Antibiotic production by intertidal sedimentary and porewater bacteria and the characterization of their prevalence in situ

In intertidal sediments, bacteria attach to sand grains in mixed-species biofilms and inhabit the surrounding porewater as free-living communities. The large densities, high species diversity, and complex spatial distribution of sediment-attached bacteria implicate inter-specific competition as a likely force in structuring sediment communities. Both sediment-attached and free-living bacteria secrete antibiotics as a common means of competition. To establish the frequency of antibiotic production, bacteria isolated from intertidal sediments and porewater were screened using a disc-diffusion assay. Among sediment-attached bacteria, 39% displayed the ability to produce antibiotics, whereas significantly fewer of the porewater-associated bacteria (23.5%) produced inhibitory compounds. Denaturing gradient gel electrophoresis (DGGE) was used to identify a selection of isolated antibiotic-producing bacteria within whole-community environmental samples. Through sequencing a region of the 16S rRNA gene, the relative abundances of 4 antibiotic producers were established to be between 4.3-9.4% of the DGGE community profile. The high frequency of antibiotic-producing bacteria in sediments, and their significant quantitative contribution to the community composition, suggest that antibiosis likely plays a significant role in structuring benthic microbial communities.