Skin flora

Composition and functions of the skin flora

The skin flora is given by the set of microorganisms that populate our skin. In this regard, a resident flora is distinguished, which represents a habitual host of the skin of many people, and a temporary flora, given by microorganisms that can settle there but only temporarily.

Under normal conditions, the resident microbial flora is not pathogenic, while, considering the enormous quantity of micro-organisms with which it comes into contact, the skin can temporarily also host pathogenic or potentially such species.

Fortunately, our skin has numerous defenses that hinder its colonization by pathogens. Its most superficial layer, known as the stratum corneum, is constituted by a dense network of extremely flattened and closely spaced cells, so as to form a real barricade that opposes the loss of liquids and microbial penetration. It is precisely the reduced humidity that significantly limits the growth of this flora, whose density is clearly lower than in other districts, such as the oral cavity.

Furthermore, every fourteen days these cells are regularly renewed and, desquamating, they carry with them the microbes that settle in the fissures interposed between the horny scales (they are thus called the most superficial cells of the stratum corneum).

Cutaneous lipids, together with sodium chloride and immunoglobulins present in sweat, contribute to making the skin an inhospitable environment for the vast majority of microbes.

Similarly to what we have seen for the intestinal and vaginal bacterial flora, also the microorganisms that constitute the skin flora establish a relationship of mutual advantage with the organism. In fact, they hinder the colonization of pathogens by subtracting their nourishment, producing antimicrobial substances and lowering skin pH thanks to the degradation of the sebum they eat. Others, such as Staphylococcus aureus or Candida albicans, although potentially pathogenic, do not form colonies that are numerically sufficient to cause problems for the organism.

Just as the composition of the intestinal microbial flora is influenced by the current and previous dietary habits of the individual, the skin flora is also sensitive to climatic conditions, to the degree of personal hygiene, to the composition and quantity of sebum and sweat, and to numerous others factors that can influence its degree and type.

The typical colonization sites are the sebaceous glands, which produce an oily mass called sebum, and the hair follicles associated with them; instead the colonization of the sweat glands is more difficult, due to the antiseptic action of lactic acid, sodium chloride and antibodies present in sweat. The anaerobes populate the deepest portion of the hair follicles and the sebaceous glands, while the staphylococci, together with Pytirosporum sp ., Settle in their most superficial tract.

Generally speaking, the more humid and sebum-rich areas, as well as the areas close to the skin orifices, are richer in microbes. Among these microorganisms, there is a small bacterium GRAM - anaerobic, called Propionibacterium acnes, particularly fond of sebum. From the hydrolysis of the cutaneous lipids from it operated, they originate free fatty acids that enter the derma, irritating it and favoring those inflammatory phenomena that are at the base of the acne.

But the real danger of the skin flora comes from the possibility that these germs can reach the bloodstream or body districts where they are not normally present. This occurrence can occur, for example, due to an injury, a surgical operation performed in an environment that is not adequately sanitized, or in the presence of a temporary decline in the immune system. In these situations there is a radical change in the environmental conditions of the skin; the presence of humidity and necrotic tissue, for example, favors the proliferation of negative GRAM pathogens, hindering the growth of GRAM + saprophytes that are the basis of normal skin flora.

Skin flora and odors

The metabolism of cutaneous lipids and sweat secretions leads to the formation of substances, such as ammonia and short-chain fatty acids, responsible for bad body odors. An alteration of the normal skin bacterial flora or its excessive growth can therefore be the basis of the unpleasant odor typical of some individuals (it is not always and only a problem of poor personal hygiene). In these cases, there are specific deodorants, called bacteriostats, which can limit, but not inhibit, the proliferation of the skin bacterial flora (since, as we have seen, this is particularly useful in preventing the establishment of pathogens) .