Two gut microbes are necessary and sufficient for normal cognition in Drosophila melanogaster

Abstract

It is widely accepted that the gut microbiome can affect various aspects of brain function, includ-2 ing anxiety, depression, learning, and memory. However, we know little about how individual microbial species contribute to communication along the gut-brain axis. Vertebrate microbiomes 4 are comprised of hundreds of species, making it difficult to systematically target individual microbes and their interactions. Here, we use Drosophila melanogaster as a simple model organism 6 to tease apart individual and combined effects of gut microbes on cognition. We used an aversive phototactic suppression assay to show that two dominant gut commensals in our lab stock, 8 Lactobacillus and Acetobacter, are necessary and sufficient for normal learning and short-term memory relative to flies with a conventional microbiome. We also demonstrate that microbes 10 did not affect their hosts' ability to detect the aversive learning stimulus (quinine), suggesting that our results were due to decreased cognition and not sensory deficits. We thus establish 12 Drosophila as a model for elucidating mechanisms of gut-brain communication at the level of individual bacterial species. 14 IntroductionWithin the last decade, there has been an explosion of studies showing that gut microbes affect 16 critical organismal functions, particularly communication along the gut-brain axis [1]. Early studies found a link between the gut microbiota and anxiety [2][3][4], depression [5], learning [6], 18 and memory [7] suggesting that gut microbes impact more than just their immediate surroundings. Since then, other studies have found that disrupting the gut microbiota through diet [7], 20 pathogens [8], antibiotics [9, 10], or probiotics [11-14] can all affect learning. Moreover, disease models for inflammatory bowel disease [15, 16], diabetes [17], and schizophrenia [18] have associ-22ated the loss of homeostasis of the gut community (dysbiosis) with learning and memory deficits in experimental animals. These deficits were ameliorated with probiotic treatment, suggesting 24 that symptoms experienced by patients may be treated with probiotics as well.If probiotic therapy is to become an effective treatment for dysbiosis of the microbial commu-26 nity, we need a better understanding of how individual community members influence cognition alone and in combination. We will then be able to target specific microbes or species combina-28 tions to more effectively address underlying issues in brain function. We know that diversity of the gut microbiota plays a role (some beneficial species have been identified), but it is not fully 30 understood how individual members of the microbiota contribute, because significantly variable communities can nonetheless be healthy [19][20][21]. 32 We chose to test learning and memory in Drosophila, because the microbiota in flies is much simpler than in vertebrates [22], allowing us to generate single and combinatorial microbial as-34 sociations that comprise the bulk of the total microbiota. In this way we can begin ...

The Gut–Brain–Microbiome Axis in Bumble Bees

“…Contrary to our prediction, the gut microbiota did not play a significant role in visual learning and memory in our assays using adult bumble bees. Our negative result contradicts the recent findings in D. melanogaster [ 12 ]. The differences in our results may stem from several possible explanations.…”

“…Alternatively, experimental details may explain the differences in our studies. DeNieu et al reared truly gnotobiotic flies by feeding them tetracycline-spiked media for multiple generations [ 12 ]. We did not quantify the microbes in our experimental bees, and it is possible that our microbe-depleted samples harbored a similar abundance of microbes as our microbe-inoculated bees.…”

“…In our previous work, we found that our protocol greatly reduced the diversity and abundance (by 10 4 times) of microbes in the gut [ 22 ], although it is unlikely that we can produce a truly gnotobiotic bee without the use of antibiotics. DeNieu et al also found that as few as two species of bacteria could rescue the detrimental effects induced by the absence of microbes [ 12 ]. It is, therefore, possible that the trace number of microbes in the guts of our microbe-depleted bees is sufficient to prevent detrimental effects on learning and memory.…”

“…While there is evidence that microorganisms that can alter host behavior, few studies have explored the brain-gut–microbiome axis in insect systems. A recent study, however, showed that learning and memory in D. melanogaster can be disrupted by knocking down the gut microbiota and can be rescued with as few as two bacterial species [ 12 ]. The mechanism and function of a gut–brain axis may, therefore, be evolutionarily conserved across vertebrates and insects.…”

Honeybee gut microbiota modulates host behaviors and neurological processes

“…By generating single bacterial associations, we intended to dissect the individual and combined effects of each core gut member in the sugar sensitivity. However, it showed that only conventionalized bees had a higher sensitivity, and individual gut members were not sufficient to improve the PER score, suggesting an integrative effect of the gut members, which is also reported for the Drosophila microbiota on the host learning 59 . Under field conditions, antibiotic treatment did not completely eliminate any core gut species but perturbed the relative abundance of the SDPs of Lactobacillus Firm-5 and Bifidobacterium, which indicates that the normal microbial community structure is required for the colony health.…”