Contribution of midgut bacteria to blood digestion and egg production in aedes aegypti (diptera: culicidae) (L.)

Abstract

Background: The insect gut harbors a variety of microorganisms that probably exceed the number of cells in insects themselves. These microorganisms can live and multiply in the insect, contributing to digestion, nutrition, and development of their host.Recent studies have shown that midgut bacteria appear to strengthen the mosquito's immune system and indirectly enhance protection from invading pathogens. Nevertheless, the physiological significance of these bacteria for mosquitoes has not been established to date. In this study, oral administration of antibiotics was employed in order to examine the contribution of gut bacteria to blood digestion and fecundity in Aedes aegypti.

Results: The antibiotics carbenicillin, tetracycline, spectinomycin, gentamycin and kanamycin, were individually offered to female mosquitoes. Treatment of female mosquitoes with antibiotics affected the lysis of red blood cells (RBCs), retarded the digestion of blood proteins and reduced egg production. In addition, antibiotics did not affect the survival of mosquitoes. Mosquito fertility was restored in the second gonotrophic cycle after suspension of the antibiotic treatment, showing that the negative effects of antibiotics in blood digestion and egg production in the first gonotrophic cycle were reversible.

Conclusions: The reduction of bacteria affected RBC lysis, subsequently retarded protein digestion, deprived mosquito from essential nutrients and, finally, oocyte maturation was affected, resulting in the production of fewer viable eggs. These results indicate that Ae. aegypti and its midgut bacteria work in synergism to digest a blood meal.Our findings open new possibilities to investigate Ae. aegypti-associated bacteria as targets for mosquito control strategies.

Figure 1

Effects of different antibiotics on protein degradation in Aedes aegypti after feeding on mouse blood. Each point represents the protein content of one mosquito midgut. A, 24 h after blood feeding; B, 36 h after blood feeding; C, 48 h after blood feeding; Co, control; Spe, spectinomycin; Gen, gentamycin; Car, carbenicillin; Kan, kanamycin; Tet, tetracycline. (*p < 0.05; **p < 0.01; ***p < 0.001). The mean ± SEM are represented by horizontal lines.

Figure 2

Effects of different antibiotics in Aedes aegypti fecundity after feeding on mouse blood. A, number of eggs laid by antibiotic-treated and untreated mosquitoes; B, number of oocytes in antibiotic-treated and untreated mosquitoes 48 h after blood feeding; C, number of oocytes during the first gonotrophic cycle in antibiotic-treated mosquitoes 48 h after blood feeding; D, number of oocytes during the second gonotrophic cycle in mosquitoes 48 h post-blood feeding after suspension of antibiotic treatment. Each point represents the number of eggs/oocytes in one female mosquito. Co, control; Spe, spectinomycin; Gen, gentamycin; Car, carbenicillin; Kan, kanamycin; Tet, tetracycline. (*p < 0.05; **p < 0.01; ***p < 0.001). Car (-), carbenicillin suspended on the second gonotrophic cycle; Tet (-), tetracycline suspended on the second gonotrophic cycle. The mean ± SEM are represented by horizontal lines.

Figure 3

Number of bacteria present in the midgut of Aedes aegypti females treated with antibiotics twenty four hours after feeding on mouse blood. Co, control; Car, carbenicillin; Tet, tetracycline. (*p < 0.05). The mean ± SEM values were obtained from three samples containing five midguts each in two independent experiments.

Figure 4

Hemolytic activities of Aedes aegypti midgut bacteria during blood digestion. A, hemolytic activities of two major bacteria isolated from Ae. aegypti midgut. Right, Enterobacter sp.; Left, Serratia sp. Halos correspond to the regions of erythrocytes lysis. B, percentage of erythrocytes in the midgut expressed in relation to the total ingested by recently fed mosquito (0 h). Co, control; Car, carbenicillin; Tet, tetracycline. (*p < 0.05; **p < 0.01; ***p < 0.001). The mean ± SEM values were obtained from three samples containing five midguts each in two independent experiments.

Figure 5

Effect of carbenicillin and tetracycline on Aedes aegypti fecundity 48 h after feeding on human blood. A, mosquitoes artificially fed on human blood. B, mosquitoes artificially fed on human blood containing 50% of lysed erythrocytes. Each point represents the number of oocytes in one female mosquito. Co, Control; Car, carbenicillin; Tet, tetracycline. The mean ± SEM are represented by horizontal lines.