Seaweed exposure modulates Escherichia coli plasmid conjugation rate

Seaweeds are a common and diverse component of coastal ecosystems and are known to be associated with <i>Escherichia coli</i> due to faecal pollution. As a biotic substrate, beach-cast seaweed may affect bacterial physiology and thereby horizontal gene transfer (HGT). Here, we test how the presence of three distinct senescing seaweed species affects <i>E. coli</i> plasmid conjugation. We allow the IncP plasmid pKJK5 to conjugate while supplying a substrate of <i>Palmaria palmata</i> (dulse), <i>Ulva lactuca</i> (sea lettuce) or <i>Fucus serratus</i> (serrated wrack). The three seaweed species induce distinct conjugative behaviours in <i>E. coli</i>: <i>U. lactuca</i> has no significant impact relative to a plastic control, the presence of <i>F. serratus</i> results in undetectable levels of conjugation and <i>P. palmata</i> promotes conjugation in a density-independent manner. This study highlights how biotic interactions can influence survival, HGT and antibiotic resistance in a human pathogen.