Medical Fun Facts Podcast

MFF0047: MALDI-TOF

If I’ve timed this correctly, tonight’s show is dropping on Monday 13 March 2017. Canberra Day!

If you’re not a microbiologist or chemist, I bet you’re thinking I’m about to launch into a show about fungi. Well, no, I’m still doing the diagnostics series of Medical Fun Facts.

MALDI-TOF is an acronym for Matrix-Assisted LASER Desorption/Ionisation Time of Flight. And, just to be a pedant, LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.

MALDI-TOF is an ionisation technique in mass spectrometry. Ionisation is when a molecule gains or loses an electron and ends up with a negative or positive charge to become an ion. Mass spectrometry is a way of sorting out ions according to their mass to charge ratio. MS is a way of figuring out chemical structures of things like peptides. It’s a chemist thing and something I’d hoped never to have to understand in any detail.

Sadly, between 2007 and 2012, I didn’t spend much time in a microbiology laboratory, and the chemists got in and altered the way medical microbiologists work. The saving grace is that this MALDI-TOF business may get surpassed by whole genome sequencing soon.

With the correct combination of laser wavelength and the right matrix, a protein can be ionised. The matrix consists of crystallised molecules which have a low molecular weight, which are acidic to act as a proton source, and which have strong optical absorption to absorb the laser radiation. The matrix is mixed with the sample, the bacteria from a single colony. The laser is usually a UV laser and ionisation occurs when the laser is fired at the matrix. The matrix absorbs the laser energy and the matrix is desorbed and ionised by this action.

Time of flight refers to the type of mass spectrometry used because the mass of the ions is relatively large. The ion’s mass to charge ratio is worked out via a time measurement. The ions are accelerated in an electric field, and the velocity depends on the mass to charge ratio. Heavy ions move more slowly. The time to reach the detector is measured and this depends on the velocity which relates to the mass to charge ratio. The ion identity can be determined based on the time of flight.

The bottom line is that an instrument may cost about a quarter of a million dollars but each time you run a specimen it costs less than 10 cents per specimen. The throughput is quick too. You can identify a bacterium from an agar plate in less than one hour. There are even moves to try to use MALDI-TOF to detect some antimicrobial resistance markers. For a general purpose medical testing microbiology laboratory, MALDI-TOF has been a huge move forward in getting fast, accurate and cost effective test results.

There are some down sides though. You still need a colony on a plate so overnight incubation is required. The technology cannot differentiate some bacterial species, e.g., it cannot accurately differentiate between Escherichia coli and Shigella species. The database is not complete and it comes in modules, so if you don’t have all the modules you may miss the identity of a bacterium or yeast. The databases are updated very regularly so as organism names are updated so are the databases. This means, we’re having to learn new bacterial and yeast names all the time. For an old bloke like me, some days it’s more than I can handle.

If you disagree with anything in these podcasts or if you would like to voice a different view, please feel free to write a comment. If I have said something incorrect I welcome correction. Please also feel free to share your comments on social media.

You can find me on Twitter and Facebook. Please follow me on Twitter and like my Facebook page.