Microorganisms, especially fungi and bacteria, are practically ubiquitous living beings . Spores can be found floating thousands of meters above sea level and in the extremely salty waters of the Dead Sea. Microorganisms populate our body . In fact, for every human cell we have, there is a bacterium; only since these are significantly smaller, the total weight of microorganisms that inhabit a human being is barely 200 grams.
A complex ecosystem lives in our intestines that helps us to digest —the microbiota , or badly called intestinal flora—; Our mouths are teeming with bacteria, and there are as many on a square centimeter of skin as there are people on the planet.
There are areas of the skin that are more preferred by bacteria and fungi than others; the genitals, the groin, the armpits or the feet present many more microorganisms than, for example, the elbow or the back of the hand. And one of the areas where more bacteria accumulate is under the nails.
Nails are hard and resistant chitin structures that are born attached to the skin of the fingers. The space between the nail and the finger is an optimal refuge for microorganisms. The same nail protects the microbial communities from possible external aggressions.
As in other areas of the body, bacteria and fungi form structures under the nails called biofilms . A biofilm, or biofilm , is a dense layer of bacteria made up of several layers, generally composed of different species. Its thickness can be up to 200 microns —2 tenths of a millimeter—. Bacteria secrete an adhesive substance , rich in polysaccharides, with which they remain firmly attached to solid surfaces. Archaea, fungi and other microorganisms also inhabit these biofilms. And the underside of the nail is an optimal place to establish these biofilms.
A biofilm is an extraordinarily resistant structure . Even when abrasive substances come into contact with it, only bacteria in the surface layers are affected. But the lower substrates are protected by the biofilm structure itself.
The way to act effectively against a biofilm is mechanical abrasion . For this reason, to eliminate the biofilms that reside under the nails, it is not enough to wash with soap and water or with hydroalcoholic gel, but it is necessary to rub under the nails, either using hard brushes or wooden sticks made specifically for it.
In many clawed predatory animals, bacteria residing under the nails are frequent sources of infection for their prey. This effect is also frequently present in bites, and generates an adaptive advantage for these animals.
The human being does not need these strategies, however, the bacteria and fungi under our nails can also be a source of infections. This is an important factor to take into account when handling food without gloves, putting on contact lenses or any other product that may be in contact with our mucous membranes, or when treating an injured person. These populations of microorganisms, at a time of low defenses, or when the nail or the underlying skin is damaged, can become opportunistic infectious agents.
Part of the bacterial population under our nails are species capable of causing health problems. The staphylococci genus ( Staphylococcus spp. ) is probably the most common, and can lead to food poisoning. Fecal bacteria Escherichia coli or species of the genus Enterobacter also abound.
Less common, but also present, are Klebsiella, Serratia or Streptococcus or the dangerous Salmonella .
Among the fungi, Trichophyton rubrum stands out, which has been defined as the most important pathogen in fungal infections of the skin, the main cause of onychomycosis — fungal infections of the nails or the skin under them. Other fungi that can appear under the nails are several species of the Candida genus —including Candida albicans— , yeasts that cause skin infections and, in some cases, sexually transmitted diseases . It is therefore important to practice good hand hygiene before engaging in sexual practices that involve hand-genital contact.
Other yeasts that do not usually cause health problems, such as those of the Saccharomices genus —such as beer yeast— are also common under the nails.
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