Submitted to: Proceedings of the Royal Society B
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/6/2014
Publication Date: 1/29/2014
Citation: Mcfrederick, Q., Mueller, U., James, R.R. 2014. Interactions between fungi and bacteria influence microbial community structure in the Megachile rotundata larval gut. Proceedings of the Royal Society B. 281:1779. Interpretive Summary: The alfalfa leafcutting bee is the second most important field crop pollinator, but is stricken by chalkbrood, a disease caused by a pathogenic fungus. Honey bees and bumble bees, like humans, have beneficial microbes that live in their guts and appear to help protect them from disease. Chalkbrood infections develop in the guts of alfalfa leafcutting bees, but whether these bees have beneficial gut microbes is unknown. We used modern, cost-effective “next-generation” DNA sequencing to answer the following questions: 1) What microorganisms occur in the intestines of alfalfa leafcutting bees; 2) if we remove bacteria or fungi from the gut, does this affect bee resistance to chalkbrood; and 3) how does the presence of the chalkbrood fungus affect gut microorganisms? We found the gut microbial communities to be quite simple. Most gut communities contained large numbers of one to five types of fungi and two types of bacteria, including a lactic acid bacterium that is common and beneficial in honey bees. The alfalfa leafcutting bee guts almost always contained the chalkbrood pathogen. When we fed bee larvae antibiotics, antifungal compounds, or infectious chalkbrood spores, we altered the microbial communities in the gut. Bacteria in the gut did not appear to have any effect on the chalkbrood fungus, and thus, bacteria probably do not protect bees from this pathogen. However, eliminating the chalkbrood pathogen led to an increase in other kinds of fungi.
Technical Abstract: Recent declines in bee populations coupled with advances in DNA-sequencing technology have sparked a renaissance in studies of bee-associated microbes. Megachile rotundata is the second, only to honey bees, as a crop pollinator, but is stricken by chalkbrood, a disease caused by the fungus Ascosphaera aggregata. To test the hypothesis that some microbes inhibit the growth of other microbes, we applied four treatments to the pollen provisions of M. rotundata eggs and young larvae: antibacterial, antifungal, A. aggregata spores, and a no-treatment control. We allowed the larvae to develop and used 454 pyrosequencing to quantify fungal and bacterial communities in the larval gut. Antifungals lowered the abundance of A. aggregata and correspondingly increased the relative diversity of surviving fungi. Antibacterials altered the bacterial, but not the fungal community. Interactions between A. aggregata and other fungi, but not between bacteria and fungi, appear to be important for fungal community structure in the larval gut. Lactobacillus kunkeei clade bacteria were common members of the larval gut microbiota, and exhibited antibiotic resistance. Because L. kunkeei is also found in honey bees and is likely transferred between bee species at flowers, the observed antibiotic resistance may have originated from exposure to pesticides applied to honey bee colonies.