Competition among Fungi and Bacteria in the Guts of Bees

An alfalfa leafcutting bee (Megachile rotundata) on an alfalfa flower. This bee species is one of many wild bees called on to help honey bees pollinate crops.

A clearer picture is emerging of microorganisms that colonize the guts of some bee species, thanks to studies by a team of scientists with the USDA Agricultural Research Service (ARS) and the University of Texas-Austin (UTA).

The effort, which examined competition among fungi and bacteria, could shed new light on how the microbes affect bee health and productivity. The findings may also yield new clues to controlling those that harm bees, like the fungus Ascosphaera aggregata. That fungus causes a disease called chalkbrood in the alfalfa leafcutting bee (Megachile rotundata) and other bee species.

The alfalfa leafcutting bee is a chief pollinator of alfalfa grown for seed. However, there are currently no treatments for chalkbrood disease, which makes the afflicted bee brood appear chalky-white and shriveled.

The pathogen infects the bees through the gut, which is home to other types of fungi and bacteria. Little is known about the bee gut microbes, however, or their dynamics. So ARS scientist Rosalind James, together with UTA colleagues Quinn McFrederick and Ulrich Mueller, designed an experiment to genetically detect and identify the presence of the microbes in response to four treatments. These consisted of pollen diets containing either 1) Ascosphaera aggregata spores, 2) a cocktail of antifungal agents, 3) a cocktail of antibacterial agents, or 4) nothing at all as a control.

Initially, the experiments were supposed to show whether these other gut microorganisms could serve as a sort of probiotic for the bees, helping to make them more resistant to the pathogen. Surprisingly, though, the results showed that when Ascosphaera aggregata levels decreased, the diversity of other fungi increased.

Conversely, when the Ascosphaera aggregata levels in the gut rose, these other fungi virtually disappeared. This suggests that Ascosphaera aggregata stymies the other fungi’s growth (rather than the other way around, as hoped), either chemically or by outcompeting them.

“Antifungals lowered A. aggregata abundance but increased the diversity of surviving fungi,” the researchers wrote in an article that appeared in the journal Proceedings of the Royal Society. “This suggests that A. aggregata inhibits the growth of other fungi in the gut through chemical or competitive interaction. Bacterial richness decreased under the antifungal treatment, suggesting that changes in the fungal community caused changes in the bacterial community. We found no evidence that bacteria affect fungal communities.”

Read more at:

Interactions between fungi and bacteria influence microbial community structure in the Megachile rotundata larval gut

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