Abstract：Objective In this study, Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was used to detect the δ-toxin, and to evaluate the role of δ-toxin production in methicillin-resistant Staphylococcus aureus (MRSA) typing and virulence expression. Methods The Bruker microflex MALDI-TOF was used to analyze collected proteins with the relative molecular weight ranging from 2,000 to 20,000 Da in a positive linear mode, and the software MALDI Biotyper 2 was used to analyze the original map of the MRSA strain. The generated list of peaks were analyzed directly by flexAnalysis and clinProTools3.0. Results A total of 83 MRSA were analyzed, and 39 (47.0%) MRSA showed peaks of (3005±5)m/z, including19 (22.9%) HA-MRSA and 20 ( 26.5%) CA-MRSA, showing no significant difference (P=0.766); 33 (39.8%)MRSA were detected with peaks at (3035±5)m/z , including 9 (10.8%) HA-MRSA and 24 (28.9%) CA-MRSA, showing the statistically significant difference between the two (P=0.003);11 (13%) MRSA were not detected with any peak at (3005±5)m/z or (3035±5)m/z, and thus all strains were HA-MRSA. As to spa typing, peaks at (3005±5)m/z were detected, 15 (18.1%) belonged to t062, 8 (9.6%) were t015, and 4 (4.8%) were t030, showing statistically significant difference (P=0); peaks at (3035±5)m/z were also found, 31 (37.3%) were t437 , 2 ( 2.4%) were t8660, showing statistically significant difference (P=0); the peak at (3035±5)m/z was the characteristic for spa type t437, and its area under the ROC curve was 0.89 (P=0). The δ- toxin was not detected in 11 strains, among which 6 were isolated from bone and joint specimens, 3 were isolated from the respiratory tract specimens, one from the secretions of chronic ulcer specimens, one isolated from the blood. In the blood plate colonies, 6 strains of MRSA colony morphology changed, while 5 colonies were normal. Conclusions MALDI-TOF MS can be used to quickly detect the δ- toxin of MRSA, and their mass spectrum peaks were at (3005±5)m/z and (3035±5)m/z. The peak at (3035±5)m/z is the mass spectrum of the δ-toxin homologous variant (HldG10S), which can quickly identify spa t437. The absence of δ-toxin is a manifestation of the dysregulation of the agr regulatory system, which is associated with chronic infections, small colony formation, and no obvious beta hemolysis.
