“Nothing is perfect..”

Apologies for the paucity of posts recently. I have had an unwell child.

MALDI-TOF (Matrix Assisted Laser Desorption & Ionisation- Time Of Flight) has revolutionised microrganism identification in the past decade, and in particular bacterial identification in the microbiology laboratory. It will continue to evolve into fields such as filamentous fungi, mycobacteria, antimicrobial resistance etc.

Rarely has such an innovation been so rapidly and comprehensively adopted by the laboratory community. And you can see why. It’s quick, it’s easy, it’s cheap (the consumables are anyway) and it’s reliable.

But it’s not perfect….

Some well known identification limitations are listed below where MALDI-TOF struggles to give the microbe a label, or at least one which fits in with our pre-defined beliefs.

Take the following well known, and not so well known examples…

  • E. coli and Shigella species
  • Streptococcus pneumoniae and Streptococcus mitis
  • Bordetella pertussis and Bordetella bronchiseptica
  • Klebsiella oxytoca and Raoultella ornitholytica
  • Shewanella putrifaciens and Shewanella algae
  • Proteus vulgaris and Proteus penneri
  • Klebsiella pneumoniae and Klebsiella variicola
  • Haemophilus haemolyticus and Haemophilus influenzae
  • Serratia ureilytica and Serratia marcescens

There are many more. Please feel free to suggest others that you have come across. There will be even be differences between different MALDI-TOF platforms, e.g. Bruker and VITEK MS.

Most of the time it does not matter too much from a clinical point of view. Some of the time, it does, and we need to try and clarify with molecular or biochemical means.

It shouldn’t surprise us however that the identification is not perfect. All identification systems are essentially typing systems in disguise, and they all have databases or libraries at the core of their function..

  • “APIs” are a typing system based on biochemical reactions.
  • MALDI-TOF is a typing system based on proteonomics.
  • 16SRNA sequencing is a typing system based on nucleic acid sequences.

Identification systems therefore suffer from the inherent problems of all typing systems. No typing system can be matched exactly against any other typing system.

For all typing systems the ability to group isolates and the ability to discriminate them is a trade-off of sorts. For MALDI-TOF, the bigger the databases get, the more the system can discriminate, and thus the more it can potentially differ from the original (textbook) identification.

So no system is perfect, including MALDI-TOF.

But it’s very, very good….


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