MALDI in microbiology

Until recently, clinical diagnostic laboratories relied mainly on conventional phenotypic methods for diagnosing infections and sometimes on gene sequencing techniques. The latest technological advances include matrix assisted laser desorption ionization flight time mass spectrometry (MALDI-TOF), an entry into daily microbiological practice.

MALDI-TOF mass spectrometry generates specific mass spectral fingerprints, which can be considered as unique signatures of microorganisms that can assist in their precise identification at the genus and species levels – with considerable potential to use for typing strains.

Bacterial identification

Several methods of using MALDI-TOF mass spectrometry are now used in microbiological diagnostic laboratories. An approach compares results to commercially available mass spectrometry signatures databases to identify sampled bacteria, and a second uses a proteome database to identify biomarker masses in bacteria from sequenced genomes . The second bioinformatic approach allows for differences in crop growth and sample processing conditions, whereas it is particularly useful in routine laboratory methods, such as diagnosis, and can differentiate between species. and subspecies.

Given its accuracy, the technology can also be applied directly to various clinical specimens, including blood, cerebrospinal fluid, urine, pleural fluid, and peritoneal fluid. The main constraint is the amount of bacteria present in the samples, due to the detection limit of the current MALDI-TOF protocols. To circumvent this disadvantage, large volumes are usually required for blood and urine samples, as well as for the use of cultures as additional enrichment of the blood.

The results observed for bacterial identification with MALDI-TOF mass spectrometry using one of the two above-mentioned approaches, the diagnostic yield and accuracy strongly depend on the bacterial taxonomy and the quality of the databases used.

One of the main advantages of using MALDI-TOF technology to identify bacteria is the quick availability of results, usually ready in less than an hour. In addition, MALDI-TOF mass spectrometry allows the accurate identification of a wide variety of bacteria with rare phenotypic traits and requiring sequencing of the 16S rRNA gene prior to the MALDI-TOF era.

Fungal identification

MALDI-TOF mass spectrometry has been developed quickly and successfully for the identification of fungi. For now, this method is mainly used for the routine identification of yeasts, but further developments are needed (especially in sample preparation protocols and database libraries) to use this approach. identification for other groups of fungi (such as dermatophytes and filamentous fungi). .

MALDI-TOF mass spectrometry is mainly due to incomplete errors, absences or reference spectra in databases, in the image of bacteria. The disadvantage is that the reference spectra currently included in the databases of commercially available MALDI-TOF mass spectrometry systems are incomplete.

In addition, the spectral signal relevant to filamentous fungi depends on the fungal phenotype – which includes the basidiospore, the fruiting body, the surface mycelium, and the substrate mycelium. In addition, when cultured on an agar plate, the vegetative mycelium has many areas corresponding to distinct ages or developmental stages. This can lead to erroneous identification and variable results of the same sample. It is therefore essential that the databases include a complete database of several MS fingerprints at different stages of development of the filamentous fungi to ensure proper identification.

Accuracy, time and profitability

Compared with conventional microbial identification methods, MALDI-TOF mass spectrometry in most cases saves substantial time in terms of engineer / technician working time (sample preparation) and lead time ( obtaining results with an automated analytical procedure).

However, the purchase of the MALDI-TOF mass spectrometry instrument is undoubtedly one of the most capital-intensive investments for the clinical microbiology laboratory. This means that appropriate cost justifications and cost analyzes should be pursued, which should include mandatory quality control steps.

applications

MALDI-TOF MS is used in a variety of industries, including biopharmaceutical, chemical, organic, metabolomic, and genomic, as well as clinical, diagnostic, and therapeutic applications. In organic chemistry applications, MALDI-TOF is used to analyze masses of nucleic acid, proteins and polymers, as well as to identify complex mixtures of oligonucleotides and small proteins, thus providing information useful to researchers in biochemistry and chemistry. MALDI-TOF MS plays a crucial role in advancing rapid diagnosis of the patient and improving health outcomes. An excellent example of this is its use in the routine classification of microorganisms in patient samples for clinical microbiology.

Conclusion

In conclusion, MALDI-TOF mass spectrometry is a fascinating new technology for rapid, efficient, economical and easy-to-use microbial identification. Soon, this instrument will be ubiquitous in diagnostic laboratories because, despite its high cost and maintenance, consumables and operating costs are much lower than conventional methods.

Review of Chloe Barnett, BSc

Further reading