Shielding the next generation: symbiotic bacteria from a reproductive organ protect bobtail squid eggs from fungal fouling

Authors: KERWIN, ALLISON - University Of Connecticut; GROMEK, SAMANTHA - University Of Connecticut; SURIA, ANDREA - University Of Connecticut; DEOSS, DISTER - University Of Connecticut; O Donnell, Kerry; FRASCA, SALVATORE - University Of Florida; SUTTON, DEANNA - University Of Texas; WIEDERHOLD, NATHAN - University Of Texas; BALUNAS, MARCY - University Of Connecticut; NYHOLM, SPENCER - University Of Connecticut

Submitted to: mBio
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/19/2019
Publication Date: 10/29/2019
Citation: Kerwin, A.H., Gromek, S.M., Suria, A.M., Samples, R.M., Deoss, D.J., O'Donnell, K., Frasca,Jr., S., Sutton, D.A., Wiederhold, N.P., Balunas, M.J., Nyholm, S.V. 2019. Shielding the next generation: symbiotic bacteria from a reproductive organ protect bobtail squid eggs from fungal fouling. American Society for Microbiology. 10:e02376-19. https://doi.org/10.1128/mBio.02376-19.
DOI: https://doi.org/10.1128/mBio.02376-19

Interpretive Summary: Many of the antibiotics that are in use today are derived from beneficial microbes. The antibiotics that bacteria produce in nature may serve to protect themselves and other organisms that they associate with. We theorized that eggs laid in an aquatic or marine environment might be protected by beneficial antibiotic-producing bacteria. To test this prediction, we studied bacteria associated with eggs of the Hawaiian bobtail squid, Euprymna scolopes. When eggs of this squid were treated with antibiotics, which eliminated most of the bacteria, the majority of the eggs died after being heavily colonized by a fungus. By way of contrast, eggs not treated with the antibiotics never became overrun by fungi and developed into healthy mature squid. To assess whether the egg-associated bacteria were producing antifungal drugs that protected the eggs, we tested extracts of the bacteria and discovered that they were producing several novel bioactive compounds that inhibited the growth of fungi and yeast. This research will be of interest to agricultural and pharmaceutical scientists, natural product chemists, and microbial ecologists interested in discovering novel antimicrobials, especially one against fungi that are broadly resistant to most antifungal currently available. Because the majority of economically important plant diseases are caused by fungi, bacteria in novel environments offer a promising source of novel antifungals to combat these pathogens.

Technical Abstract: The importance of defensive symbioses, whereby microbes protect hosts through the production of specific compounds, is becoming increasingly evident. Although defining the partners in these associations has become easier, assigning function to these relationships often presents a significant challenge. Here, we describe a functional role for a bacterial consortium in a female reproductive organ in the Hawaiian bobtail squid, Euprymna scolopes. Bacteria from the accessory nidamental gland (ANG) are deposited into the egg jelly coat (JC), where they are hypothesized to play a defensive role during embryogenesis. Eggs treated with an antibiotic cocktail developed a microbial biomass primarily composed of the pathogenic fungus Fusarium keratoplasticum that infiltrated the JC, resulting in severely reduced hatch rates. Experimental manipulation of the eggs demonstrated that the JC was protective against this fungal fouling. A large proportion of the bacterial strains isolated from the ANG or JC inhibited F. keratoplasticum in culture (87.5%), while a similar proportion of extracts from these strains also exhibited antifungal activity against F. keratoplasticum and/or the human-pathogenic yeast Candida albicans (72.7%). Mass spectral network analyses of active extracts from bacterial isolates and egg clutches revealed compounds that may be involved in preventing microbial overgrowth. Several secondary metabolites were identified from ANG/JC bacteria and egg clutches, including the known antimicrobial lincomycin as well as a suite of glycerophosphocholines and mycinamicin-like compounds. These results shed light on a widely distributed but poorly understood symbiosis in cephalopods and offer a new source for exploring bacterial secondary metabolites with antimicrobial activity.