A 38-year-old construction worker from Delhi presents with a corneal ulcer 5 days after a vegetative matter injury. Slit-lamp examination shows a central ulcer with a well-demarcated, sharp margin, copious purulent discharge, and rapid stromal infiltration with prominent neovascularization at the ulcer edge. Gram stain reveals gram-negative rods. Which feature best distinguishes this bacterial ulcer from a fungal ulcer?