Unexpected levels of antibiotic resistance in marine bacterial communities not solely explained by urban river discharge

Driven by anthropogenic pressures, antibiotic resistance has become more prevalent within urban bacterial communities over recent decades. In general, resistance is expected to spread from highly impacted areas into surrounding environments as the spatial patterns of resistance reflect the balance between the ability of dispersal and horizontal gene transfer to spread resistance, and the effects of dilution or selection driven by other environmental or ecological factors that change along these gradients. However, the strength and scale of this effect is less clear, especially in coastal environments. We investigated antibiotic resistance of bacterial communities along a transect from a freshwater site in the Los Angeles River (surrounded by high-density urban development) out into the San Pedro Basin in the Pacific Ocean. We determined the minimum concentration of twelve different antibiotics needed to inhibit the growth of bacterial communities across eight sites. We also examined community composition and the prevalence of known genes related to antibiotic resistance from anthropogenic sources in each sample using 16S rRNA sequencing and qPCR. In contrast to our expectations, marine bacteria communities displayed higher resistance than the freshwater LA River community. Rather than the presence of known resistance genes or proximity to urban impacts, patterns of resistance were more strongly associated with the higher species diversity found in marine communities. These unexpected results open new opportunities to determine why marine bacterial communities possess high resistance and the repercussions for potentially pathogenic bacteria and their impacts on the health of humans and wildlife.