A 32-year-old male farmer from rural Maharashtra presents with a painless, indurated ulcer on his left hand that has been present for 3 weeks. He reports contact with armadillos on his farm. Gram stain of the ulcer exudate shows numerous acid-fast bacilli arranged in tight clusters. Culture on Löwenstein-Jensen medium at 30°C shows slow growth of yellow colonies after 4 weeks. Which structural feature of this organism is most responsible for its resistance to standard disinfectants and antibiotics?