Antibiotic resistance is quickly becoming one of the biggest modern-day threats to human health. It has not only been observed in the clinic but in natural environments as well. Selection for antimicrobial resistant bacteria in the marine environment has been shown to be driven factors such as low concentrations of antibiotics entering the environment through discharge from wastewater treatment plants and run off from agricultural sites. However, antimicrobial resistance is likely to not be solely due to anthropogenic pollution, as it is an ancient mechanism and has been found in environments with minimal human exposure. Here we investigated whether natural antimicrobial producers, i.e. seaweeds, select for antibiotic resistant bacteria.
We used both culture-based and molecular techniques to characterise the bacterial communities associated with different seaweed species, focusing on the human pathogens Vibrio, E. coli and S. aureus. Vibrio was harboured by all the seaweeds tested but E. coli and S. aureus were not. For the first time, we tested if Vibrio isolated from seaweed are locally adapted to their host macroalgae using a novel seaweed media assay. Our results showed Vibrio did not display local adaption. We tested the resistance profiles of bacteria isolated from seaweeds and found Vibrio showed cross-resistance to antibiotics and natural antimicrobial, in the form of methanolic seaweed extracts. We can conclude seaweeds harbour antibiotic resistant bacteria, but specific species of seaweeds do not select for specific antibiotic resistance.
We quantified the prevalence of a biomarker for antibiotic resistance, the intI1 gene, and found seaweed select for antibiotic resistant bacteria independent of anthropogenic pollution, suggesting seaweed-associated bacterial resistance is an intrinsic mechanism. Using metagenomics, we characterised possible antimicrobial resistance genes associated with different seaweed species from which we were able identify eflamycin, aminocoumarin and fluoroquinolone resistance genes on all the seaweeds tested. Two of the antibiotic classes are produced by Streptomyces, which is present on seaweeds. Suggesting resistance on seaweeds is selected for by bacterial community or the genes characterised show cross-resistance to seaweed antimicrobials
