Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS), as a high-throughput, low-cost, and efficient mass spectrometry technology, is currently widely used in microbiology laboratories. The MALDI-TOF MS technology platform we have established can provide support for food microbiology, clinical microbiology, water quality testing, veterinary microbiology and pharmaceuticals.
MALDI-TOF MS combines both MALDI and TOF technologies. It can identify bacteria in a short time and shorten the time required for microbial identification. It is a fast and reliable mass spectrometry technique.
Figure 1. MALDI-TOF MS methodology. (Hou.; et al. 2019)
Before analyzing the mass of the sample, it must be ionized in the ion source. MALDI is an ionization technology developed from laser desorption (LD). The mixed liquid of the sample solution and the matrix aqueous solution is placed on the metal sample plate, and then the ionization and desorption of the sample and the matrix material are triggered by the irradiation of the pulsed laser. The analyte molecule and the ionized matrix undergo a charge transfer reaction during the desorption process, thereby ionizing the analyte molecule. MALDI is suitable for dissociating biological macromolecules such as DNA, protein, peptide and sugar, and has the advantage of not requiring a lot of pre-processing work.
Figure 1. Schematic diagram of MALDI. (Lavigne.; et al. 2013)
After the analyte molecules undergo ionization and ion transfer, these ions will be detected and measured in the mass analyzer. The TOF analyzer consists of a mass filter, a flight tube and a collision cell. A high-voltage pulse is applied to accelerate the ions into the flight tube. The ion mirror at the end of the flight tube reflects ions and transports them to the detector. Measuring the time it takes for the ion to reach the detector can determine the mass-to-charge ratio (m/z) of the ion. TOF analyzer has high resolving power and can provide accurate analyte information.
There are differences in the sequence and size of ribosomal proteins and some housekeeping proteins (usually 2-20kDa) between different bacterial species. These ribosomal proteins account for about 60-70% of the dry weight of microbial cells. MALDI-TOF MS can obtain the peptide mass fingerprint through the detection of microbial samples, and compare it with peptide fingerprint libraries to achieve species level identification of microorganisms. MALDI-TOF MS is a simple and convenient microbial identification technology, which greatly shortens the time required for microbial identification.
As a proteomics technology, MALDI-TOF MS can not only be used as a tool for microbial identification, but also as an efficient and sensitive tool for strain typing. For example, MALDI-TOF MS can be used for typing and characterization of beta hemolytic streptococci, streptococci group A, Vibrio spp., Y. enterocolitica, etc.
MALDI-TOF MS is also often used for antimicrobial susceptibility testing (AST). The resistance of microorganisms to β-lactam antibiotics depends on the expression of β-lactamase, which can hydrolyze the β-lactam ring and thereby inactivate this antibiotic. This hydrolysis process caused an 18 Da mass change, and this mass change caused by the degradation of antibiotics can be detected by MALDI-TOF MS technology. In addition, MALDI-TOF MS can be used to find specific biomarkers of antibiotic resistant strains, as well as phenotypic antibiotic resistance analysis of bacterial strains, so as to obtain AST results in a shorter time.