See also: vaginal bacterial flora, intestinal bacterial flora, oral bacterial flora
Generality
The human bacterial flora consists of numerous microbial species that colonize exposed areas, such as the skin, or communicating with the outside, such as the oral cavity, the gastrointestinal tract, the respiratory tract, the vagina and the lower urinary tract.
The situation changes radically at the time of delivery, when the newborn comes into contact with the microbes coming from the genitourinary tract of the mother. In the following hours and days, the germs transmitted by people and the environment with which the young body comes into contact will be installed. From this moment on, the aforementioned body areas will begin to acquire their complex "ecosystem", consisting of different microbial species.
What at first glance might seem to be a passive process, is actually a complex and delicate system, strongly regulated by a constraint made of mutual advantages. The human body supplies nourishing substrates to its own bacterial flora, which in turn protects it from pathogens, preventing the development of other microorganisms in the same habitat. In these complex interactions, the immune system represents an attentive spectator, ready to intervene if the balance breaks. Bacteria that are normally not harmful can indeed become so when they multiply without control or migrate to other areas of the body.
Food shortages, traumatic injuries, prolonged antibiotic therapies or a temporary lowering of the immune defenses can cause an alteration of the human microbial flora.
Skin bacterial flora
As you can imagine, the body surface is exposed to a great variety of potential colonizers, from which it is protected by adopting various defensive strategies (lipids present in sebum and in the hydrolipidic film, poor hydration, frequent replacement of external cell layers, acid pH and sweat immunoglobulins). For this reason, bacterial settlements are concentrated near the skin orifices and in the more humid areas, such as the axilla or the interdigital folds of the feet. The decomposition of cutaneous lipids and glandular secretions is responsible for the bad smells that, look at random, become more intense in the above mentioned skin areas. Similarly, alterations of the normal skin bacterial flora can be responsible for unpleasant odors, which are not always an indication of bad personal hygiene.
Supported by an excessive secretion of sebum, the proliferation of some microorganisms and in particular of Propionibacterium acnes, favors the onset of inflammatory processes that occur with the appearance of pimples and acne.
Bacterial flora of the respiratory system
The bacterial flora of the upper airways is very similar to the oral but less abundant. As you descend along the respiratory tree, the concentration of these microorganisms further decreases, until it disappears at the pulmonary alveoli.
The mucus, secreted by the muciparous respiratory glands, helps to protect the organism from pathogens, trapping them inside and neutralizing them through the antibodies it is equipped with.
Bacterial flora of the digestive system
The digestive tract is colonized by an impressive number of microorganisms, which are particularly numerous also from the qualitative point of view. In the oral cavity we find the so-called bacterial plaque, a sort of patina adhered to the surface of the teeth on which the bacteria develop. The main causes of caries are Streptococcus mutans and Lactobacillus acidophilus . The body defends itself from their cariogenic attack through saliva, but little can be done when a diet too rich in sugars is accompanied by poor oral hygiene.
The bad smell (halitosis) can be, also in this case, a sign of the presence of certain bacterial colonies, whose metabolism produces volatile sulphurous substances with an unpleasant smell.
In the oral cavity of healthy people small colonies of pathogens such as Candida albicans can also be found. However, these microorganisms are present in numerically insufficient quantities to carry out their pathogenic activity. When their virulence increases, for example due to a temporary drop in the body's defenses, they can give rise to specific pathological conditions (in the specific case of the lily of the valley).
In the stomach, the presence of microorganisms is strongly limited by gastric acidity. The exception is Helicobacter pylori which can cause the formation of an ulcer in the long run.
