Honeybee Exposure to Veterinary Drugs: How Is the Gut Microbiota Affected?

Several studies have outlined that a balanced gut microbiota offers metabolic and protective functions supporting honeybee health and performance. The present work contributes to increasing knowledge on the impact on the honeybee gut microbiota of the three most common veterinary drugs (oxytetracycline, sulfonamides, and tylosin). The study was designed with a semi-field approach in micro-hives containing about 500 honeybees. Micro-hives were located in an incubator during the day and moved outdoors in the late afternoon, considering the restrictions on the use of antibiotics in the open field but allowing a certain freedom to honeybees; 6 replicates were considered for each treatment. The absolute abundance of the major gut microbial taxa in newly eclosed individuals was studied with qPCR and next-generation sequencing. Antimicrobial resistance genes for the target antibiotics were also monitored using a qPCR approach. The results showed that the total amount of gut bacteria was not altered by antibiotic treatment, but qualitative variations were observed. Tylosin treatment determined a significant decrease of α- and β-diversity indices and a strong depletion of the rectum population (lactobacilli and bifidobacteria) while favoring the ileum microorganisms (Gilliamella, Snodgrassella, and Frischella spp.). Major changes were also observed in honeybees treated with sulfonamides, with a decrease in Bartonella and Frischella core taxa and an increase of Bombilactobacillus spp. and Snodgrassella spp. The present study also shows an important effect of tetracycline that is focused on specific taxa with minor impact on alfa and beta diversity. Monitoring of antibiotic resistance genes confirmed that honeybees represent a great reservoir of tetracycline resistance genes. Tetracycline and sulfonamides resistance genes tended to increase in the gut microbiota population upon antibiotic administration. IMPORTANCE This study investigates the impact of the three most widely used antibiotics in the beekeeping sector (oxytetracycline, tylosin, and sulfonamides) on the honeybee gut microbiota and on the spread of antibiotic resistance genes. The research represents an advance to the present literature, considering that the tylosin and sulfonamides effects on the gut microbiota have never been studied. Another original aspect lies in the experimental approach used, as the study looks at the impact of veterinary drugs and feed supplements 24 days after the beginning of the administration, in order to explore perturbations in newly eclosed honeybees, instead of the same treated honeybee generation. Moreover, the study was not performed with cage tests but in micro-hives, thus achieving conditions closer to real hives. The study reaches the conclusion that the most common veterinary drugs determine changes in some core microbiota members and that incidence of resistance genes for tetracycline and sulfonamides increases following antibiotic treatment.