Honey stores of Apis mellifera colonies are replaced with sugar water by beekeepers, which may result in malnutrition. Nutritional supplements have been developed, but the importance of bacterial probiotics and vitamins is poorly understood. Given that supplementary feeding with vitamins and probiotics enhances worker weight and longevity, this would suggest a feasible approach to mitigate winter colony losses. Here, we conducted a laboratory hoarding cage study with freshly emerged winter bees, which were treated with the antibiotic tetracycline to reduce gut bacteria and subsequently assigned to feeding regimes: sucrose only, sucrose + pollen, probiotics (low and high dosage), probiotics + pollen (low and high dosage), or b-vitamins (low and high dosage), (N=8 treatments, 29 workers/cage x8 replicates). In parallel, controls remained on their frame (=Frame) to establish their gut microbiota and were subsequently fed with sucrose only or sucrose + pollen (N=2 treatments, 29 workers/cage x4 replicates). Higher body weights were found in workers fed pollen, and in notably the Frame Sucrose + Pollen group, confirming a role of gut bacteria in protein digestion. Furthermore, both Frame groups and the antibiotic-treated workers fed with probiotic low + pollen survived longer than all other groups, highlighting an inherent host-microbial relationship. In contrast, high dosages of both probiotic and b-vitamins significantly reduced life span compared to their low concentration counterparts, probably due to dysbiosis and toxicity, suggesting that the outcome was dose-dependent. These results highlight that bacterial supplementation can alter longevity with advisable caution since harmful concentrations appear to exist.