High-sugar diet leads to loss of beneficial probiotics in housefly larvae guts.

The housefly (Musca domestica) is a common insect species with only a few recurrent bacterial taxa in its gut microbiota, because the numerous microbial acquisition routes in its septic habitats can favor transient microbes. Here, we investigated the role of the diet on the microbiota and the developmental success of a housefly strain reared on three substrates. We used a control wheat bran-based substrate, and added clotted cream and sucrose to make a high-fat, and a high-sugar substrate, respectively. The conducted survey revealed that, in contrast to the high-fat diet, the high-sugar diet caused lower developmental success and less diverse microbiota, in which several lactobacilli were replaced with Weissella bacterial phylotypes. Cultures with sucrose as the sole carbon source confirmed that a Weissella confusa strain, isolated from larvae, could utilize sucrose more efficiently than other tested lactic acid bacteria; a result also supported by gene function prediction analysis. Enhancing the rearing substrate with Limosilactobacillus fermentum and Lactiplantibacillus plantarum strains, which were isolated from control larvae, could not only revert the negative effect of the high-sucrose diet on development, but also increase the gut bacterial diversity. In our study, we show that the microbiota shifts in response to the high-sucrose diet did not benefit the host, that showed lower developmental success. In contrast, high-sucrose favored specific components of the microbiota, that continued to be enriched even after multiple generations, outcompeting beneficial bacteria. Also, microbiome manipulation showed the potential of probiotics to rescue host performance and restore the microbiome.