MALDI-TOF MS subject of PHE draft standard

MALDI-ToF mass spectrometry allows analysis of biomolecules (DNA, proteins, peptide and sugars) and large organic molecules (polymers, dendrimers and other macromolecules) which tend to be fragile and fragment when ionised by more conventional ionisation methods.

The technique can analyse protein composition of a microbial cell and has emerged as a new technology for species identification, particularly bacteria and fungi.

It is a soft ionisation method, with ionisation triggered by a laser beam.  

MALDI-TOF MS benefits​

It is popular because of reproducibility, speed and sensitivity with results available within minutes, or hours, rather than days.

With prior knowledge of microorganism type (through conventional and supplementary tests); users find it easier to know whether a bacterium or yeast is being tested. Without this, identification attempts usually fail, according to a consultation document.

Simplicity of sample preparation and low costs of consumables make the method well suited for routine and high throughput use despite initial substantial cost, said PHE.

A consultation on the draft standard (SMI test procedure 40) by PHE’s Microbiology Services standards unit is open until October 5​.

Methods under the Test Procedures category of UK Standards for Microbiology Investigations (SMIs) are mostly prescriptive recommendations relating to conventional laboratory procedures, said PHE.

However, SMI TP40 is an acknowledgement that it is in widespread use in many clinical laboratories for identification and characterisation of microbiological pathogens.

Standardisation of the diagnostic process through the application of SMIs helps to assure the equivalence of investigation strategies in different laboratories across the UK and is essential for public health surveillance, research and development activities.

Companies that use MALDI-TOF in the food industry include Shimadzu, Bruker, bioMérieux, SCIEX and Waters.

Limitations of the technique​

A drawback is with the currently available commercial platforms.

“A number of well-established commercial manufacturers use their own algorithms, databases, software, and interpretive criteria for microbial identification, thereby making numerical data (that is, spectral scores) between these different commercial systems not directly comparable​,” said PHE.

Failure to identify some organisms may occur due to growth media used.

A limitation is the spectral interference due to the presence of spores in some organism species, for example Clostridium species.

“Another limitation of this technique is the inability of the mass spectrometry spectra to differentiate similar or closely related organisms such as E.coli and Shigella species…”​ ​

Steps in the procedure involve a pure bacterial colony (typically single) picked from a culture plate to a spot on a MALDI-TOF MS target plate using a wooden or plastic stick, pipette tip, or loop - known as a direct smear application, the spot on the target plate is overlaid with 1–2 μL of matrix. 

Following drying at room temperature, the plate is placed in the ionisation chamber of the mass spectrometer for analysis and a mass spectrum is generated and automatically compared against a database of mass spectra by the software, resulting in identification of the organism.