Acquisition of microbes by the neonate, which begins immediately during birth, is influenced by gestational age and mother’s microbiota and modified by exposure to antibiotics 1 . In neonates, prolonged duration of antibiotic therapy is associated with increased risk of sepsis after 4 days of life, known as late-onset sepsis (LOS) 2 , a disorder critically controlled by neutrophils 3 , but a role for the microbiota in regulating neutrophil behavior in the neonate has not been described. We exposed pregnant mouse dams to antibiotics in drinking water to limit transfer of maternal microbes to the neonates. Antibiotic exposure of dams decreased the total number of microbes in the intestine, altered the structure of intestinal microbiota and changed the pattern of microbial colonization. These changes were associated with decreased numbers of circulating and bone marrow neutrophils and granulocyte/macrophage restricted progenitor cells in the bone marrow. Antibiotic-exposure of dams attenuated the postnatal granulocytosis by reducing the number of interleukin (IL) 17-producing cells in intestine and consequent production of granulocyte colony stimulating factor (G-CSF). Relative granulocytopenia contributed to increased susceptibility of antibiotic-exposed neonatal mice to Escherichia coli K1 and Klebsiella pneumoniae sepsis, which could be partially reversed by administration of G-CSF. Restoration of normal microbiota, through TLR4- and MYD88-dependent mechanism, induced accumulation of IL17-producing type 3 innate lymphoid cells (ILC) in the intestine, promoted granulocytosis, and restored the IL17-dependent resistance to sepsis. Specific depletion of ILCs prevented the IL17- and G-CSF-dependent granulocytosis and resistance to sepsis. These data support a role for the intestinal microbiota in regulation of granulocytosis and host resistance to sepsis in the neonates.
