A 38-year-old contact lens wearer from Delhi presents with acute corneal ulcer. Gram stain shows gram-negative rods, and culture grows Pseudomonas aeruginosa. A second patient, a 45-year-old farmer, presents with a chronic corneal ulcer following ocular trauma. Fungal culture grows Aspergillus fumigatus. Which single feature most reliably distinguishes the expected clinical course of the Pseudomonas ulcer from the Aspergillus ulcer?