Unravelling the drivers of antibiotic resistance in urban rivers

Abstract

Rivers are increasingly burdened by wastewater discharges (treated or otherwise), which introduce antibiotic-resistant bacteria (ARB), antibiotic resistant genes (ARGs) and chemical contaminants into the aquatic ecosystems. This scenario is particularly alarming because it creates optimal conditions for the acquisition, proliferation, and dissemination of antimicrobial resistance (AMR) within the aquatic microbial communities. The escalation of AMR is a serious public health threat, also referred to as a “silent epidemic” by WHO. AMR occurs when bacteria acquire the ability to resist the effects of antibiotics, either through mutation or by acquiring resistance genes from other bacteria via horizontal gene transfer (HGT). The mechanisms of AMR acquisition are complex and multifaceted but are exacerbated when under selection pressure. Several environmental factors influence these mechanisms, including the presence of selective pressures like antibiotics, heavy metals, and other pollutants, which can co-select for ARGs. Additionally, physical factors such as temperature, pH, and nutrient availability also play a crucial role in shaping the microbial community structure and influencing the dynamics of AMR acquisition. In river systems, microbial communities act as our first line of defence against incoming ARB, resisting invasion through competitive exclusion and resource limitation; however, exposure to environmental stressors can disrupt this stability, heightening the potential for AMR proliferation, especially in regions with inadequate wastewater treatment infrastructure. In India, where an estimated 78.7% of urban sewage is discharged untreated or inadequately treated into rivers, these water bodies become breeding grounds for ARBs and ARGs, making them a potential global “hotspot” for AMR. The interaction between invading ARBs from wastewater and the native microbial communities in rivers can lead to significant shifts in the resistome - the collection of all resistance genes in a microbial community. Understanding the environmental and microbial factors that drive AMR in these settings is crucial for developing strategies to mitigate its proliferation and dissemination. This thesis aims to explore these dynamics in detail to elucidate the mechanisms behind the increase of AMR in river microbial communities and inform potential intervention strategies.