## 1 Introduction
Moreover, the transfer of resistance from animal pathogens to human ones is a possible threat that has to be considered [19]. Finally, residues of antibiotics contaminate beehive products, especially honey, affecting quality and safety for human consumption [11; 20; 21]. Due to lack of metabolism in the beehive, an elimination of residues within a certain period of time, as defined for other food producing animals, cannot be established in apiculture. Hence, the persistence of residues of antibiotics in honey leads to significant commercial problems [10]. Investigation of alternative, more sustainable methods to prevent and control honey bee bacterial diseases is needed. One of the most studied methods is the selection of honey bee colonies with hygienic behavior of adult bees toward infected larvae [22]. Other strategies are based on the use of natural substances, including: essential oils, plant extracts, propolisols, royal jelly, non-conventional natural molecules, bacterioxins [23; 24; 25; 26; 27; 28; 29; 30; 31; 32] and antagonistic bacteria [7; 33; 24; 32; 34; 7; 15; 23]. The last may act as biocontrol agents representing a promising alternative to antibiotics. Frequently, to this scope, autochthonous strains isolated from honeybees are used [35; 36; 37]. Even if the microbiota associated with honeybees is not fully known, it is mainly composed of yeasts, Gram-positive bacteria (such as _Lactobacillus_ spp., _Bacillus_ spp., _Brevibacillus_ spp., _Streptococcus_ and _Clostridium_) and Gram-negative or Gram variable bacteria (_Achromobacter_, _Citrobacter_, _Enterobacter_, _Erwinia_, _Escherichia coli_, _Flavobacterium_, _Klebsiella_, _Proteus_ and _Pseudomonas_) [38; 39; 40; 41; 42]. [PERSON] [43] reported that these bacteria were likely to be endemic in the alimentary tract of adult bees and were dependent neither on seasonal factors nor on nutritional factors. The ability of the spore-forming species to inhibit different fungi and bacteria, by secreting a broad range of bioactive compounds, that include peptides, lipoproteins,bacteriocins, and bacteriocin-like inhibitory substances, has been well documented [15]. More in detail, _Bacillus subtilis_, _Bacillus pumilus_, _Bacillus licheniformis_, _Bacillus cereus_, _Bacillus megaterium_ and _Brevibacillus lateosporus_ have been reported as normal microorganisms associated with honeybees and honey [40; 44; 45], characterized by high inhibitory capacities against _P. larvae_, as evidenced through in vitro studies by [PERSON] and [PERSON] [15]. More generally, antagonistic activity against _P. larvae_ showed by _Bacillus_ and _Brevibacillus_ species has been confirmed by several studies [29; 33; 46]. The microbial group currently of major interest for honeybee probiotic purposes is represented by lactic acid bacteria (LAB) [47; 48; 49; 50; 51]. _Lactobacillus_ and _Bifidobacterium_, the most important genera within LAB, are commonly beneficial commensals and are widely used as probiotics to modulate the composition of the microflora in order to protect the host from infections. LAB are in fact known to be good producers of antimicrobial substances such as organic acids, hydrogen peroxide and antimicrobial peptides [52]. Multiple factors are involved in the mechanisms associated with antibacterial activity in probiotic LAB and there is a clear variation in the production of antimicrobial agents including common organic acids, proteins, peptides, enzymes, and bacteriocins between the different LAB species [7; 53]. A novel flora of LAB composed of _Lactobacillus_ and _Bifidobacterium_ has recently been identified in the honey stomach of honeybees [52; 53]. A mutual dependence between honeybees and the novel LAB flora was hypothesized: the LAB obtaining nutrients, the honeybees and the honey in turn being protected by the LAB from harmful microorganisms. Previously, [PERSON] and [PERSON] [54] observed a strong immune response in larvae fed with a mix of _Lactobacillus_ and _Bifidobacterium_ species, with the enhanced production of antibacterial peptides.