A patient with a nail discoloration lasting over a year, unresponsive to over‑the‑counter antifungal treatments, now reports the nail is green rather than black; what is the likely cause and how should it be managed?

Green Nail Discoloration: Pseudomonas Bacterial Infection

This is bacterial green nail syndrome (chloronychia) caused by Pseudomonas aeruginosa, not a fungal infection, and requires immediate cessation of antifungal treatment and initiation of antibacterial therapy.

Diagnosis

The greenish (rather than black) nail discoloration that has persisted over a year and failed to respond to antifungal treatment is pathognomonic for Pseudomonas aeruginosa infection 1. The British Association of Dermatologists guidelines explicitly state that "bacterial infection, particularly when due to Pseudomonas aeruginosa, tends to result in green or black discoloration of the nails" 1.

Key Clinical Features to Confirm:

• Classic triad: Green discoloration, proximal paronychia, and distal onycholysis 2
• Color spectrum: Ranges from pale green to dark green, sometimes appearing almost black 2
• Single nail involvement: Typically affects one or two nails 3
• Moisture exposure history: Look for occupational or domestic water exposure, recent manicures, or trauma 4, 5

Critical Pitfall:

The failure to respond to antifungal treatment over a year should have immediately triggered reconsideration of the diagnosis. Bacterial infections can coexist with fungal infections and require treatment in their own right 1.

Essential Investigations

Obtain bacterial culture and antimicrobial susceptibility testing from nail clippings to confirm P. aeruginosa and guide targeted therapy 4. This is critical because:

• Multidrug-resistant strains exist 6
• Co-infections with other bacteria (e.g., Burkholderia cepacia, Achromobacter xylosoxidans) can occur 4, 6
• Empiric treatment may fail without susceptibility data

Treatment Algorithm

Immediate Management:

1. Mechanical debridement: Cut back the detached nail plate as far as possible 2
2. Topical antiseptic: Apply 2% sodium hypochlorite (dilute bleach) solution to the nail bed twice daily 2
3. Moisture control: Wear latex gloves over cotton gloves for all water-related household tasks 2

Definitive Antibiotic Therapy (guided by culture):

First-line topical options:

• Ciprofloxacin topical solution (otic formulation can be used off-label) 5
• Ozenoxacin 1% cream daily for 12 weeks—particularly effective for multidrug-resistant strains 6
• Nadifloxacin topical 3

Systemic therapy (for severe cases or treatment failure):

• Oral levofloxacin or ciprofloxacin based on susceptibility testing 4
• Consider combination oral + topical therapy for complex or co-infections 4

Treatment Duration:

Continue therapy for 12 weeks minimum to ensure complete eradication and prevent recurrence 6.

Why Antifungals Failed

The British Association of Dermatologists guidelines emphasize that nondermatophyte mould infections should be suspected "when previous antifungal treatment has failed on several occasions" 1. However, in this case, the green color definitively indicates bacterial rather than fungal or mould etiology. The nail surface in bacterial infection does not become soft and friable as in fungal infections 1.

Monitoring and Follow-up

• Clinical improvement should be visible within 4-6 weeks with appropriate antibacterial therapy
• Repeat culture if no improvement after 4 weeks to assess for resistance or co-infection
• Patient education on moisture avoidance and nail trauma prevention to prevent recurrence