To avoid potential adverse side effects of chemical plant protection products, microbial pest control products (MPCP) are commonly applied as biological alternatives. This study aimed to evaluate the biosafety of a MPCP with the active organismBacillus thuringiensisssp.aizawai(strain: ABTS-1857). An in-hive feeding experiment was performed under field-realistic conditions to examine the effect ofB. thuringiensis (B. t.)on brood development and the bacterial abundance of the core gut microbiome (Bifidobacterium asteroids,Gilliamella apicola, the group ofLactobacillusandSnodgrasella alvi) inApis melliferaworker bees. We detected a higher brood termination rate and a non-successful development into worker bees of treated colonies compared to those of the controls. For the gut microbiome, all tested core members showed a significantly lower normalized abundance in bees of the treated colonies than in those of the controls; thus, a general response of the gut microbiome may be assumed. Consequently, colony exposure toB. t.strain ABTS-1857 had a negative effect on brood development under field-realistic conditions and caused dysbiosis of the gut microbiome. Further studies withB. t.–based products, after field-realistic application in bee attractive crops, are needed to evaluate the potential risk of these MPCPs on honey bees.