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 may enhance worker weight and longevity, this might 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 obtained post-emergence 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 × 8 replicates). In parallel, another age cohort of bees 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 × 4 replicates). The most beneficial effects on body weights were found in workers given ad libitum access to pollen, notably in the Frame Sucrose + Pollen group, confirming the inherent importance of post-emergent gut flora inoculation and the 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 a fundamental host-microbial relationship. On the other hand, our current treatments alone, post-tetracycline, did not yield any positive results. In contrast, high dosages of both probiotic and b-vitamins significantly reduced lifespan compared to their low concentration counterparts, probably due to dysbiosis and toxicity, suggesting that the outcome was dose-dependent. These results highlight that bacterial and b-vitamin supplementation can alter longevity with advisable caution since harmful concentrations appear to exist.