Antimicrobial Resistant Genes and Organisms as Environmental Contaminants of Emerging Concern: Addressing Global Public Health Risks
Haley Sanderson, ... Steven N. Liss, in Management of Emerging Public Health Issues and Risks, 2019
22.214.171.124 Wastewater Treatment Plants as a Point of Control
WWTPs are a significant point source for AMRDs and antimicrobials. WWTPs are relatively nutrient-rich, heavily contaminated environments that receive waste from a variety of AMRD-loaded environments, including hospitals, industrial and agricultural sites and release both solid and liquid by-products that can disseminate AMRDs. Influent can be contaminated with a variety of pollutants, including antimicrobial agents, pharmaceuticals, personal care products, and heavy metals, which can accumulate within WWTPs. Many microbial and chemical contaminants in wastewater cannot be degraded by the treatment process or inactivated through disinfection of the effluent. For those contaminants that can be degraded, the resulting metabolites may still have antimicrobial or selective activity. WWTP effluent and solid waste products not only have a high prevalence of AMRDs but also release selective agents into the receiving environments (Jury et al., 2011).
The nature of biological treatment can also encourage the dissemination of AMRD into the environment and within the wastewater microbiome. Microorganisms are found in a variety of states in WWTP including in planktonic form, flocs suspended in the wastewater, and biofilms attached to solid surfaces (Sustaric, 2009; Sheng et al., 2010). The presence of microorganisms in flocs and in biofilms may be significant in establishing the basis of why WWTPs are considered both hot spots for gene transfer and possible reservoirs for AMRD.
Andersen (1993) found that microbial community composition in a WWTP affected AMR coliforms. Additionally, different WWTPs have different efficiencies for the removal of AMRO. Both operational conditions and design can influence the fate of AMRDs in WWTPs (McKinney et al., 2010; Novo and Manaia, 2010; Chen and Zhang, 2013). There have been numerous studies to determine which treatment systems and operational conditions impact AMRDs. For instance, Kim et al. (2007) found that both organic loading and growth rate resulted in the amplification of tetracycline resistance in WWTPs using biological treatment processes. Christgen et al. (2015) used metagenomic approaches to compare the fate of AMRDs in anaerobic, aerobic, and anaerobic-aerobic sequence bioreactors (AASs). AASs and aerobic reactors were superior to anaerobic reactors in reducing AMRD abundance, particularly aminoglycoside, tetracycline, and beta-lactam determinants. Sulfonamide and chloramphenicol AMRD levels were unaffected by treatment, and a shift from target-specific AMRDs to AMRDs associated with multidrug resistance was seen in influents and effluents from all WWTP. The AASs used 32% less energy than aerobic reactors and favorably reduced AMRD abundance. The chemical properties of the wastewater, including chemical oxygen demand (COD), ammonia (NH3–N), suspended solids (SS), dissolved oxygen, and temperature, can impact the fate of different AMRDs. For instance, Du et al. (2014) found that the COD was highly correlated with the fate of tetW, intI1, and sul1. Yuan et al. (2014) indicated that most AMROs and AMRDs were positively related to COD and SS of raw sewage and negatively correlated to the corresponding variables in the effluent.
Similarly, the choice of disinfection method can impact the fate of AMRDs in WWTPs. Disinfection may not reduce the abundance of AMRDs and AMROs in the effluent as Munir et al. (2011) observed when studying the presence of tetracycline and sulfonamide and their resistance determinants within five WWTPs in Michigan. The use of chlorination and UV radiation for disinfection is common, and the effectiveness of these strategies for removing AMRD varies as a result of multiple factors. For instance, Zhang et al. (2015) compared the inactivation of AMRDs in municipal wastewater effluent by chlorination, UV radiation, and sequential UV/chlorination. They found that chlorination was more effective than UV radiation in removing AMRDs from WWTP effluent and that its efficiency was affected by NH3–N concentration. The presence of high NH3–N in wastewater corresponds to a decline in AMRD removal (Zhang et al., 2015). Free chlorine was also more effective than combined chlorine, and a combination treatment of UV irradiation followed by chlorination showed higher AMRD removal efficiency than UV or chlorination alone.
The methods of evaluation of AMRDs in WWTPs can make the comparison of studies difficult. In some studies, a culture-dependent step is crucial and the focus is mainly on the detection and fate of AMROs (Reinthaler et al., 2003; Schwartz et al., 2003; Garcia-Armisen et al., 2011; Slekovec et al., 2012), whereas others use a combination of culture-dependent and -independent techniques and have shifted the focus to MGE (Tennstedt et al., 2003; Ma et al., 2013). Studies that use a combination of culture-dependent and -independent techniques provide more comprehensive information than studies that use only one technique or the other. Culture-dependent studies provide valuable information about the expression of AMRDs but neglect the impact of extracellular and unexpressed AMRDs (Matthews et al., 2010), whereas culture-independent methods may not account for the function of those genes.
The complexity of the engineered system can obscure the influence of antimicrobials on the spread and prevalence of AMR both within the WWTP and in their effluent. This can make elucidating the factors and mechanisms responsible for the increased prevalence of AMRDs more difficult and determining their relative importance is a constantly evolving area of research. The high degree of gene transfer that can occur in WWTPs and the high prevalence of AMR in microorganisms isolated from wastewater would suggest that WWTPs are a point source for AMR-related environmental contamination. An engineered system, like a WWTP, may be ideal for environmental public health monitoring, and surveillance efforts and management strategies could be developed that are targeted at reducing the release of AMRDs into water and soil environments.