Knowledge of the progression of fecal microbial colonization patterns could be important in determining an infant's predisposition to bacterial invasion. However, there is little detailed information regarding the overall fecal microbial milieu in extremely premature infants. We serially characterized stool microflora in 29 infants of birthweight <1000 gm. Specimens were collected on days 10, 20, and 30 (± 2) postnatally, and quantitative aerobic and anaerobic cultures were performed.

Stool bacterial patterns changed between days 10-30, with Gram (-) organisms increasing over time. The most commonly identified bacteria on day 10 were S. epidermidis, S. haemolyticus, E. faecalis, and K. pneumoniae; by day 30, predominant species were E. faecalis, E. coli, S. epidermidis, E. cloacae, K. pneumoniae, and S. haemolyticus. Lactobacillus spp. were only identified in a single infant. On day 30, more stools were colonized by E. cloacae and E. coli compared to day 10 (p<.05). Candida spp. decreased significantly from day 10 to day 30. Total number of bacterial species/stool increased over time in breastmilk-fed infants (2.50±0.34 on day 10; 3.13±0.38 on day 20; and 4.27±0.45 on day 30 (p<.001); most of this increase was accounted for by Gram (-) species. Species number in formula-fed infants did not increase significantly over time. Absolute total number of bacteria/gm stool also increased significantly over time, primarily in breastmilk-fed infants, again resulting from increased Gram (-) organisms. There was an inverse relationship between days on antibiotics in the first month and number of bacterial species (r=.491; p=.007) and total organisms/gm stool(r=.482;p=.008) on day 30. Gestational age, birthweight, maternal steroid or antibiotic therapy, PROM, and mode of delivery did not correlate with number of fecal bacterial species.

These results indicate that the gut of extremely low birthweight infants is colonized by a paucity of bacterial species and only rarely contains lactobacilli. Breastmilk feeding and reduction of antibiotic exposure are key elements in increasing fecal microbial diversity, which in turn may protect the sparsely colonized immature gut from overgrowth by potentially pathogenic species.