Antibiotic resistant bacteria and genes in drinking water

Abstract

Antimicrobial resistance is now considered a growing public health challenge around the world. This research aimed to investigate the presence and identity of antibiotic-resistant bacteria (ARBs) and antibiotic resistance genes (ARGs) in drinking water supplies and to better understand the ability of water treatment processes to control ARBs and ARGs, as well as the potential reactivation of antibiotic resistance following disinfection. In the first part of this research, a screening survey for ARBs and ARGs was carried out in tap waters across London, one of the first such studies to do so. For this purpose, a sampling programme focusing on the resistance against six antibiotics was conducted via six sampling rounds of tap water sampled from residential properties. Subsequently, sets of culture-based (viable bacterial counts on nutrient media supplemented with antibiotics) and molecular tools (detection and quantification of ARGs from environmental DNA using quantitative real-time PCR) were applied, followed by species identification using a biochemical method (API 20NE identification). Bacteria that were resistant to vancomycin, erythromycin, amoxicillin, ciprofloxacin, tetracycline, and trimethoprim were found at all sampling points, with the percentage of the bacterial populations displaying trimethoprim resistance the highest throughout all sampling rounds. Seasonal differences did not affect the percentage of resistant bacteria significantly, although the percentage of resistant bacteria tended to elevate during winter (i.e. December to January). Antibiotic resistance genes bla-TEM1, tet(A), sul1, mph(A) and dfrA7 were commonly present, with the sul1 gene the most common. This study was the first to find the presence of the resistance gene mph(A) in municipal drinking water. Also, antibiotic-resistant opportunistic pathogen species, including Pseudomonas aeruginosa, Acinetobacter baumannii, Stenotrophomonas maltophilia and Burkholderia cepacia, were detected. A series of laboratory experiments to evaluate the impact of chlorine and UV disinfection on ARBs and ARGs and their regrowth/reactivation was conducted in the second part of this research. ARGs inactivation required much higher UV and chlorine doses than ARB inactivation. The bla-TEM1, tet(A), sul1, and mph(A) genes could not be completely inactivated under the typical disinfection doses applied in water treatment. On the other hand, the application of sequential UV disinfection followed by chlorination at typically applied doses significantly reduced the studied ARGs and had synergistic effects compared to single disinfectant use. Antibiotic-resistant Escherichia coli and Pseudomonas aeruginosa demonstrated the ability to regrow in phosphate buffer saline after chlorine disinfection of 1 to 5 mg/L and UV dose of 3 to 10 mJ/cm2. However, it is noteworthy that this UV dose is much lower than that normally applied in water treatment. Overall, the research has highlighted the great challenge posed by antimicrobial resistance in indigenous microbial populations commonly found in water supplies. It is evident that UV and chlorination are limited in their potential to control ARBs and ARGs on their own, though synergies were shown in their effectiveness as disinfectants when applied sequentially. The finding that some of the ARBs commonly found in tap waters are also opportunistic pathogens adds weight to the general advice that is currently given to those who are immune-compromised/suppressed (e.g. infants, hospital patients) that they should provide supplemental treatment to tap water before use (e.g. boil or filter it). Water companies should apply multiple disinfection barriers in drinking water treatment plants where possible, as well as maintain sufficient chlorine residuals in drinking water distribution networks to minimise re-growth of ARBs and spreading of ARGs during distribution.