Green discoloration of the nail plate is the most common change in nail color of the fingernails. Yellow-green, green, and green-black discolorations of the nail plate are caused by pseudomonas bacteria, which can grow in the cleft between the nail plate and nail bed at the base of the nail. Pseudomonas bacteria ordinarily does not thrive on the skin and under nails because skin that is dry and cool inhibits the growth of microorganisms. Conditions have to be just right for bacteria to grow and produce the pigment pyocyanin, which causes the green discoloration.

Interestingly, pseudomonas infections of the nail usually develop first on the dominant thumb. Right-handed persons are most likely to develop the green discoloration on their right thumbnail, while lefties are more likely to develop it on their thumbnail. This is because thumbnails and fingernails are used as tools of counter-pressure. Whether picking up tiny objects or buttoning clothing, our nails enable us to grasp objects and make fine manipulations. Each time nails perform these functions, counter-pressure is applied between the thumbnail and one of the other four fingernails. Consequently, the thumbnail is subject to more frequent trauma throughout the day than any other digit. As a result, there is a tendency for the nail to separate from the nail bed on the thumbnail of the dominant hand, creating the ideal warm, damp environment for bacterial growth.

Fortunately, the bond between the nail plate and nail bed is strong, and nail separation rarely occurs. It is only when the nails are compromised that the nail plate and bed partially separate and infection develops at the site of separation.

How might nails be compromised? As a result of the “fulcrum” effect, long nails are more subject to trauma than short nails. Anyone who has ever tried to pry open a can understands the fulcrum effect. It is much easier to pry open a can with a long tool, such as a screwdriver, than with a small one, such as the edge of a coin. The longer instrument affords greater leverage and applies greater pressure with the same amount of effort as applied with a shorter object. The fulcrum principle applies to nails, as well. Longer nails experience greater counter-pressure than shorter nails. Routine activities are more likely to subject longer nails to injury and separation. As a result, the damaged tissue is more susceptible to bacterial invasion.

Nail enamels and artificial extension products can also compromise the nails. Under normal circumstances the nail plate is porous, meaning it will absorb water. When the hands are removed from water and dried, residual water evaporates from the nail bed through the nail plate. Evaporation allows nails to stay dry, which discourages the growth of pseudomonas bacteria and yeast (including the common candida albicans). People who have their hands constantly in water and wear artificial extensions are at highest risk for developing a nail infection.

Why doesn’t everyone who wears artificial nails or nail polish get a nail infection? Frankly, I’m not sure. How frequently the hands are immersed in water and how often the nails are traumatized are two important factors. Age seems to play a role as well, since this infection is more common in older persons. Poor circulation and underlying health conditions, such as diabetes, are also a factor.

You can help clients avoid pseudomonas bacteria infections by encouraging them to keep their nails at a moderate length and not to use them as tools. Also recommend that clients wear rubber gloves with cotton inserts to do wet work such as washing dishes.

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