Exploring the role of opportunistic pathogens in honeybee health: the case of Serratia marcescens synergy with Nosema ceranae

Honeybee's population losses show periodic fluctuations in intensity at global level, with their decline often driven by specific pathogens, such as the gut microsporidia Nosema ceranae and a multitude of viruses. However, many of these pathogens alone rarely bring honeybee colonies to death. Indeed, negative synergies with other pathogens or environmental factors often occur and get the colonies to collapse. The study of honeybees' pathogen synergies at the colony level is a little explored aspect and has focused especially on the pathogens-pesticides negative effects. However, other biological synergies, especially with pathogen and non-core opportunistic gut bacteria such as Serratia marcescens, is rarely explored and may explain important drawbacks in colony losses and may help beekeepers with a more accurate diseases management. To achieve this goal, our research wants to explore the development dynamics of {S. marcescens} within the co-infection of N. ceranae, determining the incidence of S. marcescens on the survival rate of honeybees and proposing possible solution (e.g. beneficial bacteria uptake and medicaments) on controlling these pathogens. Briefly, different Serratia strains were isolated from weak honeybees and inoculated into new-born, healthy honeybees' in co-infection with N. ceranae. Infected honeybees were reared in cages, and the impact of the multiple infections on their survival was observed in controlled lab conditions. Quantitative PCR was used to quantify Serratia and N. ceranae in the honeybee gut and hemocoel. Moreover, by feeding specific additives based on beneficial bacteria such as Lactobacillaceae or antimicrobial agents such as fumagillin, a possible co-infection control strategy was tested. Our results, showed that all tested Serratia strains were found capable to cross the gut epithelium and proliferate in the hemocoel and the co-infection with N. ceranae facilitated the surge of Serratia population within the gut, leading honeybees to a rapid death. Beneficial microbes from flowers, particularly Apilactobacillus kunkeei, effectively reduced Serratia load, suggesting it as a potential organic microbial resource management, easily applicable in commercial apiaries. These findings underscore the complexity of microbial interactions in the honeybee, recalling the need for wider research in determining multifactorial interactions between pathogens and environmental bacteria within the honeybee’s gut.