Bacterial Diversity in Meconium of Preterm Neonates and Evolution of Their Fecal Microbiota during the First Month of Life

A nurse in a neonatal intensive care unit collects the very first stool a premature baby will ever pass. It is dark, sticky, and almost tar-like. It has a name: meconium. For decades, the scientific assumption was that it was sterile — a blank slate, untouched by bacteria, the first chapter of a gut microbiome waiting to be written by the outside world. Jiménez and colleagues set out to test that assumption. What they found was that the first chapter had already been written. The stakes for getting early gut colonization right are high, especially in preterm infants. The microbial community that establishes itself in those first weeks shapes barrier function, nutritional processing, and immune development. If it goes wrong, or if it is shaped by the wrong forces, the risk of serious disease climbs. Preterm infants are uniquely exposed: they are born before the colonization process that term infants undergo, placed immediately into a hospital environment, and treated with the antibiotics and interventions that define neonatal intensive care. Jiménez and colleagues wanted to know what bacteria actually colonize these infants, starting from the very first sample they could collect, and how that community changes over the first three weeks of life. To do that, they followed fourteen preterm infants with a mean gestational age of 28 weeks and a mean birth weight of 1,288 grams. Seven were delivered by Cesarean section. All but one received antibacterial prophylaxis for at least the first three days of life. Seven required mechanical ventilation. The team collected the first spontaneously passed meconium from each infant — sometimes arriving within minutes of birth, and sometimes not until day five — and then took fecal samples roughly weekly over the next three weeks. Two complementary methods were used to analyze those samples, and this methodological choice matters. Culture-based techniques grow bacteria on selective media in the lab; they identify and quantify living organisms, allowing researchers to study individual isolates in detail. But culture has a blind spot — many bacteria cannot be grown in laboratory conditions at all. So the team also used molecular methods: PCR-denaturing gradient gel electrophoresis, or DGGE, and a DNA microarray called the Human Intestinal Tract Chip, or HITChip. The HITChip uses more than 4,800 oligonucleotide probes targeting regions of the 16S ribosomal RNA gene — a kind of universal bacterial barcode — to detect over a thousand distinct bacterial variants, including ones that would never grow on a petri dish. When the team compared the two approaches, both pointed in the same broad directions. HITChip consistently detected greater diversity. Shannon-Weaver diversity indices for meconium ranged from 0.00 to 1.38 by culture, but from 2.09 to 4.28 by microarray. The molecular method was simply more sensitive. Both told the same story; HITChip told it in more detail. Now, the finding that overturns the assumption: meconium was not sterile. Every infant in this cohort had detectable bacteria in their first stool. The community that was there had a recognizable shape. Firmicutes — and specifically the class Bacilli — were the dominant bacterial group. By culture, Staphylococcus came out on top, with mean counts around 6.5 log10 colony-forming units per milliliter. HITChip flagged strong signals for bacteria related to Lactobacillus plantarum and Streptococcus mitis. These are not classic pathogens. Lactobacillus is a genus associated with gut health — the kind of organism you see promoted in probiotic products. Its prominence in meconium is notable. Its presence before the infant has had any meaningful contact with the outside world raises the question of where it came from. The authors point to previous evidence of similar organisms in umbilical cord blood and amniotic fluid and suggest that bacteria may reach the fetal gut through in utero swallowing of amniotic fluid. Not all of this first community, in other words, was put there by the neonatal intensive care unit. Every infant's meconium profile was different in its details — inter-individual variability was high. However, the group-level pattern was consistent and distinct from what came next. What came next was a rapid takeover. Over the following weeks, the Firmicutes-dominated community in meconium gave way to a very different one. By the first and second weeks of life, Proteobacteria were in charge. Escherichia coli, absent from every meconium sample, appeared in six infants in the first week alone, with mean counts approaching 8.9 log10 colony-forming units per milliliter, and kept climbing in prevalence through week three — a shift significant at a p-value of 0.003. Klebsiella pneumoniae followed a similar trajectory, jumping from one meconium detection to eight first-week fecal samples with mean counts of 9.18. HITChip results confirmed the picture at the community level, showing Enterococcus, Escherichia, Klebsiella, and Yersinia-related organisms dominating third-week feces. And then there was Serratia marcescens. It was not detected in a single meconium sample, but then it was present in four infants by the first week and seven by the second week. Mean counts in those second-week samples ran to 9.16 log10 — a level that, in a healthy adult, might be manageable, but in a 28-week preterm infant with an immature immune system represents a very different situation. The change over time was significant at a p-value of 0.009. These are not harmless commensals. Escherichia, Klebsiella, and Serratia are organisms known to cause invasive disease in preterm infants, as the paper puts it. The gut that was colonized by Lactobacillus-related bacteria at birth was, within weeks, colonized by the kind of bacteria that cause neonatal sepsis and necrotizing enterocolitis. What drove that shift? The neonatal intensive care unit itself. The authors are careful to frame their findings as associations rather than proven causation, but the associations are pointed. Serratia prevalence was strongly linked to hospital-related factors — specifically antibiotherapy and mechanical ventilation. Consider the mechanism: broad-spectrum antibiotics, given to nearly every infant in this cohort from the first days of life, kill off early colonizers and open ecological space. What fills that space tends to be whatever is present in the hospital environment — and Serratia, Klebsiella, and their relatives live there. Mechanical ventilation introduces additional risk and correlates with the most extreme prematurity. The feeding context matters too: infants received maternal breast milk, donor milk, or preterm formula, all of which carry different microbial and nutritional profiles. The diversity data confirm the pattern. HITChip-measured diversity was significantly higher in fecal samples than in meconium, with a p-value of 0.016 — meaning the gut community grew more complex over time. But complexity in this case meant more Proteobacteria, not more Lactobacillus. A richer microbiome, but one skewed toward hospital-acquired organisms. What the Jiménez study ultimately shows is that the newborn preterm gut is not a blank slate waiting for its first microbial instruction. There is already a community in place at birth — one that looks nothing like what the neonatal intensive care unit environment subsequently produces. That first community, dominated by Bacilli and Lactobacillus-related organisms, is displaced within weeks by a Proteobacteria-dominated succession that tracks with clinical interventions and hospital exposure. The meconium and the feces are telling different stories, and understanding that difference is where the clinical implications live. Interventions aimed at reshaping that trajectory — probiotic supplementation, tighter antibiotic stewardship, and optimized feeding strategies — remain to be tested in controlled studies. The paper establishes the pattern, not the remedy. But the pattern is clear enough to be actionable as a research target. A preterm gut seeded with Klebsiella and Serratia in its first weeks is starting from a fundamentally different immunological position than one seeded with Lactobacillus. The first stool a premature baby passes is a message about where that infant has already been, microbiologically. The fecal samples that follow are a record of where the hospital took them next. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.