Use of antibiotics for treatment and prevention of bacterial infections in human, agri- and aquaculture as well as livestock rearing leads to antibiotic pollution of fresh waters and these anti-biotics have an impact on free-living bacteria. While we know which antibiotics are most common in natural environments like rivers and streams, there is considerable uncertainty regarding antibi-otics’ interactions with one another and the effect of abiotic factors like temperature. Here, we used an experimental approach to explore the effects of antibiotic identity, concentration, mixing and water temperature on the growth of Pseudomonas fluorescens, a common, ubiquitous bacterium. We exposed P. fluorescens to the four antibiotics most commonly found in surface waters (ciprofloxacin, ofloxacin, sulfamethoxazole and sulfapyridine) and investigated antibiotic interactions for single and mixed treatments at different, field-realistic temperatures. We observed an overall dependence of antibiotic potency on temperature, as temperature increased efficacy of ciprofloxacin and ofloxa-cin with their EC50 lowered by >75% with a 10°C temperature increase. Further, we show that mix-tures of ciprofloxacin and ofloxacin, despite both belonging to the fluoroquinolone class, exhibit low-temperature-dependent synergistic effects in inhibiting bacterial growth. These findings high-light context-dependence of antibiotic efficacy. They further suggest antibiotic-specific off-target ef-fects that only affect the bacteria once they enter a certain temperature range. This has important implications as freshwater systems already contain multi-drug antibiotic cocktails and are changing temperature due to environmental warming.
|Published - 8 Aug 2022