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Background Gastrointestinal function depends on the normal formation of the enteric nervous system (ENS) during fetal and postnatal development. Prior research in an outbred strain of mice (NIH Swiss) has shown that the absence of the gut microbiome in germ-free (GF) mice results in morphological and functional abnormalities of the ENS compared to specific pathogen free (SPF) mice, including an alteration in proportion of nitrergic neurons. Increasing research has been suggesting that the genetic background of the host can impact the host response to the GF state. Aims We tested the hypothesis that the absence of the microbiome in an inbred mouse strain (C57BL/6) could influence the development of the ENS during early postnatal life. Methods C57BL/6 GF and SPF mice were sacrificed at postnatal day 3 (P3) and P28 (n=4–5 per group). Ileum and colon were collected at P3 and P28 and processed for whole mount preparations. The neuronal network in the myenteric plexus was visualized by immunohistochemistry using antibodies against the pan-neuronal marker PGP9.5. Neuronal cell bodies and nitrergic neurons were identified by immunolabeling with antibodies to the neuronal marker HuC/D and to neuronal nitric oxide (nNOS). Nerve fibre density was quantified by measuring the percentage of PGP9.5-positive pixels (μm2) compared to the whole field using an image analysis program (Volocity; reported as %). Proportions of nitrergic to myenteric neurons were determined by manually counting (blinded) the number of nNOS-positive neurons and dividing by the total number of HuC/D-positive cells per field (reported as %). Results We found a significant increase in nerve density at P3 in the GF compared to SPF mice in both ileum (43% vs. 37%; p=0.03) and colon (45% vs. 39%; p=0.03). No significant differences, however, were identified between GF and SPF mice at P28 in either ileum (27% vs. 25%; n.s.) or colon (31% vs. 32%; n.s.). At P3, no significant differences in proportion of nitrergic neurons were seen in the GF compared to SPF ileum (27% vs. 27%; n.s.). Conclusions In contrast to earlier observations in the NIH Swiss mice, in which GF mice had decreased nerve density and an increase in nitrergic neurons at P3, our findings reveal an opposite response in nerve density in the C57BL/6 mice and no change in nitrergic neurons. These results suggest that the genetic strain of the mouse model can influence the host response to changes in the microbiome. Further studies are needed to further elucidate potential underlying mechanisms. Funding Agencies NSERC

A120 INFLUENCE OF THE MICROBIOTA ON THE POSTNATAL DEVELOPMENT OF THE ENTERIC NERVOUS SYSTEM IS DEPENDENT ON MOUSE STRAIN