Intermediate concentrations of synthetic antibiotics drive drastic changes in aquatic bacterial communities

The spread of resistances to commonly used synthetic antibiotics (AB) in natural environments is one of the major threats to human health. A large number of publications investigated the relations between different human pathogens and the spread of AB resistances, but a comparably low number of studies targeted the ecological effect of the presence of synthetic AB on nonâEUR pathogenic bacteria in waters. We tested the longâEUR term impact of a cocktail of synthetic antibiotics of commercial use (Imipenem, Levofloaxcin, and Tetracycline) on artificial bacterial communities miming a very simplified natural freshwater community. The experimental setup consisted in a 24 daysâEUR long continuous culture system where the bacterial communities were expose to three different AB concentrations: no AB (control), subâEUR lethal AB concentration (comparable to anthropized European lakes), and in AB concentration 10 folds higher (comparable to heavily contaminated water treatment plants). Bacterial number and morphological distribution were daily monitored (epifluorescence microscopy), while community composition (CARDâEUR FISH) and antibiotic resistance were assessed biâEUR weekly. Bacterial production, and thus abundance, dropped by 75% in the presence of AB, independently by their concentration, while bacterial community composition resulted highly affected by intermediate AB concentration, with reversal in the proportion of the dominant groups (A. hydrophila and Brevundimonas sp.). These results, for the first time focussing on the ecological consequences of AB on a experimental bacterial community, highlight the potential risk posed by low AB concentrations in waters. Their effect is not only limited to the spread of potentially pathogenic strains, but it extends to modification of interspecific ecological interactions, weakening the natural resistance of the aquatic communities to the success of allochtonous, and potentially dangerous, strains.