Matrix-Assisted Laser Desorption/Ionization Time Of Flight Mass Spectrometry (MALDI-TOF MS)
The instrument in the image belongs to the Department of Biomedical Sciences and Veterinary Public Health at the Swedish University of Agricultural Science (SLU). Lise-Lotte Fernström (+46 18 672389) and Lars Frykberg are responsible for the equipment and it is possible to get samples analyzed.
The principle of MALDI-TOF
Mass spectrometry based on the MALDI-TOF (or Maldi only) means that the sample to be analyzed, is adsorbed to some type of carrier material (matrix). The sample is then irradiated with laser UV light, so that the molecules in the sample are broken into positively charged fragments (ionization), which are thrown towards a detector. The time it takes for the fragment to reach the detector (time of flight) is measured. The time is dependent on fragment size and charge. Also very large molecules (proteins and nucleic acids) can be fragmented and ionized in this way. Large molecules give rise to many fragments and a characteristic mass spectrum, which can be used for identification.
The use of MALDI-TOF MS for identification of bacteria
One can perform these analyzes directly on bacterial colonies and the resulting mass spectrum is then compared with stored mass spectra of known bacterial species. Thus, you will get an analytical response within a minute and the method is considered to be very reliable. The more mass spectra of known bacteria you have to compare with, the safer the method will be.
MALDI-TOF MS is already used in some laboratories for veterinary bacteriology and many researchers believe that this technique will be tomorrow's routine method for identification of bacteria. The instrument is still very expensive, but material costs are low.
The software for the Maldi system contains a database with information about which fragments one can possibly get from a particular bacterium. This information can be used by the system to compare with unknown samples and calculate a score value for each sample. The score value is a measure of the probability that a particular sample will represent a particular bacterial species/strain. A score value between 0.000 and 1.699 means that the unknown sample is unlikely to represent the bacterium in question. A score value between 1.700 and 1.999 means that it is likely that the unknown sample represents the same genus as the bacterium in question. A score value between 2.000 and 3.000 means that it is highly likely that the unknown samples represent the bacterium in question.