The experiment was performed using three N. gonorrhoeae clinical isolates and N. gonorrhoeae strain ATCC 49226 was used as a reference strain. Tandem mass spectra were processed by PEAKS Studio version 8.5 (Bioinformatics Solutions Inc., Waterloo, Canada). PEAKS DB was set up to search the database assuming trypsin as the digestion enzyme and searched with a fragment ion mass tolerance of 0.05 Da and a parent ion tolerance of 7 ppm. Carbamidomethylation (C) and iTRAQ 4plex (K, N-term) were specified as the fixed modification. Oxidation (M), Deamidation (NQ), and Acetylation (Protein N-term) were specified as the variable modifications. Peptides were filtered with a 1% false discovery rate (FDR) and were required to be a unique peptide. PEAKSQ was used for peptide and protein abundance calculation. Normalization was performed by averaging the abundance of all peptides, and medians were used for averaging.