can promote development in honeybee gut: an underrated threat for bees?

The genus harbors opportunistic pathogenic species, among which is pathogenic for honeybees although little studied. Recently, virulent strains of colonizing the mite's mouth were found vectored into the honeybee body, leading to septicemia and death. also occurs as an opportunistic pathogen in the honeybee's gut with a low absolute abundance. The population seems controlled by the host immune system, but its presence may represent a hidden threat, ready to arise when honeybees are weakened by biotic and abiotic stressors. To shed light on the pathogen, this research aims at studying 's development dynamics in the honeybee body and its interactions with the co-occurring fungal pathogen . Firstly, the degree of pathogenicity and the ability to permeate the gut epithelial barrier of three strains, isolated from honeybees and belonging to different species (, , and ), were assessed by artificial inoculation of newborn honeybees with different doses (10, 10, and 10 cells/mL). The absolute abundance of in the gut and in the hemocoel was assessed in qPCR with primers targeting the gene. Moreover, the absolute abundance of was assessed in the gut of honeybees infected with at different development stages and supplied with beneficial microorganisms and fumagillin. Our results showed that all tested strains could pass through the gut epithelial barrier and proliferate in the hemocoel, with being the most pathogenic. Moreover, under cage conditions, better proliferates when a infection is co-occurring, with a positive and significant correlation. Finally, fumagillin and some of the tested beneficial microorganisms could control both and development. Our findings suggest a correlation between the two pathogens under laboratory conditions, a co-occurring infection that should be taken into consideration by researches when testing antimicrobial compounds active against , and the related honeybees survival rate. Moreover, our findings suggest a positive control of by the environmental microorganism in a in vivo model, confirming the potential of this specie as beneficial bacteria for honeybees.