Evaluation of occupational health risks from bacterial contaminants at wastewater treatment plants in Gauteng Province, South Africa

Abstract

Background: Workers at wastewater treatment plants (WWTPs) are routinely exposed to several health hazards, including bacteria due to frequent exposure to untreated wastewater. These occupational exposure increases the risk of infections, particularly respiratory and gastrointestinal illnesses. Aim: This study aimed to provide data on the potential health risks associated with occupational exposure to selected respiratory and enteric bacterial pathogens among South African WWTP workers. Methodology: 16S rRNA sequencing was used to screen and identify bacterial pathogens in wastewater and bioaerosol samples. Quantification of selected bacterial pathogens was determined using both culture method and qPCR. Subsequently, metagenome-assembled genomes (MAGs) Legionella pneumophila and Aeromonas spp. were reconstructed to identify antibiotic resistance genes (ARGs) and spontaneous mutations in key ARGs conferring resistance to commonly used drugs in L. pneumophila. Self-administered questionnaires were used to investigate the prevalence of chronic respiratory and gastrointestinal health issues among WWTP workers. Finally, quantitative microbial risk assessment (QMRA) was used to estimate the risk of infection associated with occupational exposure to L. pneumophila and A. hydrophila. Key results: A total of 36 pathogenic genera were identified during the screening of influent samples. Culturable L. pneumophila was detected in 92% of the samples (2 to 5.4 log10 MPN/100 mL). Mycobacterium spp. had 100% detection rate, with concentrations ranging from 2.8 -8.9 log10 GC/mL. Enteric pathogens A. butzleri and A. hydrophila were detected in 96% and 82% of samples, respectively, with comparable GC levels. Bioaerosols from mechanical aeration tanks (72%, 26/36) had a relatively higher richness and diversity of airborne bacterial pathogens than diffused aeration tanks (17%, 6/36). In L. pneumophila, 138 nonsynonymous SNVs were identified in genes associated with antimicrobial resistance, with gyrA and gyrB showing the highest prevalence of variants (85 and 17, respectively). In Aeromonas spp., two (aph(6)-Id and aph(3’’)-Ib) and five (blaMOX−4, blaOXA−1143, blaOXA−724, cepH, and imiH) ARGs conferring resistance to aminoglycosides and β-lactams were identified, respectively. Commonly reported chronic respiratory symptoms were wheezing (23.5%) and shortness of breath (7.4%) among WWTP workers compared to the control group (16.9 and 0%, respectively). Legionella pneumophila infection risks were vii minimal for 0.5-hr but exceeded the USEPA threshold for exposure durations of 2-hrs or more across all scenarios and WWTPs. Similarly, infection risks posed A. hydrophila for the 0.5-hr exposure time were acceptable across all scenarios. Conclusion: Findings of this study suggest that WWTP workers are at increased risk of respiratory and gastrointestinal health issues, potentially linked to occupational exposure to bacterial pathogens. The study emphasises the urgent need for improved PPE practices, targeted safety training, and increased awareness to mitigate these risks. Therefore, the combined data (hazard identification, health surveys, and QMRA) generated in this study can contribute to the understanding of microbial exposure risks at WWTPs, offering a foundation for risk mitigation strategies to safeguard worker health.