A 70-year-old man with a history of diabetes presents with severe pain in his right ear. The patient was diagnosed with external otitis. Further tests suggested that the patient suffered bone and nerve damage. Clinical laboratory analysis showed that the isolated microorganism produced a distinct blue pigment as well as an ADP-ribosylation toxin. What is the most likely causative agent?

External otitis or swimmer's ear is an inflammation of the outer ear and the ear canal. In the virulent form of the disease-malignant external otitis, which occurs in people with diabetes, damage to the cranial nerves and bone may occur. External otitis is caused by either bacterial or fungal pathogens. Bacterial pathogens include Pseudomonas aeruginosa, Staphylococcus aureus, Staphylococcus epidermidis, and Enterococcus faecalis. Fungal pathogens include Candida albicans and Aspergillus spp. The majority of bacterial external otitis is caused by P. aeruginosa. P. aeruginosa produces pyocyanin, which is a blue pigment that catalyzes the production of superoxide and hydrogen peroxide. Pyocyanin also stimulates the release of IL-8 (CXCL8 in humans). P. aeruginosa also produces exotoxin A, which ADP-ribosylates EF-2 in the eukaryotic cell leading to the cessation of protein synthesis and cell death. Neither pyocyanin nor exotoxin A is produced by the other pathogens listed (S. aureus, S. epidermidis, E. faecalis, and C. albicans).