dominated by ribosomal proteins (~2–20 kDa)
- The spectrum is compared with a reference library; a score expresses confidence at genus and species level
Performance
- Turnaround: minutes per isolate after growth
- Cost per identification: very low once the instrument is purchased
- Excellent for most aerobic bacteria, yeasts, anaerobes; improving for moulds and mycobacteria (needs extraction)
Known weak points
- Shigella vs Escherichia coli — essentially indistinguishable
- Streptococcus pneumoniae vs other mitis-group streptococci
- Some closely related Acinetobacter, Burkholderia, and Enterobacter species
- Database coverage determines what can be identified at all
Clinical Importance
- Collapsed identification time from ~24 h to minutes, enabling much earlier targeted therapy and de-escalation (Antimicrobial Stewardship)
- Direct application to positive blood culture broth shortens sepsis workflows further
Resistance detection
Not a susceptibility test. Research and some commercial approaches use spectra for resistance signals — carbapenemase hydrolysis assays and machine learning on raw spectra — see Proteomics and MALDI Bioinformatics and AI Diagnostics in Microbiology. Routine practice still needs Antimicrobial Susceptibility Testing.
Related MOCs
Active Recall Questions
- Which protein class dominates the identifying spectrum?
- Name two species pairs MALDI-TOF cannot reliably separate.
- Does MALDI-TOF report susceptibility? Explain.