Antimicrobial resistance and virulence signatures of Listeria and Aeromonas species recovered from treated wastewater effluent and receiving surface water in Durban, South Africa

BACKGROUND: Treated wastewater effluent has been found to contain high levels of contaminants, including disease-causing bacteria such as Listeria and Aeromonas species. The aim of this study was to evaluate the antimicrobial resistance and virulence signatures of Listeria and Aeromonas spp. recovered from treated effluents of two wastewater treatment plants and receiving rivers in Durban, South Africa. METHODS: A total of 100 Aeromonas spp. and 78 Listeria spp. were positively identified based on biochemical tests and PCR detection of DNA region conserved in these genera. The antimicrobial resistance profiles of the isolates were determined using Kirby Bauer disc diffusion assay. The presence of important virulence genes were detected via PCR, while other virulence determinants; protease, gelatinase and haemolysin were detected using standard assays. RESULTS: Highest resistance was observed against penicillin, erythromycin and nalidixic acid, with all 78 (100 %) tested Listeria spp displaying resistance, followed by ampicillin (83.33 %), trimethoprim (67.95 %), nitrofurantoin (64.10 %) and cephalosporin (60.26 %). Among Aeromonas spp., the highest resistance (100 %) 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 naladixic acid and trimethoprim. Among Listeria spp., 26.92 % were found to contain virulence genes, with 14.10, 5.12 and 21 % harbouring the actA, plcA and iap genes, respectively. Of the 100 tested Aeromonas spp., 52 % harboured the aerolysin (aer) virulence associated gene, while lipase (lip) virulence associated gene was also detected in 68 % of the tested Aeromonas spp. CONCLUSIONS: The presence of these organisms in effluents samples following conventional wastewater treatment is worrisome as this could lead to major environmental and human health problems. This emphasizes the need for constant evaluation of the wastewater treatment effluents to ensure compliance to set guidelines.