PS 48-242 - Characterizing antibiotic resistance between marine and freshwater bacterial communities, from the Los Angeles River to the Pacific Ocean

Abstract: Antibiotics have revolutionized modern medicine, dramatically decreasing human morbidity and mortality from bacterial infections. However, the continued use and overuse of antibiotics in humans and agriculture have led to an increase in antibiotic resistant bacteria. This is especially important in densely populated areas, where antibiotics readily enter the environment through sewage spills caused by storms and surface runoff. Already in February 2023, 24,000 gallons of untreated sewage have spilled into the Pacific Ocean from the greater Los Angeles (LA) metropolitan area this year. In this study, we aimed to understand how antibiotic resistance in bacteria changed between freshwater and marine systems along a transect with increasing distance from shore. First, we collected water samples at 8 different sites in the LA metropolitan area, with 1 site in the LA River and 7 other sites in the San Pedro Channel of the Pacific Ocean at increasing distances from land. We then characterized the bacterial communities in these samples, including their overall composition (16S rRNA sequencing) and occurrence of antibiotic resistance genes (qPCR). We also experimentally tested their sensitivity to 12 different antibiotics, providing insight into their function. We predicted that freshwater communities, and communities from sites closer to the LA river, would exhibit higher antibiotic resistance from exposure to anthropogenic sources of antibiotics than marine bacteria.

Surprisingly, marine bacteria actually exhibited antibiotic resistance that was either similar to or higher than that of freshwater communities in 9 out of the 12 antibiotics tested. This ran counter to our expectation that communities more heavily influenced by anthropogenic factors would be most likely to harbor antibiotic resistance. We also ruled out the possibility that observed differences might be explained by adaptation to other stressors. While other studies suggest that antibiotic resistance can emerge as a side effect of adaptation to warmer temperatures, we found no significant differences between the thermal tolerance of bacterial communities at each site. Our results suggest further research is needed to understand why marine communities are so resistant to antibiotics, whether this is due to anthropogenic impacts or the ecology of the bacterial communities themselves, and whether similar patterns occur in other places across gradients of urban influence. Results from our study and future work to characterize the prevalence of antibiotic resistant marine bacteria in urban coastal systems will better inform policy decisions regarding public health and wastewater management.