SummaryThe effects of orally administered gentamicin and colistin on stool bacterial flora and overall antibiotic sensitivity patterns were evaluated in 100 newborns at risk for neonatal necrotizing enterocolitis. Gentamicin (2.5 mg/kg q6h) and colistin (1 mg/kg q6h) were administered to randomly selected groups of 50 newborns for 3 wk after birth during an 11-month study period. Stools were collected on days 1, 11, and 21 and cultures were grown under aerobic conditions on three different media.Staph. epidermidiswas the most common predominant organism in both antibiotic groups, whereasE. coliandKlebsiellawere the most common Gram-negative bacteria isolated. Seventeen % of these Gram-negative species were resistant to colistin and 9% to gentamicin, with a gradual increase occurring during the 3-wk period. On the basis of 980 positive cultures from all sites in babies in the nursery during the 11-month study,E. colisensitivity to kanamycin and gentamicin ranged between 92% and 100% except for one month midway through the study when sensitivity to kanamycin was at 80% and then returned to the 92–100% range. Klebsiella sensitivity to both aminoglycosides remained greater than 95% throughout. The incidence of neonatal sepsis remained consistent at seven to nine per 1000 live births during the study. One baby of 50 in the gentamicin group developed necrotizing enterocolitis at 5 wk of age; 0/50 in the colistin group had necrotizing enterocolitis (not significant).SpeculationThe results suggest that the use of oral antibiotics in a limited segment of the newborn nursery population may not have significantly adverse effects on the overall incidence of bacterial resistance to antibiotics and incidence of sepsis.