<p>Source data underlying plots shown in the study.</p>

Yet, the nutrient environment it experiences within the gut remains poorly characterized. In particular, little is known about the spatial distribution of nutrients across the microbial community, as resolving such fine gradients in vivo has been technically challenging.

Here, we engineer the native honeybee symbiont <i>Snodgrassella alvi</i> as a living biosensor to quantify the bioavailability of the dietary sugar arabinose within the gut. By expanding the genetic toolkit for <i>S. alvi</i> through chromosomal integration of high-burden genes and a suite of low-strength promoters, we achieve stable multi-gene expression without compromising host colonization. The resulting biosensor generates a specific, dose-dependent fluorescent response to arabinose in the living host, enabling visualization of sugar gradients across gut-associated bacterial biofilms at micron-scale resolution.