Characterization of a Staphylococcus aureus USA300 protein signature using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

Staphylococcus aureus strain type USA300 is an important human pathogen. Matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry has been used successfully for rapid identification of S. aureus at the genus and species levels. Here, it was hypothesized that MALDI-TOF could be used to identify USA300 organisms at the strain level. A genetic algorithm model using ClinProTools software (Bruker Daltonics) was built using 47 isolates of USA300 S. aureus and 77 non-USA300 S. aureus isolates. Three mass/charge peaks (5932, 6423 and 6592) were found to be discriminators between the groups of isolates. The model was validated using 224 test isolates: 197 of 224 test isolates were correctly classified as 'USA300 family' or 'non-USA300'. The sensitivity of the model was 0.87, with a specificity of 0.89, positive likelihood ratio of 8.19 and negative likelihood ratio of 0.15. The three-peak intensity MALDI-TOF model designed and tested in the current study can be used to rapidly identify USA300 family S. aureus isolates with reasonable sensitivity and specificity.