Global and local trajectories of infant gut microbiota development in Bangladesh across the COVID-19 pandemic.

The establishment of the gut microbiota in early life is fundamental to lifelong health, yet this process remains poorly explored outside Western and high-income settings. We investigated microbial maturation in a non-Western context, by longitudinally characterizing the gut microbiota of 20 healthy infants from urban Bangladesh (n = 984 fecal samples) across the first 2 years of life. Microbiota development followed a three-staged successional pattern characterized by a progressive increase of microbiota diversity, closely resembling patterns described in Western populations. Comparative analysis with a Belgian infant cohort showed broadly conserved maturation dynamics but population-specific differences in core bacteria, including Segatella, which was characteristic of Bangladeshi infants. Segatella's role in the developing gut remains unclear, as high abundances were associated with disease in this cohort. Transient disruptions in maturation also coincided with episodes of illness. The second year of this cohort coincided with the onset of the COVID-19 pandemic where differences in the relative abundance of several key taxa were detected. These insights expand our understanding of healthy infant gut microbiota development across populations and emphasize the need to consider sociocultural and environmental factors, including global disruptions, in shaping early-life microbial ecosystems.The first years of life represent a critical period during which the gut microbiota is established, with lasting implications for long-term health. However, current knowledge of this process is derived largely from studies conducted in Western and high-income populations even though gut microbiota composition is known to vary across geographic locations. In this study, we longitudinally characterized the gut microbiota development in a healthy infant cohort from urban Bangladesh, showing that geographically distinct populations can harbor different microbial communities while following similar developmental patterns. We further observe that illness can temporarily influence microbiota maturation, and that part of the microbiota maturation period in this cohort overlapped with a major societal disruption, the COVID-19 pandemic. Together, these findings illustrate the importance of geographic, cultural, and environmental context when defining healthy gut microbiota trajectories and contribute to a more inclusive, globally representative framework for understanding early-life microbiota development.