Prevalence of listeria pathogens in effluents of some wastewater treatment facilities in the Eastern Cape province of South Africa

Wastewater discharges may contain health compromising pathogens and carcinogenic and/or chemical substances that could compromise the public health and impact negatively on the environment. The present study was conducted between August 2007 and July 2008 to evaluate the Listeria abundance (as free-living and plankton associated species) and physicochemical qualities of the final effluents of three wastewater treatment facilities in the Eastern Cape Province of South Africa selected to represent typical urban, peri-urban and rural communities and the impact of the discharged final effluents on their respective receiving watershed, as well as to elucidated the in vitro antibiotic susceptibilities and resistance genes profile of Listeria species isolated from the final effluents. The suitability of the secondary effluent of the urban treatment facility (as a case study) for use in agriculture and aquaculture with reference to recommended standards was also determined. Wastewater samples were collected from the raw sewage, secondary effluent, final treated effluent, discharge point, 500 m upstream discharge point, and 500 m downstream discharge point from all three locations on a monthly basis throughout the study period. Listeria abundance in the final effluents and the receiving watersheds varied between 2.9× 100 and 3.52 × 105cfu/ml across the sampled locations. Free-living listerial density across the sampled locations ranged between 0 and 3.2 × 103cfu/ml while counts of Listeria species attached to large (180 μm) planktons varied from 0 to 1.58 × 105 cfu/ml and those of the 60 and 20 μm categories were in the range of 0 to 1.32 × 103 cfu/ml and 0 to 2.82 × 105 cfu/ml respectively. Listeria abundance did not vary significantly with location and season; there was however, significant (P < 0.05; P < 0.01) variance in Listeria abundance with plankton sizes across the locations. Free-living Listeria species were more abundant in the rural and urban xii communities than plankton attached Listeria species; whereas the reverse was the case in the peri-urban community. Prevalence of Listeria in terms of total counts was 100 percent across all sampled locations. Free-living Listeria species showed prevalence ranging from 84-96 percent across the sampling locations; while Listeria species attached to large (180 μm) planktons exhibited prevalence ranging from 75 percent to 90 percent. The prevalence of medium-sized (60 μm) plankton associated Listeria species varied between 58 percent and 92.5 percent; whereas those of Listeria species attached to small (20 μm) planktons ranged from 65-100 percent across all three communities. Listeria prevalence was generally a reflection of the turbidity of the water system, with free-living Listeria species being more prevalent than plankton associated cells in the relatively less turbid rural and urban waters compared to the more turbid peri-urban waters where plankton attached cells were more prevalent in comparison with their free living counterparts The final treated effluent quality fell short of recommended standards for turbidity, chemical oxygen demand and phosphate across all three communities. In addition, the final effluent of the rural treatment plant also fell short of recommended standard for NO3, while that of the urban treatment plant did not comply with acceptable limits for dissolved oxygen and nitrite. Other physicochemical parameters were compliant with set standards after treatment. An inverse relationship was observed between chlorine residual and listerial density across the sampled facilities; the effect of chlorine was however not enough to eliminate the pathogen from the water systems. At the urban treatment plant and its receiving watershed, pH, temperature, EC, turbidity, TDS, DO, and nitrate varied significantly with season and sampling point (P < 0.05; P < 0.01). Salinity also varied significantly with sampling point (P < 0.01), while COD and nitrite varied significantly with season (P < 0.05). Although, the treated effluent fell within recommended water quality standard for pH, TDS, nitrate and nitrite, it fell short of stipulated standards for other parameters. Whereas the microbial quality of the secondary treated effluent at this (urban) facility fell short of recommended standard after secondary treatment, its physicochemical quality were generally compliant with recommended standards for reuse wastewater in agriculture and aquaculture. Listeria pathogens isolated from effluents of the rural wastewater facility were sensitive to 11 (55 percent) of the 20 test antibiotics, and showed varying (7-71 percent) levels of resistance to 8 antibiotics; whereas those isolated from the peri-urban community showed sensitivity to 6 (30 percent) of the 20 test antibiotics, and varying (6-94 percent) levels of resistance to 12 antibiotics; while the urban effluent isolates were sensitive to 3 (15 percent) of the 20 test antibiotics, and showed varying (4.5-91 percent) levels of resistance to 17 antibiotics. Multiple antibiotic resistances involving 78.5-100 percent of isolates and antibiotics combination ranging from 2-10 antibiotics was observed across the sampled locations. Penicillin G and ampicillin showed remarkably high (64-91 percent) phenotypic resistance across the three sampled facilities. Other antibiotics, to which isolates showed significant resistance, were linezolid (22-88 percent); erythromycin (43-94 percent) and sulphamethoxazole (7-94 percent). Two of the 14 Listeria strains isolated from the rural effluents were positive for ereA and sul1 antibiotic resistance genes; while sulII genes were detected in five of the 23 Listeria isolates from the urban effluent and none was detected in isolates from the peri-urban community. The presence of antimicrobial resistance genes in the isolates did not correlate with phenotypic antibiotic resistance. The current study demonstrated that Listeria pathogens easily survived the activated sludge treatment process as free-living and plankton attached entities and suggests that municipal wastewater treatment plants are a significant source of multiple resistant Listeria pathogens in the South African aquatic milieu. While the physicochemical quality of the urban final effluent suggests that it is a major source of pollution to the receiving watershed, the secondary effluent quality demonstrated a great potential for use in agriculture and aquaculture