Enterobacteria of the neonate. Normal colonization and antibiotic-induced selection.

Enterobacterial digestive tract colonization of neonates under different conditions during their first days of life was studied. Our objective is to determine the frequency of antibiotic resistance under different conditions (hospitalized newborns treated or not with antibiotics, newborns in a maternity unit, mothers treated or not) thereby permitting a better appreciation of the role played by the 'selection pressure' created by antibiotics commonly used in perinatology. The method used was the culture of graded newborn fecal dilutions on selective media, some of which contained antibiotics. Cultures were made every day during the 1st week and 2 weeks of age. By using this technique, enterobacteria can be quantified as a function of their variety and of their resistance to different antibiotics. In the normal neonate under the normal conditions, the most predominant enterobacterium is an Escherichia coli sensitive to antibiotics; these infants also had several varieties of resistant enterobacteria, frequently acquired from their mothers. An antibiotic treatment of the mothers following delivery had no effect on the enterobacteria of the newborns. In a hospital setting, treated neonates rapidly acquired resistant species of enterobacteria. Nontreated neonates, initially often colonized with sensitive enterobacteria, acquired resistant enterobacteria during their first days of life.