Treated wastewater effluent as a potential source of emerging bacterial pathogens in surface water.
Nzimande, Siphephile Bukelwa Thembelihle.
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Rivers and estuaries are the major sources of the earth’s fresh water, and are also the major recipient of treated wastewater effluent discharge, thus leading to their deterioration. This study was aimed at evaluating the microbiological and physico-chemical quality of final effluents of two independent wastewater treatment plants within the eThekwini Municipality as well as their effect on the receiving water bodies. Water samples were collected over a 12 month sampling period at designated points: before and after tertiary treatment, as well as upstream and downstream of the receiving rivers. Bacterial analyses of collected samples were conducted using standard membrane filtration methods and selective media, allowing for the presumptive enumeration of prevalent emerging bacterial pathogens. Analysis of the samples revealed that the effluent samples were not of acceptable standards for most of the parameters analysed throughout the sampling period. Percentage bacterial reduction varied between 19.5 – 99.9%, 23.3 – 99.9%, 8.2 – 99%, 29.1 – 99.9%, and 2.4 – 99% for Aeromonas spp., Pseudomonas spp., Listeria spp., Yersinia spp. and Legionella spp. respectively after chlorination. Temperature, pH, dissolved oxygen, biochemical oxygen demand, chemical oxygen demand, total dissolved solids values ranged between 12 – 27 °C, 6.41 – 7.88, 4.01 – 7.66, 1.40 – 9.61, <10 – 309.06, and 200.03 – 710.00 mg/L, respectively, while turbidity and total suspended solids ranged between 6.48 – 71.02 NTU and 0.01 – 5.93 mg/L respectively, across the sampling period. Presumptive Aeromonas spp. and Listeria spp. recovered from the water samples were further identified using biochemical tests and PCR methods, followed by the construction of antibiograms of all confirmed isolates, with some isolates showing resistance to a number of commonly used antibiotics. For the tested Aeromonas species, complete resistance was observed against ampicillin, penicillin, vancomycin, clindamycin and fusidic acid, followed by cephalosporin (82%), and erythromycin (58%), with 56% of the isolates found to be resistant to nalidixic acid and trimethoprim, the antibiotic resistance index (ARI) ranging from 0.25 – 0.58. Listeria spp. displayed the highest resistance against penicillin, erythromycin and nalidixic acid, with all 78 (100%) tested species displaying resistance, followed by ampicillin, trimethoprim, nitrofurantoin and cephalosporin with 83.33%, 67.95%, 64.10% and 60.26%, respectively. The ARI for the Listeria spp. ranged between 0.13 (resistance to 3 test antibiotics) – 0.5 (resistance to 12 of the test antibiotics). Characterization of the virulence gene markers and enzymes in Aeromonas and Listeria species confirmed the level of potential pathogenicity of the isolates. Of the 78 tested Listeria spp., a total of 26.92% (21) were found to contain virulence genes, 14.10% (11), 5.12% (4) and 21% (17) of these species were found to harbour the actA, plcA and iap genes respectively, while 11.54% (9) contained more than one virulence gene. Of the 100 tested Aeromonas spp., 52% harboured the aer, while 68% tested positive for the lip virulence associated gene. In addition, up to 35% of the Listeria spp. were positive for haemolysin enzyme and negative for gelatinase and protease, while 57%, 81% and 100% of the Aeromonas spp. were positive for haemolysin, gelatinase and protease enzyme. The prevalence of these emerging pathogens in treated effluent presents a potential threat to the health of surrounding communities, considering that they are not included in the current guidelines and therefore cannot be monitored. The obtained results further highlight the need for revised standards which include emerging pathogens.