Abstract
Wastewater treatment plants (WWTPs) are major reservoirs of antibiotic-resistant bacteria (ARB) which are transported to the natural environment with discharged effluents. Samples of untreated wastewater (UWW) and treated wastewater (TWW) from four municipal WWTPs and samples of river water collected upstream (URW) and downstream (DRW) from the effluent discharge point were analyzed in the study. The total counts of bacteria resistant to β-lactams and tetracyclines and the counts of antibiotic-resistant Escherichia coli were determined. Antibiotic-resistant bacteria, including antibiotic-resistant E. coli, were removed with up to 99.9% efficiency in the evaluated WWTPs. Despite the above, ARB counts in TWW samples were high at up to 1.25x105 CFU/mL in winter and 1.25x103 CFU/mL in summer. Antibiotic-resistant bacteria were also abundant (up to 103 CFU/ml) in URW and DRW samples collected in winter and summer. In both UWW and TWW samples, the counts of ARB and antibiotic-resistant E. coli were at least one order of magnitude lower in summer than in winter. The study revealed that despite the high efficiency of bacterial removal in the wastewater treatment processes, considerable amounts of ARB are released into the environment with TWW and that the percentage of ARB in total bacterial counts increases after wastewater treatment.
Highlights
Bacterial resistance to antibiotics poses a considerable challenge in medicine, and it is not limited to pathogenic microorganisms
The predominant antibiotic-resistant bacteria (ARB) were ampicillin-resistant bacteria that accounted for 26–91% of total bacterial counts in untreated wastewater (UWW) samples, whereas summer samples of UWW were characterized by a predominance of cefuroxime-resistant bacteria (6–28%)
An increase in the proportions of ARB in treated wastewater can be attributed to the presence of mobile genetic elements in bacterial cells, including integrons, plasmids and transposons, which play a key role in the transmission of antibiotic resistance genes (ARGs) [19,20,21]
Summary
Bacterial resistance to antibiotics poses a considerable challenge in medicine, and it is not limited to pathogenic microorganisms. Research studies investigating the antibiotic resistance of environmental strains revealed that the natural environment, including soil and aquatic ecosystems, are often reservoirs of clinical bacterial strains with increased resistance to antibiotics [1]. Antibiotic-resistant environmental bacteria can transfer antibiotic resistance genes (ARGs) to human pathogens, and the resulting infections are increasingly difficult or even impossible to treat with the available antibiotics [2]. Prolonged exposure to even low concentrations of antibiotics in the sewage system contributes to the transfer of ARGs, exerts selective pressure on sewage-borne microorganisms [3] and leads to the spread of antibiotic resistance among bacteria in WWTPs’ wastewater and in the natural environment [4]
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