Differential Expression of Antioxidant Enzymes in Chlorine-Resistant Acinetobacter and Serratia spp. Isolated from Water Distribution Sites in Mumbai: A Study on Mechanisms of Chlorine Resistance for Sustainable Water Treatment Strategies

Chlorination is a widely used process for disinfecting drinking water, but the emergence of chlorine-resistant bacteria has become a significant concern. While previous research has focused on identifying chlorine-resistant organisms, there has been limited investigation into the mechanisms behind chlorine resistance. Some bacterial isolates that display resistance to chlorine treatment may protect themselves using various mechanisms, including biofilm production, antibiotic resistance, horizontal transfer of antibiotic resistance genes, or producing antioxidant enzymes. Given that chlorination employs hypochlorous acid (HOCl), which is an extremely potent oxidizing agent, the most critical mechanism to investigate is antioxidant enzymes. This study investigated the antioxidant profile of eight chlorine-resistant isolates (three of the Serratia sp. and five of the Acinetobacter) after chlorine exposure. The profiles, both between and within species, were noticeably different. Among the isolates, Acinetobacter junii NA 3-2 showed a significant increase in the specific activity of superoxide dismutase, catalase, and ascorbate peroxidase after exposure to 20 ppm chlorine. In the guaiacol peroxidase (GPX) assay, only isolates belonging to Serratia marcescens showed GPX activity, and Serratia marcescens 3929-1 showed significant increase after exposure to 20 ppm of chlorine. None of the isolates belonging to Acinetobacter spp. showed GPX activity. Additionally, almost all control samples exhibited some enzyme activity, which may explain their survival against chlorine treatment in reservoirs. Principal component analysis revealed no strain-dependent similarities, while the balance of scavenging enzymes changed, as demonstrated in the heat map. Thus, this study suggests that antioxidant enzymes may be one mechanism of protection for some bacterial species against oxidative stress from chlorination, resulting in chlorine resistance. Understanding the mechanism of chlorine resistance is critical to identifying potential solutions. This study highlights the need to consider more modern approaches to disinfecting drinking water.