Matrix assisted laser ionization-time of flight mass spectrometry MALDI-TOF MS is a new method used in microbiological diagnostics, which allows shortening the time of identification of pathogen from the culture obtained from the moment of its growth. The principle of MALDI-TOF MS is based on the analysis of a unique protein profile, specific for a particular species, to allow diagnosis. This technique requires a small number of cells of microorganisms, it is possible to start analysis at the time of obtaining visible growth on the medium, which is usually possible within 16-24 hours after seeding material. Currently, there are cases reported of the use of MALDI-TOF MS in rapid microbiological diagnostic testing, wherein identification of species was performed with cultures incubated for approximately 4-6 hours (1).

Sample analysis using MALDI-TOF MS starts by connecting the test culture with a matrix solution on a surface of specially-designed slides. Depending on the manufacturer, up to 96 samples can be placed on one slide. After the application of the test strain, usually using a disposable plastic loop, less frequently, a toothpick or a pipette tip, and adding the matrix, the slide is left to dry and then to be placed in the analyzer. A single sample preparation and control time does not take more than a minute. A number of processes take place in the analyzer. First, absorption of laser energy by the matrix, which is then converted into heat energy. Under the influence of the heat generated a small amount of the matrix is abruptly heated and then undergoes desorption (evaporation) together with the test material. At the same time, under the influence of the laser, the remainder of the matrix is ionized, and the protons resulting in the process combine with the test substance. The resulting charged particles are suppressed in the electromagnetic field. The time of flight (TOF) of particles to the detector is dependent on the mass-to-charge ratio (m/z). Thus, particles with a smaller m/z reach the detector sensor more quickly than those with higher molecular mass and charge. In this way, a mass spectrum is obtained, which is then compared with the standard mass spectra contained in the database, as developed on the basis of reference strains. Databases containing reference spectra are developed by apparatus manufacturers and are being continuously expanded. Some systems allow you to manually add some standard spectra (2).

The purpose of the study was to compare the results of the identification of Enterobacteriaceae strains obtained by MALDI Biotyper apparatus (Bruker Daltonics) and a microbiological analyzer Vitek 2 (bioMèrieux).

The study involved 313 strains of Gram-negative Enterobacteriaceae cultured from clinical specimens derived from patients of the Infant Jesus Teaching Hospital in Warsaw.

Bacterial cultures were handled for 18-24 hours on the following bases: Columbia Agar with 5% sheep erythrocytes and MacConkey. Identification of microorganisms was performed simultaneously on two devices: a microbiological analyzer Vitek 2 (bioMèrieux), based on the biochemical characteristics of bacteria (identification cards of the ID-GN), and using the MALDI-TOF MALDI Biotyper (Bruker Daltonics), based on mass spectrometry (MS) with time-of-flight measurement. Alpha-cyano-4-hydroxycinnamic acid (Bruker Daltonics), dissolved in a mixture composed of: water, trifluoroacetic acid, acetonitrile, ratios of 19:1:20 (all HPLC grade reagents (Sigma)), was used as matrix for the MS analyzer.