Development of the human intestinal microbiota, the concept probiotics and their relationships with human health

Probiotics are live microorganisms which, when ingested in adequate numbers, confer health benefits to the host. They originate mostly from the colonic and vaginal microbiota of humans although a number of strains originate from the environment. The human fetus is colonized after birth by bacteria of maternal fecal and vaginal origin and by microorganisms from the environment. Maternal milk contains a varied microbiota, mainly lactobacilli and bifidobacteria. After weaning the resident microbiota becomes more complex and by 2 years of age it is composed of some 1500 species with 1014 microorganisms. During the colonization of the digestive tract of premature infants low birth weight, immaturity of the defenses and artificial feeding may lead to necrotizing enterocolitis. This inflammatory condition involves mainly the terminal ileum and the colon and may result in necrosis and perforation of the wall with subsequent peritonitis. Anoxia and abnormal colonization are important associated factors. Probiotic administration is associated with a decreased risk of this condition and decreases of its morbidity, mortality and sequelae if the treatment is started early. The positive effects are associated with more than one species of probiotics. The risk of septicemia associated with probiotics has been widely discussed. Studies in Helsinki, Finland, demonstrated that the results of comparing the frequency and etiology of septicemia during the 10 years after the introduction of probiotics with the results in the 10 years previous to their introduction were not different. Septicemia due to probiotics is infrequent and most cases are associated with extreme prematurity, failure of the intestinal barrier function, heart valve disease, severe shock and congenital immune deficiencies; patients with these conditions should be closely watched if they consume probiotics. However, patients with HIV and AIDS benefit from the consumption of these microorganisms. It has nor been demonstrated that probiotics play a role in the genesis of obesity.