A 20-year-old male who works as a truck driver presented to the skin and venereal disease OPD with a genital ulcer, which was painless for the last 10 days. He had unprotected sexual contact with a commercial sex worker about 2 weeks previously. On examination the ulcer was found to be circumscribed, indurated and partially healed. The inguinal lymph nodes were enlarged. What would be probable cause of infection on the basis of history and genital lesion?