Lurking Just Beneath the Surface: Antibiotic Resistance Among Fe(III)-Reducing Bacteria and Other Members of Freshwater Surface Microlayer Bacterial Communities

English Abstract:

Bacterial communities that included Fe(III)-reducing bacteria (IRB) were enriched from the surface microlayer (SML; a trace metal, nutrient, and microbially enriched interfacial microenvironment) and at 0.5 m depth in two freshwater littoral environments (wastewater-impacted Sunnyside Beach; pristine Coldspring Lake). The presence of IRBs was permitted at these well-oxygenated depths by the availability of O₂-depleted floc-based microhabitats. Enriched communities were expected to be highly resistant to metals and antibiotics due to the ability of IRB to dissolve metal-rich Fe minerals and thus be highly exposed to metals. However, inhibition of soluble Fe(III) reduction (i.e. IRB activity) occurred at significantly lower (P < 0.05) concentrations of environmentally relevant trace metal or antibiotic cocktails compared to the inhibition of bulk Fe(III)-reducing enrichment community metabolic activity. Further, Cu exposure selectively inhibited Fe(II) production by solid Fe(III)-reducing enrichment community members, which were specifically expected to be highly resistant to trace metals, and was associated with a decrease in the relative abundance of known IRB genera (Aeromonas and Clostridium) in 16S rRNA gene clone libraries derived from enrichments. Interestingly, unlike trace metal resistance, antibiotic resistance was significantly higher (P < 0.05) among IRB in soluble Fe(III)-reducing enrichment communities from the SML relative to those from 0.5 m depth, pointing to the selective promotion of antibiotic resistance in SML communities independent of metal and antibiotic resistance co-selection mechanisms. SML soluble Fe(III)-reducing enrichment communities also possessed a greater variety of antibiotic resistance gene-carrying mobile genetic elements and opportunistic pathogens compared to enrichments from 0.5 m depth in both lakes, identifying the SML as a potential reservoir and disseminator of antibiotic resistant pathogens within aquatic systems.