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The identification bottleneck
In a food laboratory, detecting a suspect colony is only half the work. The other half is knowing what it is. For decades that question was answered with batteries of biochemical tests, commercial strips and serology, which can take one or two additional days per isolate. MALDI-TOF mass spectrometry changed that stretch of the analysis: from a pure colony, it delivers an identification in minutes.
This article explains how the technique works, where it fits in the workflow of a food laboratory, how it is validated to confirm pathogens and which questions it cannot answer.
What MALDI-TOF means
The acronym describes the two parts of the process. MALDI (matrix-assisted laser desorption/ionization) is the way of turning the sample's molecules into ions: the microbial material is mixed with a chemical matrix that absorbs the energy of a laser and transfers it to the proteins, which are released and become charged. TOF (time of flight) is the way of measuring them: the ions are accelerated in a vacuum tube and reach the detector in a time that depends on their mass-to-charge ratio. The lightest ones arrive first.
The result is a spectrum: a series of peaks that works as a protein fingerprint. According to the review by Singhal and colleagues (2015), identification is based on the pattern of highly abundant ribosomal proteins, in the range of 2 to 20 kDa. Because those proteins change little within a species and differ between species, the profile makes it possible to recognize the microorganism.
What the laboratory workflow looks like
1. Start from a pure colony
MALDI-TOF does not work on the food or on the enrichment broth. It needs an isolate: a colony grown on a plate, ideally separate from others. Singhal and colleagues note that the technique cannot identify organisms directly from mixed populations. That is why MALDI-TOF comes after plate culture, not instead of it.
2. Prepare the sample
There are three levels of preparation, chosen according to the type of microorganism:
- Direct transfer: a minimal portion of the colony is spread on the instrument's metal plate and covered with the matrix. Singhal and colleagues indicate that it works well for many Gram-negative bacteria.
- Direct transfer with formic acid: a drop of formic acid is added before the matrix to help break more resistant cell walls. According to the same review, it improves the identification of Gram-positive bacteria and fungi.
- Full extraction: a protocol with ethanol and formic acid that releases the proteins before depositing them. Microorganisms such as mycobacteria require additional specific procedures.
3. Acquire the spectrum and compare it
The instrument generates the spectrum and the software compares it with a database of reference spectra. The result is an identification accompanied by a match score; each system defines the score ranges it considers reliable at genus or species level. A low score is not an identification, and the laboratory procedure must say what to do in that case: repeat with extraction, subculture or resort to another method.
The age of the culture matters
A detail that is often overlooked is that the protein profile changes with the physiological state of the cell. Janiszewska and colleagues (2023) studied the identification of spore-forming Bacillus species isolated from cow's milk, sauerkraut and pickled cucumbers. They found that the optimal incubation time varied between 1 and 12 hours depending on the strain, with fewer spores between 5 and 8 hours, and that after 24 hours of incubation only about 47% of the samples were successfully identified. In that study, direct transfer and extraction gave comparable results.
The practical lesson is that, for spore-forming microorganisms, an "old" plate can give low scores even though the instrument and the database work well. It is relevant when working with Bacillus cereus or with Alicyclobacillus, both spore formers.
MALDI-TOF as a confirmation method
In food microbiology, the most important regulatory use of MALDI-TOF is to confirm presumptive pathogen isolates. For a laboratory to use it for that purpose, the method has to be validated.
The AOAC 2017.09 method
The official method AOAC 2017.09, based on a commercial MALDI-TOF system, covers the confirmation and identification of Salmonella spp., Cronobacter spp. and other Gram-negatives from selective and non-selective agars, and was later extended to Campylobacter. The collaborative study published in the Journal of AOAC International (2019) included, for Salmonella, 15 collaborators from 15 service laboratories in Europe. The authors report 100% correct identifications at genus level for Salmonella and Cronobacter from each type of agar, and results equivalent to the confirmation procedures specified in each reference method.
The FDA BAM
Chapter 5 of the FDA Bacteriological Analytical Manual (BAM) accepts identification by MALDI-TOF according to AOAC OMA 2017.09 as one of the routes for confirming presumptive Salmonella isolates, along with the combination of serology and biochemistry and other validated instrumental methods. We develop it in presumptive vs. confirmed result.
ISO 16140-6 and ISO 16140-3
Within the ISO framework, the validation of alternative confirmation and typing methods is covered by ISO 16140-6:2019. And as with any validated method, each laboratory must demonstrate that it applies it correctly under its own conditions. For that purpose, Amendment 1 of ISO 16140-3 was published in 2025, adding a specific protocol for the verification of validated microorganism identification methods. We explain the difference between validating and verifying in validation and verification of methods according to ISO 16140.
What MALDI-TOF cannot do
- It does not detect in the sample. It needs a pure isolate. It does not replace enrichment or PCR screening; it comes in afterwards.
- It only recognizes what is in the database. Singhal and colleagues remind us that a new isolate can only be identified if the database contains the corresponding reference profiles. An uncommon spoilage microorganism may remain unidentified.
- It has difficulty with very close species. The same review cites as examples that it cannot reliably distinguish Escherichia coli from Shigella, or Streptococcus pneumoniae from S. mitis, without additional tests.
- It does not replace routine typing. Identifying an isolate as Salmonella does not say which serovar it belongs to. Serotyping remains a separate step, and for linking cases in an outbreak the standard is whole genome sequencing (WGS).
Where it contributes most in a food plant
In addition to pathogen confirmation, MALDI-TOF is useful in tasks where many isolates accumulate and the question is "what is this":
- Spoilage: identifying the yeast or bacterium isolated from a swollen package or an altered beverage guides the search for the root cause much more than a total count, as we saw in spoilage microorganisms.
- Environmental monitoring: identifying recurring isolates from the same point in the plant shows whether the same species repeats or whether each finding is different.
- In-house strain collections: keeping a record of the species found in the plant helps compare findings over time.
Singhal and colleagues cite a study in which identification by MALDI-TOF cost a few dollars per sample, compared with about 100 dollars for 16S rRNA gene sequencing, with results in minutes or hours instead of the 24 to 48 hours of conventional methods. The cost of the instrument is high, so the technique tends to be concentrated in high-volume or service laboratories, which offer it to plants that do not have the equipment themselves.
MALDI-TOF, PCR and WGS: three different questions
These techniques complement each other because they answer different questions. PCR answers whether a microorganism's genetic material is in the sample or in the enrichment. MALDI-TOF answers what species an already isolated colony is. WGS answers whether two isolates are, in practice, the same strain. A well-organized laboratory uses each one at the point in the workflow where its answer is the one needed.
Conclusion
MALDI-TOF identifies a microorganism from the fingerprint of its ribosomal proteins and reduces to minutes a step that used to take one or two days per isolate. In food, its most relevant regulatory use is the confirmation of pathogens with validated methods such as AOAC 2017.09, accepted by the FDA BAM, with ISO 16140-6 and the 2025 amendment of ISO 16140-3 as the validation and verification framework. Its limits are also clear: it needs a pure colony, depends on the database, has difficulty with very close species and does not replace typing.
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Frequently asked questions
Can MALDI-TOF analyze a food directly?
No. It needs a pure colony grown on a plate. The technique does not identify organisms from mixed populations, so it is used after culture, to identify or confirm isolates.
Is MALDI-TOF useful for confirming Salmonella?
Yes, with a validated method. The AOAC 2017.09 method covers the confirmation of Salmonella spp. from selective and non-selective agars, and the FDA BAM accepts it as one of the confirmation routes for presumptive isolates.
Why can an isolate give a low score?
Because of a mixed colony, little biomass, insufficient preparation for that type of microorganism, a culture that is too old (with spores, for example) or because the species is not in the database. The procedure should indicate whether to repeat with extraction, subculture or use another method.
Does MALDI-TOF replace serotyping or WGS?
No. It identifies the species or genus, but not the Salmonella serovar or the genetic relationship between isolates. For that, serotyping and whole genome sequencing are used.