Hoenigsberger, M., Pretzer, C., Rahimi, M.J., Kopchinskiy, A.G., Parich, A., Laciny, A., Metscher, B., Chan, C.M., Lim, L.B.L., Salim, K.A., Zettel, H., Druzhinina, I.S. & Schuhmacher, R.

Strong antimicrobial and low insecticidal activity of mandibular gland reservoir content in Bornean “exploding ants” Colobopsis explodens Laciny & Zettel, 2018 (Hymenoptera: Formicidae)

Minor workers of some ant species belonging to the Colobopsis cylindrica (Fabricius, 1798) (COCY) species group can suicidally eject their sticky and potentially toxic mandibular gland reservoir content (MGRC) to ward off putative arthropod opponents. Since the MGRC can also be ejected non-suicidally as droplets at the mandible base, it was hypothesized that the secretion also serves other roles than just defense. In former studies, a range of potentially antimicrobial compounds has been identified in the MGRCs of different COCY species, which is why a function in shaping of the ant-associated microbiome has been proposed. Here, we aimed to assess the putative insecticidal and antimicrobial properties of the MGRC of the COCY species Colobopsis explodens Laciny & Zettel, 2018. For this, we conducted in-situ confrontation assays with C. explodens and sympatric insects. In in-vitro studies, individuals of Acheta domesticus (Linnaeus, 1758) and Atta sexdens (Linnaeus, 1758) were treated with the isolated MGRC as well as with its identified dominant phenolic constituents, that is, 1-(2,4,6-trihydroxyphenyl) ethanone (monoacetylphloroglucinol, MAPG) and 5,7-dihydroxy-2-methylchromen-4-one (noreugenin) in separate approaches. To determine a possible antimicrobial effect, the MGRC as well as MAPG and noreugenin were tested individually on Trichoderma spp., Candida sp., Bacillus velezensis, and Escherichia coli. We showed that neither the naturally expelled secretion nor the isolated MGRC or its dominant compounds caused acute lethality of the tested insects. In contrast, antimicrobial assays with the MGRC resulted in growth inhibition of some microorganisms. When the antimicrobial activity of the major constituents was further assessed, MAPG, but not noreugenin, induced profound growth inhibition. The results suggest that the MGRC of C. explodens does not primarily act via deadly toxins, but that rather its adhesive properties are mainly responsible for rendering an arthropod opponent innocuous. The demonstrated antimicrobial potential of the MGRC further supports the hypothesis about its role in influencing the microbe community associated with COCY ants.

Open access, licensed under CC BY 4.0. © 2020 The Author(s).

Hoenigsberger, M., Pretzer, C., Rahimi, M.J., Kopchinskiy, A.G., Parich, A., Laciny, A., Metscher, B., Chan, C.M., Lim, L.B.L., Salim, K.A., Zettel, H., Druzhinina, I.S. & Schuhmacher, R.

Strong antimicrobial and low insecticidal activity of mandibular gland reservoir content in Bornean “exploding ants” Colobopsis explodens Laciny & Zettel, 2018 (Hymenoptera: Formicidae)

Minor workers of some ant species belonging to the Colobopsis cylindrica (Fabricius, 1798) (COCY) species group can suicidally eject their sticky and potentially toxic mandibular gland reservoir content (MGRC) to ward off putative arthropod opponents. Since the MGRC can also be ejected non-suicidally as droplets at the mandible base, it was hypothesized that the secretion also serves other roles than just defense. In former studies, a range of potentially antimicrobial compounds has been identified in the MGRCs of different COCY species, which is why a function in shaping of the ant-associated microbiome has been proposed. Here, we aimed to assess the putative insecticidal and antimicrobial properties of the MGRC of the COCY species Colobopsis explodens Laciny & Zettel, 2018. For this, we conducted in-situ confrontation assays with C. explodens and sympatric insects. In in-vitro studies, individuals of Acheta domesticus (Linnaeus, 1758) and Atta sexdens (Linnaeus, 1758) were treated with the isolated MGRC as well as with its identified dominant phenolic constituents, that is, 1-(2,4,6-trihydroxyphenyl) ethanone (monoacetylphloroglucinol, MAPG) and 5,7-dihydroxy-2-methylchromen-4-one (noreugenin) in separate approaches. To determine a possible antimicrobial effect, the MGRC as well as MAPG and noreugenin were tested individually on Trichoderma spp., Candida sp., Bacillus velezensis, and Escherichia coli. We showed that neither the naturally expelled secretion nor the isolated MGRC or its dominant compounds caused acute lethality of the tested insects. In contrast, antimicrobial assays with the MGRC resulted in growth inhibition of some microorganisms. When the antimicrobial activity of the major constituents was further assessed, MAPG, but not noreugenin, induced profound growth inhibition. The results suggest that the MGRC of C. explodens does not primarily act via deadly toxins, but that rather its adhesive properties are mainly responsible for rendering an arthropod opponent innocuous. The demonstrated antimicrobial potential of the MGRC further supports the hypothesis about its role in influencing the microbe community associated with COCY ants.

Open access, licensed under CC BY 4.0. © 2020 The Author(s).