Evaluation of matrix-assisted laser desorption ionization-time of flight mass spectrometry in identification of methicillin resistance in clinical isolates of <i>Staphylococcus aureus</i>

Objectives: The present study was planned to identify specific peaks in mass to charge ratio (m/z) spectra of Staphylococcus aureus isolates as seen on Vitek MS (matrix-assisted laser desorption ionization-time of flight mass spectrometry [MALDI-TOF MS] Technology, bioMerieux, USA), which can recognize the bacteria as methicillin-resistant S. aureus (MRSA) or methicillin-sensitive S. aureus (MSSA). Materials and Methods: Clinical isolates identified as S. aureus by MALDI-TOF MS were included. Manual Antimicrobial susceptibility of study isolates was performed using cefoxitin as a surrogate marker for methicillin resistance. This was then confirmed by the minimum inhibitory concentration of cefoxitin using Vitek-2 Compact (bioMérieux, USA). An equal number of MRSA and MSSA isolates were randomly selected for evaluation of their m/z spectra as obtained earlier on MALDI-TOF MS during bacterial species identification. Statistical analysis: All the data were entered into MS Excel and presented as percentages or proportions. Chi-square test was applied as a test of significance. P &lt; 0.05 was taken as statistically significant. Results: One hundred isolates each of MRSA and MSSA were included. It was found that some m/z peaks were common to both MRSA and MSSA (3007, 5033, and 6890), whereas some other peaks were seen in MRSA (2417, 4822, and 5545; P &lt; 0.0001) and some others in MSSA (2190 and 2229; P &lt; 0.0001). Conclusions: The presence/absence of different m/z peaks can give a clue about its methicillin resistance status, thus reducing the turnaround time to report by ~24 h.