(53h) Antibiotic Pumps In Bacteria Export Endocrine Disrupting Chemicals During Wastewater Treatment

Along with the presence of pharmaceuticals and endocrine disrupting chemicals (EDCs) in the environment, drug resistance can be found in wastewater effluent, receiving waters, and produced biosolids. The classes of resistance genes found in wastewater and treated effluent vary significantly due to the nature of antibiotics present.  Some classes include: b-lactam resistance (ampicillin), polyketide resistance (tetracycline), and aminoglycoside resistance (kanamycin).    It was recently discovered that the major antibiotic pumps AcrAB-TolC and AcrAD-TolC in e. coli export human hormones in addition to the common substrates antibiotics, bile salts, and surfactants.  AcrAB-TolC and AcrAD-TolC belong to a class of antibiotic pumps known as the Resistant Nodulation and Division (RND) exporters.  RND proteins span the entire inner and outer membranes of gram-negative bacteria and consist of an inner membrane component, a membrane fusion protein, and an outer membrane channel.  During export, an antibiotic (or other chemical) enters the protein complex through periplasm, and is pumped out of the bacterium. 

To determine if multidrug resistance efflux pumps export endocrine disrupting chemicals in addition to common substrates, minimum inhibitory concentrations (MIC) were measured. The MIC is the concentration of a chemical at which no growth of a bacteria strain is observed.  Two major Resistant Nodulation and Division (RND) efflux pumps, AcrAB-TolC and MexAB-OprM, were expressed in Escherichia coli and exposed to EDCs.  These include hormones, synthetic hormones, alkylphenols, platsticizers, and phytoestrogens. Hormones such as 17β-estradiol, estrone, and estriol, showed no inhibitory effect on the E. coli with the chemical concentration up to 100 mg/L. Meanwhile, the tripartite complexes of AcrAB-TolC and MexAB-OprM efflux pumps were revealed as important functions in resistance of 17α-ethynylestradiol, octylphenol, nonylphenol, bisphenol-A.

The presence of EDC resistant bacteria in local wastewater effluent and river samples was qualified.  Water samples were plated on LB agar with appropriate concentrations of EDCs determined from MIC tests above (17α-ethynylestradiol, octylphenol, nonylphenol, bisphenol-A). Results showed that various EDC-resistant bacteria survive UV disinfection, and reach the Jordan River, which feeds into the Great Salt Lake.  Such bacteria have also been detected in fresh-water tributaries leading to the Jordan River.

Bacteria are ever-adapting to chemical stressors in the environment, leading to biochemical responses such as drug resistance.  The persistence of endocrine disruptors in the environment supports the findings of this research, in that EDCs are substrates of multidrug efflux pumps.  Since EDCs are exported from the bacterial cell, they may not reach oxidizing proteins which would eliminate them from wastewater.  One reason for the lack of degradation during secondary wastewater treatment could be that EDCs are exported from cells before coming into contact with degrading enzymes.