Effects of chlorination on the inactivation and reactivation of Escherichia coli K12 and its ampicillin resistance gene

Antibiotic resistance genes (ARGs) and antibiotic resistant bacteria (ARB) are an emerging threat to public health and are identified as contaminants in aquatic environments. This study investigated the effect of chlorine on the inactivation and reactivation of Escherichia coli K12 and its ampicillin resistance gene. Inactivation of K12 was achieved with exposure to chlorine for 10 minutes, which were then allowed to reactivate in the dark for 24 hours. Bacterial populations were enumerated using heterotrophic plate counts containing concentrations of ampicillin, which caused further inactivation. The results showed that nearly 100% of the bacterial population was inactivated by ampicillin regardless of chlorination. Upon reactivation, a sharp decrease in bacterial concentration is observed between the control (0 mg/L Ampicillin) and any ampicillin concentration in unchlorinated samples. However, after chlorination, there is a much less decrease, suggesting that bacteria that survived chlorination were more resistant to ampicillin. Both inhibitory concentrations (MIC50 and MIC90) were greater than 16 mg/L Amp for at 1 mg/L Cl2. At higher chlorine concentrations, the MIC90 shifted from 0-4 mg/L to 4-8 mg/L ampicillin