Regulation of microbial populations by coral surface mucus and mucus-associated bacteria
Caribbean populations of the elkhorn coral Acropora palmata have declined due to envi- ronmental stress, bleaching, and disease. Potential sources of coral mortality include invasive microbes that become trapped in the surface mucus and thrive under conditions of increased coral stress. In this study, mucus from healthy A. palmata inhibited growth of potentially invasive microbes by up to 10-fold. Among cultured bacteria from the mucus of A. palmata, 20% displayed antibiotic activity against one or more tester strains, including the pathogen implicated in white pox disease. A novel mucus- mediated selection for coral symbionts revealed a discrete subset of bacteria and selected for isolates that produce antibiotics. This result suggests that coral mucus plays a role in the structuring of beneficial coral-associated microbial communities and implies a microbial contribution to the antibacterial activity described for coral mucus. Interestingly, antibiotic activity was lost when mucus was collected during a summer bleaching event. Isolates from apparently healthy A. palmata tissue during this event lacked antibiotic-producing bacteria and were dominated by members of the genus Vibrio, including species implicated in temperature-dependent bleaching of corals worldwide. This indicates an environmental shift from beneficial bacteria, and variability in the protective qualities of coral mucus, which may lead to an overgrowth of opportunistic microbes when temperatures increase. Finally, coral mucus inhibited antibiotic activity and pigment production in a poten- tially invasive bacterium, illustrating that coral mucus may inactivate mechanisms used for bacterial niche establishment.
Community proteogenomics reveals insights into the physiology of phyllosphere bacteria
Aerial plant surfaces represent the largest biological interface on Earth and provide essential services as sites of carbon dioxide fixation, molecular oxygen release, and primary biomass production. Rather than existing as axenic organisms, plants are colonized by microorganisms that affect both their health and growth. To gain insight into the physiology of phyllosphere bacteria under in situ conditions, we performed a culture-independent analysis of the microbiota associated with leaves of soybean, clover, and Arabidopsis thaliana plants using a metaproteogenomic approach. We found a high consistency of the communities on the 3 different plant species, both with respect to the predominant community members (including the alphaproteobacterial genera Sphingomonas and Methylo bacterium) and with respect to their proteomes. Observed known proteins of Methylobacterium were to a large extent related to the ability of these bacteria to use methanol as a source of carbon and energy. A remarkably high expression of various TonB-dependent receptors was observed for Sphingomonas. Because these outer membrane proteins are involved in transport processes of various carbohydrates, a particularly large substrate utilization pattern for Sphingomonads can be assumed to occur in the phyllosphere. These adaptations at the genus level can be expected to contribute to the success and coexistence of these 2 taxa on plant leaves. We anticipate that our results will form the basis for the identification of unique traits of phyllosphere bacteria, and for uncovering previously unrecorded mechanisms of bacteria-plant and bacteria-bacteria relationships.
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