Green nails. Now there’s something you may see occasionally in the podiatry clinic. The most common cause is an infection of the nail unit by the bacterium Pseudomonas aeruginosa. The species is a gram negative, facultative anaerobic bacterium that can lead to the discolouration of the nail as it produces a characteristic green-black pigments of pyocyanin, pyoverdin and pyorubin.
Pseudomonas infection causing green discolouration to the nail and paronychia
The bacterium is an interesting one as bugs go. Naturally you find it in the soil and in water. The infection superficially will only thrive in a damp environment hence it is seen most often in the fingernails of workers who undertake a lot of daily wet work. On the foot you may see it colonise around a damp wound or in macerated web spaces or within the nail unit. It is unusual to find more than two nail infected with the bacteria. Pseudomonas possesses various enzymes that allow it to invade the epidermis and digest keratin, hence it can dissolve nail causing discolouration.
Under the nail it thrives, similarly, where there is moisture. Typically, the toenail will be onycholytic permitting the ingress of water in which the bacterium can thrive leading to discolouration of the nail plate and potentially paronychia. Onycholysis can be due to a number of issues but on the feet typically its traumatic but can be observed in circulatory disorders, onychomycosis, patients with psoriasis or those taking drugs such as doxycycline.
Diagnosis
In most cases the diagnosis is straight forward due to the green discolouration in one or two nails but where there is doubt a swab or nail clipping sent to the laboratory for culture can be helpful. Other bacterial agents which can lead to green discolouration can also include Staphylococcus aureus and Klebsiella sp. (1). Negative swab cultures are not unusual as the actual infection may be some distance from the pigment. Woods light (read earlier blog here) occasionally shows a yellow-green fluorescence. Another test includes immersing a nail clipping in distilled water for 24 hours. A colour change to green suggests a Pseudomonas infection.
Treatment of the Nail
Despite the infection being recognised there is very little research investigating the most effective treatment. When onychomycosis is suspected, this should also be confirmed mycologically and treated as often the dermatophyte infection creates the environment conducive to pseudomonas infection. The bacterial infection will respond to antibiotics such as ciprofloxacin and gentamicin. These can be given orally but can used topically where appropriate. My colleagues use antibiotic eye drops applied behind the infected nail.
Interdigital co-infection with interdigital tinea and Pseudomonas. Note the green edge to the lesion.
Nail removal is rarely indicated but has been used as a treatment for the condition and requires some topical treatment to clear the nail bed of any residual infection after avulsion. Other agents which are worth considering include hypochlorous solution (read earlier blog here). Anecdotally, I have fenestrated infected nails using the Clearanail device and instructed the patient to apply daily Clinisept+ (hypochlorous solution) to the nail. Diluted vinegar has been suggested but care is needed as vinegar, if not sufficiently diluted, can cause irritation to the skin. Chlorohexidine as a topical antiseptic treatment should be avoided as resistance to the antiseptic has been observed with some strains of pseudomonas (2, 3).
References
1. Chiriac A, Brzezinski P, Foia L, Marincu I. Chloronychia: green nail syndrome caused by Pseudomonas aeruginosa in elderly persons. Clin Interv Aging. 2015;10:265-7.
2. Tag ElDein MA, Yassin AS, El-Tayeb O, Kashef MT. Chlorhexidine leads to the evolution of antibiotic-resistant Pseudomonas aeruginosa. Eur J Clin Microbiol Infect Dis. 2021;40(11):2349-61.
3. Davies A, Roberts W. The cell wall of a chlorhexidine-resistant Pseudomonas. The Biochemical journal. 1969;112(1):15P-P.