Assessment of antibiotic activity in surface water of the lower Taieri Plain and impacts on aquatic bacteria in Lake Waipori, South Otago, New Zealand

Abstract

Veterinary antibiotics are widely used in New Zealand and around the world to treat bacterial infections and for non-therapeutic use as growth promoters. There is growing concern that unmetabolised antibiotics pass through stock and enter the environment, facilitating antibiotic resistance in pathogenic bacteria and harming aquatic ecosystems. Bioassay experiments showed that filtered water from an agricultural drain reduced the abundance of aquatic bacteria in Lake Waipori, a shallow coastal lake, whereas most other planktonic organisms were not inhibited. This result was consistent with the presence in the drain water of bacteriolytic antibiotics (e.g., penicillin). Further experiments were conducted to examine the effects of admixtures of water from the same agricultural drain both on the abundance and respiration rate of aquatic bacteria in the lake. One of four experiments showed inhibition of bacterial respiration consistent with the presence of bacteriostatic antibiotics (e.g., tetracyclines, aminoglycosides, macrolides) in the drain water. No further significant bacteriolytic effects were observed. A mixture of commonly used antibiotics was not consistently effective at reducing bacterial abundance or inhibiting bacterial respiration in lake water. Although we found evidence of antibacterial activity in the drain water this was intermittent, indicating either that antibiotic inputs to the drain were also intermittent, that other environmental factors intermittently affected the potency of antibiotics in the drain water, and/or that aquatic bacteria exhibited temporally variable resistance to antibiotics.