Disinfection potential of moringa oleifara to eliminate antibiotic-resistant bacteria in wastewater in comparison to chlorination.
Adewoge, Aderonke Latifat
Adewoge, Aderonke Latifat
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Abstract
The levels of antibiotic-resistant Enterobacteriaceae (Escherichia coli, Vibrio cholera, Salmonella typhimurium, Shigella flexneri and Proteus mirabilis) found to be present in discharged effluents of four wastewater treatment plants (WWTPs) in Tshwane Municipality in Gauteng are of significant health concern. These WWTPs discharge their effluents into nearby rivers which are of economic importance to the immediate communities living downstream. This situation exposes individuals and livestock that consume or use water from these rivers to health risks associated with partially decontaminated wastewater.
Therefore, the purpose of this study was to evaluate the reactivation of bacteria previously inactivated by chlorine and ultraviolet disinfection, which entailed assessing the efficiency of the current disinfection agents being used in eliminating antibiotic-resistant bacteria (ArB). Our aim was achieved by monitoring the dynamics (movement) of ArB response to disinfection in WWTPs from the point downstream of the secondary settling tank (SST) (prior to disinfection) to the point of discharge (disinfected effluents). In addition, the disinfection potential of Moringa oleifera (M. oleifera) extracts on ArB were investigated, and comparative experiments were conducted to determine whether there is a difference in disinfection efficiency between M. oleifera extracts and chlorine at equal concentrations.
To achieve the aims of this study, the investigations were divided into different stages. The first stage involved collection of M. oleifera leaves and seeds, phytochemical screening and gas chromatography-mass spectrometry (GC-MS) profiling of the extracts of M. oleifera. The GC-MS profiling showed that our extracts contained similar antioxidants and natural products cited in previous studies, but the most prominent natural product in our extracts was found to be palmitic acid (hexadecanoic acid). The second stage involved wastewater sampling, and a compilation of data on the physicochemical (pH, dissolved oxygen (DO) redox potential, chemical oxygen demand (COD), turbidity) and microbiological parameters (faecal coliforms, E. coli, and total coliforms) of wastewater. From the data collected in this section, an overview of the compliance level of each WWTP was achieved in comparison to standard limits prescribed by the Department of Water Affairs. Only the pH parameter was found to be within acceptable limits in the four WWTPs evaluated. Results of the statistical analysis also revealed a statistically significant correlation between the redox potential values of the final effluent and the disinfection efficiency on ArB.
The third stage of the study consisted of two subsections of which the first involved antimicrobial susceptibility testing (AST) of the heterotrophic bacterial population of wastewater samples. In the second subsection, the detection and confirmation of housekeeping genes of the antibiotic-resistant isolates obtained from plates during AST experiments was achieved by real-time polymerase chain reaction (PCR). Disinfection results obtained from the sampled WWTPs showed that disinfection at WWTP-A and WWTP-D was very effective in terms of antibiotic-resistant Escherichia coli, where both displayed a disinfection efficiency of 81.8%. For WWTP-B, a disinfection efficiency of 36.4% was achieved, and for WWTP-C, it was found to be 54.5%. Also, the disinfection efficiency against antibiotic-resistant Vibrio cholerae was observed to be 27.3% for WWTP-B, 45.5% for WWTP-A and 63.6% for WWTP-C, respectively, but very high for WWTP-D, which exhibited the same disinfection efficiency as for antibiotic-resistant E. coli (81.8%). Disinfection efficiency against antibiotic-resistant Salmonella typhimurium (hereinafter referred to as Salmonella typhimurium) at WWTP-A, WWTP-B and WWTP-C was found to range between 90 and 100%, while WWTP-D exhibited the lowest efficiency of 54.54%. Antibiotic-resistant Shigella flexneri was found to be the best respondent to disinfection in all four WWTPs as the disinfection efficiency was found to range from 81.8% to 100%. Antibiotic-resistant Proteus mirabilis was the poorest respondent to disinfection in all four sampled WWTPs, as the disinfection efficiency was found to range between a minimum value of 27.3% (WWTP-A) and a maximum value of 45.45 % (WWTP-C).
The fourth stage involved the investigation of antibiotic-resistant genes (ArGs) conferring resistance to the isolates found to be present in the final effluents by means of AST. However, this study focused on the detection of only six antibiotic resistant genes, namely blaCMY-2 (β-lactamase gene), strA (streptomycin), tetM (tetracycline), sul1 (sulfamethoxazole), cat1 (chloramphenicol) and parC (ciprofloxacin). Out of the 103 final effluent samples screened for their resistance genes, only 45 samples were positively identified for the six genes investigated. The following resistance genes were detected in wastewater samples: parC gene in 13 samples, tetM gene in 9, cat1 gene in 6, strA in 8, sul1 gene in 5, and blaCMY-2 in 4 samples. The parC gene was the most prevalent in the wastewater samples assayed as it was detected in 13 samples. In the fifth stage, four extracts of M. oleifera (two extracts of leaves and two extracts of seeds) were evaluated for their disinfection potential in an 8.5% saline suspension (synthetic wastewater). The suspension also contained a consortium of five different antibiotic-resistant bacteria (isolated antibiotic-resistant E. coli, Vibrio cholerae, Salmonella Typhimurium, Shigella flexneri and Proteus mirabilis) at a concentration of 1.5 × 108 bacterial cells. The disinfection data indicated that M. oleifera exhibited excellent antibacterial and disinfection potential on ArB. It was observed that a disinfection efficiency of between 75 and 100% was achieved within 1 h of contact time between M. oleifera extracts and the synthetic wastewater. Consequently, all the extracts of M. oleifera were determined as possessing significantly better disinfection efficiency than chlorine which implies that they can effectively be used as a substitute to chlorine by their activity as a potent disinfectant for inhibiting ArB. Our results therefore support the possibility of using any of the four extracts of M. oleifera in place of chlorine, in order that the acquisition of resistance traits by selective pressure induced by chlorination in bacteria can be eradicated in WWTPs.
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Date
2019-03-05
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Tshwane University of Technology
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Keywords
Antibiotic-resistant, Salmonella typhimurium, Moringa oleifera, Wastewater treatment plants, Tshwane Municipality, Gauteng