## 4 Discussion
ROS can damage both host and microbial cells, and previous studies showed that compounds that conferred protection against ROS were produced in the bee midgut during _V. ceranae_ infection [45; 46; 47]. Interestingly, _V. ceranae_ spores were not detected in any bee bread samples, even those collected from colonies demonstrating high _V. ceranae_ spore counts. Nevertheless, microbial diversity was reduced in group D of bee bread samples. Therefore, we assume that _V. ceranae_ infection might affect bee bread microbiota indirectly through altered microbial inoculation by honey bees. During pollen collection and storage, bees add small amounts of regurgitated nectar as well as glandular secretions to pollen that eventually is transformed to bee bread during fermentation. Therefore, it is plausible that bees with high _V. ceranae_ spore numbers inoculate collected pollen with modified microbial communities of lower diversity as compared with those with low _V. ceranae_ spore numbers. Accordingly, be-pollen fermentation could lead to microbial communities characterized by lower diversity. Regarding the relative abundance of specific genera (bacterial and fungal), no significant change was observed among control group A and group B. A previous study reported a neutral or even positive effect of _V. ceranae_ infection on the core bee bacteriophage [48]. [PERSON] et al. briefly reviewed similar studies and compared these data with their own findings. Overall, they concluded that _V. ceranae_ infection does not detrimentally affect the homeostasis of the honey bee and its core bacteriophage [48]. A rather stable core bacteriophage, also observed in our study, could lead to milder infection and potentially lower mortality. However, further field or laboratory investigation should be designed in order to confirm whether bees showing severe Nosemosis, from collapsed colonies or those about to collapse, experience dysbiosis of the core bacteriophage. Regarding the bee mycobiome, observed high variability between samples, even in the same group (f.i., lower _V. ceranae_ spore count), did not allow reliable comparisons, in accordance with previous recent studies [12; 13]. It is evident that a higher number of bee samples derived from a larger colony pool should be sequenced and analyzed. Nevertheless, it was demonstrated that the rather abundant mycobiome, as reflected by significant diversity, should be the focus of future microbiota studies, which at present assess mainly the core bacteriophage. In bee bread samples, some significant differences were described, as reflected by relative abundance of specific fungal and bacterial genera. For instance, _Rosenbergiella_ spp. were relatively abundant (reaching up to 10%) in bee bread samples collected from infected colonies (group D) as compared with non-infected (0-0.77% in group C). _Rosenbergiella_ spp. were often identified in nectar of pollination-dependent plants, producing several metabolites present in nectar (D-lactic acid, acetic acid and others). Depending on its composition, nectar might affect insect lifespan [49]. Similarly, bacteria and their metabolites produced during BB fermentation may affect honey bee lifespan. However, this hypothesis should be confirmed by further investigation. The genus _Podosphaera_ was absent in bee bread samples collected from colonies in group D, while the genus _Blumeria_ demonstrated significantly decreased relative abundance in the same group. Interestingly, both genera include species that are known plant pathogens, causing powdery mildew on leaves. _Blumeria graminis_ infests grasses and cereals in a highly specialized way. On the other hand, _Podosphaera astericola_ and _Podosphaera panosa_ (primarily affecting roses and _Pruus_ spp.) were the two species identified in our study [50]. These fungi could end up in bee bread through pollen collection from infected plants. It is known that bee-collected pollen undergoes fermentation that lowers pH and leads to production of antimicrobial compounds, thus preventing pathogen growth.