Assessing microbial contaminant abundance and antibiotic resistance within tributaries of Johnson Bay, Maryland Coastal Bays

Water quality in Maryland’s Coastal Bays has been declining dramatically in recent years due to land runoff and excess nutrients. In tributaries of Johnson Bay, abundances of bacteria are high considering relatively low-density land usage. We hypothesize that many of the microbial contaminants may be from runoff from farms as well as agricultural fields where manure is applied as fertilizer. We also hypothesize that microbes from these sources will exhibit resistance to common antibiotics used in sub-therapeutic administration in nearby feeding operations. This study seeks to assess antibiotic resistance and abundance of isolated indicator species of bacteria (e.g. Escherichia coli and Enterococcus spp.). Surface water samples were collected at the head, mid, and mouth of three creeks that flow into Johnson Bay in order to measure water quality parameters and determine bacterial abundance. Specific agar plates were used to assess bacterial contaminant abundance and a modified chromogenic agar impregnated with four different antibiotics (tetracycline, ciprofloxacin, gentamicin, and oxacillin) was used to assess E. coli antibiotic resistance. Preliminary data indicate that enterococci abundances (41.1x10³-6.32x10⁵ mL ^-1) at a majority of sites were in excess of EPA primary contact standards and correlate with nitrogen inputs. E. coli exhibited the highest resistance to oxacillin (52-100% resistance) whereas tetracycline, ciprofloxacin, and gentamicin had lower resistance (0-11% resistance). These findings may help target contaminant sources helping inform management agencies about improvements needed and the possibility for the control of contaminants that can affect human health.