How MALDI-TOF-MS makes mycobacterium diagnosis faster and more accurate

The laboratory I work in has plenty of instruments that help us characterise and identify microorganisms causing diseases in patients. One of my current projects is to validate an instrument called "matrix-assisted laser desorption ionization-time of flight mass spectrometer" (MALDI-TOF-MS) in identifying members of the Mycobacterium species. Many of these organisms are opportunistic, meaning they only cause illnesses in people whose immune systems are not strong enough to fight infections. Mycobacterium leprae is known for causing leprosy, but we cannot grow this bacterium in culture media, so we cannot isolate it. Mycobacterium tuberculosis complex, on the other hand, is a group of several species of Mycobacterium that causes tuberculosis. Tuberculosis is a disease that killed 1.6 million people in 2021 alone. It is a leading cause of death globally, second only to COVID-19 since the pandemic began.

Identifying the Mycobacterium species that has infected a patient is challenging because (1) this group of organisms require additional inactivating treatments, such as exposure to high temperatures (e.g., 121ºC for 15 minutes), to ensure that they cannot cause disease to the laboratorians; and (2) the method of choice for identifying mycobacteria (high-performance liquid chromatography, HPLC) has a limited library of chromatograms. This method can tell if a specimen has M. tuberculosis and other mycobacteria based on mycolic acid chromatograms. However, it won't be able to identify mycobacteria whose mycolic acid profiles are not included in the library.

Enter MALDI-TOF-MS. 

This technique separates ribosomal proteins by ionizing them and allowing them to travel in a time-of-flight tube. The separation of proteins is based on mass-to-charge ratios, if I understand correctly. The concentration of each separated protein then provides the mass spectrum, which is unique to each mycobacterium. The mass spectrum is compared to a library of mass spectra via a computer software. The software bases the identification on a "confidence score".

The MALDI-TOF-MS has several advantages over the HPLC. First, the library used by the MALDI-TOF-MS software is more extensive than the library used for HPLC. This is important because it lessens the number of misidentification events (particularly of organisms that are highly related). Second, if the inactivation of mycobacteria and the extraction of ribosomal proteins are conducted using a commercially available kit, the method becomes less error-prone. This can be attributed to the one-time inoculation of biomass into a microfuge tube. In comparison, several sample transfers during the sample preparation for HPLC increase the chances of creating errors during the preparation. Third, the streamlined methodology (from the kit) is much shorter than the sample preparation for HPLC.

I'm excited to say that the MALDI-TOF-MS just might live to see the day when more microbiologists in the laboratory can use the instrument routinely.