Physiological responses of insects to microbial fermentation products: Insights from the interactions between Drosophila and acetic acid

Abstract

Acetic acid is a fermentation product of many microorganisms, including some that inhabit the food and guts of Drosophila. Here, we investigated the effect of dietary acetic acid on oviposition and larval performance of Drosophila. At all concentrations tested (0.34-3.4%), acetic acid promoted egg deposition by mated females in no-choice assays; and females preferred to oviposit on diet with acetic acid relative to acetic acid-free diet. However, acetic acid depressed larval performance, particularly extending the development time of both larvae colonized with the bacterium Acetobacter pomorum and axenic (microbe-free) larvae. The larvae may incur an energetic cost associated with dissipating the high acid load on acetic acid-supplemented diets. This effect was compounded by suppressed population growth of A. pomorum on the 3.4% acetic acid diet, such that the gnotobiotic Drosophila on this diet displayed traits characteristic of axenic Drosophila, specifically reduced developmental rate and elevated lipid content. It is concluded that acetic acid is deleterious to larval Drosophila, and hypothesized that acetic acid may function as a reliable cue for females to oviposit in substrates bearing microbial communities that promote larval nutrition.

Keywords: Acetic acid; Acid load; Drosophila; Fermentation products; Gut microbiota; Larval development; Microbiome.

Fig. 1

Effect of acetic acid on egg deposition over 24 h by Drosophila. (A) Variation in oviposition with acetic acid concentration in no-choice assay. ANOVA for acetic acid concentration: F_3,32 = 8.57, p<0.001). (B) Oviposition preference over 24 h between the control substrate (acetic acid-free food) and test substrate containing acetic acid at three concentrations (the acetic acid-free substrate treatment is the negative control, with mean value that does not differ significantly from zero, p>0.05). ANOVA for oviposition index, calculated as described in Methods section: F_3,32 = 27.0, p<0.001). Treatments with different letters are significantly different by Tukey’s post hoc test.

Fig. 2

Effect of acetic acid on the performance of gnotobiotic larvae bearing A. pomorum and axenic larvae. A. Development time to pupation with statistical analysis in Table S1. B. Weight of wandering larvae (ANOVA: acetic acid F_3,88 = 1.80, p>0.05, bacteria F_1,88 = 28.54, p<0.001, interaction F_3,88 = 3.65, p=0.016).

Fig. 3

Effect of acetic acid supplement of food on food consumption by final-instar larvae (ANOVA: acetic acid F_3,72 = 21.52, p=0.01, bacteria F_1,72 = 5.01, p=0.028, interaction F_3,72 = 0.57, p>0.05).

Fig. 4

Variation in abundance of the bacterium Acetobacter pomorum with acetic acid concentration. A. Abundance in food at day-6 after hatching of the larvae ANOVA: +/no Drosophila F_1,86 = 6.20, p=0.015; acetic acid concentration F_3,86 = 38.3, p<0.0001, interaction F_3,86 = 1.57, p>0.05. B. Abundance in flies at day-5 post eclosion. ANOVA: sex: F_1,173 = 3.886, p=0.05; acetic acid: F_3,173 = 54.52, p<2e-16; interaction: F_3,173 = 1.817, p>0.05.

Figure 5

Effect of dietary acetic acid on adult Drosophila. The insects were associated with Acetobacter pomorum and raised from egg to 5 days p.e. adulthood on the test diets. A. Weight per fly. ANOVA: sex: F_1,264.50 = 2557, p<2e-16; acetic acid: F_3,264.45 = 2.49, p>0.05; interaction: F_3,264.51 = 2.12, p>0.05. B. TAG content of flies. ANOVA: sex: F_1,264.11 = 3.886, p<2.2e-16; acetic acid: F_3,264.10 = 54.52, p<2.2e-16; interaction: F_3,264.11 = 1.817, p<2.2e-16.