Abstract
The effects of streptomycin (STM) on the development of antibiotic resistance in an aerobic-biofilm reactor was explored by stepwise increases in STM doses (0–50 mg L−1), over a period of 618 days. Totally 191 bacterial isolates affiliated with 90 different species were harvested from the reactor exposed to six STM exposures. Gammaproteobacteria (20–31.8%), Bacilli (20–35.7%), Betaproteobacteria (4.5–21%) and Actinobacteria (0–18.2%) were dominant, and their diversity was not affected over the whole period. Thirteen dominant isolates from each STM exposures (78 isolates) were applied to determine their resistance prevalence against eight classes of antibiotics. Increased STM resistance (53.8–69.2%) and multi-drug resistance (MDR) (46.2–61.5%) were observed in the STM exposures (0.1–50 mg L−1), compared to exposure without STM (15.3 and 0%, respectively). Based on their variable minimum inhibitory concentration results, 40 differentiated isolates from various STM exposures were selected to check the prevalence of nine aminoglycoside resistance genes (aac(3)-II, aacA4, aadA, aadB, aadE, aphA1, aphA2, strA and strB) and two class I integron genes (3′-CS and IntI). STM resistance genes (aadA, strA and strB), a non-STM resistance gene (aacA4) and integron genes (3′-CS and Int1) were distributed widely in all STM exposures, compared to the exposure without STM. This new culture-based stepwise increasing antibiotic approach reveals that biological systems treating wastewater with lower STM dose (0.1 mg L−1) could lead to notably increased levels of STM resistance, MDR, and resistant gene determinants, which were sustainable even under higher STM doses (> 25 mg L−1).
Highlights
The broad use of antibiotics in the fields of medical therapy, animal husbandry, and plant disease control during the past 65 years has resulted in the rapid growth and global prevalence of antibiotic resistance
Our previous study showed that the presence of high concentrations of antibiotics in a biological wastewater treatment plants (WWTP) could lead to the occurrence of tetracycline resistance and multi-drug resistance (MDR) in treated wastewater and downstream rivers (Li et al 2010)
The remaining distributed bacterial classes were Alphaproteobacteria (0–13.3%), Flavobacteria (0–18.2%) and Sphingobacteria (0–5.2%). Bacterial genera such as Bacillus, Pseudomonas, Aeromonas, Microbacterium and Acinetobacter were dominant, and bacterial species such as Aeromonas veronii, Bacillus anthracis, Chryseobacterium lactis, Comamonas testosteroni, Lactococcus chungangnensis and Microbacterium maritypicum were predominantly present in almost all STM exposures
Summary
The broad use of antibiotics in the fields of medical therapy, animal husbandry, and plant disease control during the past 65 years has resulted in the rapid growth and global prevalence of antibiotic resistance. Similar results have been observed in antibiotic production WWTPs accepting penicillin and spiramycin production wastewater (Li et al 2009, 2011; Liu et al 2014). Most of these studies have been derived from investigative results of full-scale real WWTPs, and do not reveal the influence of antibiotic concentrations. Some effort has been made to investigate the influence of minimum antibiotic concentrations on the transfer of MDR under controlled experimental conditions Under such studies, concentrations of tetracycline and sulfamethoxazole, as low as 10–100 μg L−1 were found to promote MDR through the horizontal dissemination of mobile resistance elements (Jutkina et al 2016; Kim et al 2014). Pure cultures and overnight culture times have often been adopted for these experiments, which may not be able to reveal the impact of antibiotic concentrations on the wastewater treatment bacteria
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