## 3 Results
### Effects of Antibiotics on D. melanogaster
#### 3.1.2 Antibiotics Do Not Impact Mortality or Feeding of Adult \(D\). melanogaster  
I turned to the adult stage to examine direct effects of antibiotic treatments on the fly itself. Indeed, the aforementioned large fitness effect on axenic larvae does not necessarily impede the use of adult _Drosophila_ as a gut microbiome model in antibacterial research. To this end, we followed the feeding rate and survivorship of conventional and axenic adults during capillary feeding, with similar treatments as in the development assay, offering a daily recording of daily liquid food (+antibiotic) uptake and mortality (Figure 1B).  
Overall, feeding rates were low on the first day and gradually increased over time (Figure 1B) to reach a daily food uptake of about 1.5 \(\mu\)L fly\({}^{-1}\). Such a feeding profile potentially implies a form of adaptation in flies as they shift from the spatially abundant, solid nutrient agar to a spatially restricted and liquid-type food and resembles previously reported results [94; 95]. No significant differences were found in the feeding patterns  
Figure 1: Strong, direct effects of antibiotics on _Drosophila melanogaster_ survival and food uptake are limited to the dechorionated, larval stage. (**A**) Antibiotics elicit strong larval mortality in axenic, antibiotic-treated groups. Conventional eggs (CONV, full bars) or dechorionated eggs with eradicated microbiome by bleach treatment (AX, dashed bars) were deposited on the surface of nutrient agar containing either no antibiotic (gray) or ampicillin (red), amikacin (blue) or ofloxacin (yellow) at different concentrations (shades). The ratio of the cumulative number of pupae by day 14 to the estimated number of eggs at the start was plotted (means \(\pm\) stdevs with \(n=\pm 800\) per group across 20 vials as gray points). Linear mixed model (lme) with post hoc tests for significant differences with the untreated control (Dunnett test per AX and CONV groups, in bar) and between AX-CONV per treatment (Tukey, above bar) (ns, nonsignificant; *, \(p<0.05\); **, \(p<0.01\)). (**B**) Antibiotics have no effect on daily food uptake in adult flies. Uptake of antibiotic-free or -containing liquid food was recorded daily for AX (dotted line) and CONV (full line) flies using capillaries (means, \(n=\pm 60\) per group across 10 vials shown as full and half points for CONV and AX). Post hoc tests on an lme with the untreated control (Dunnett) and between AX-CONV per treatment (Tukey) were nonsignificant for all contrasts.  
of axenic groups compared to their respective conventional counterpart per treatment or between treated groups and their untreated controls (Figure 1B). Therefore, antibiotic-containing food does not change the daily food uptake and food uptake is not different between axenic and conventional flies.  
Fly mortality in the capillary feeding setup, in contrast to the large differences for larval mortality in the development assay, only showed minor, statistically insignificant differences (Figure S1E) but also indicated less, instead of more, mortality in the axenic groups. Together, these data confirm that adult _D. melanogaster_ flies are receptive for oral, high-dose antibiotic treatments.