A 35-year-old woman with a history of contact lens use presents with a 3-day history of severe ocular pain, photophobia, and purulent discharge. Slit-lamp examination shows a central corneal ulcer with a well-demarcated border, dense infiltration, and anterior chamber reaction. The ulcer is progressing rapidly despite topical ciprofloxacin. Which investigation is most appropriate to guide antimicrobial therapy?