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Assessment of inactivation of bacterial and fungal pathogens in aquatic matrices

Papadopoulou Konstantina

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Summary

Water is covering 71% of the Earth's surface, is the cornerstone of life and maintains the balance of the planet. Unfortunately, anthropogenic activity has led to a significant degradation of this precious resource. Water pollution and contamination are serious health and environmental problems. The present study focuses on water contamination, more specifically on the inactivation of pathogenic microorganisms, such as the bacterium Acinetobacter baumannii and the fungus Candida albicans, known for their high resistance to antibiotics. The purpose of the research is to investigate the conditions under which these microorganisms are inactivated, applying two disinfection methods: chlorination with sodium hypochlorite (NaOCl) and ultraviolet radiation (UVC). The experimental conditions include the use of different aqueous matrices, such as NaCl solution (0.8% w/v), water from Lake Agia and wastewater after secondary treatment from the DEYAX and DEYAVA facilities. This study aims to develop effective strategies for addressing microbial water pollution and protecting public health.Initially, a series of chlorination experiments were conducted with the aim of evaluating the effectiveness of sodium hypochlorite (NaOCl) against the two microorganisms. For each microorganism, eight repetitions of the experiment were performed, while studying the effect of two different disinfectant concentrations for each of the four different aqueous matrices. Specifically, higher concentrations of NaOCl, 5 mg/L and 10 mg/L in terms of available chlorine (Cl2), were applied to the wastewater, while lower concentrations, 0.5 mg/L and 1 mg/L Cl2, were used in the remaining aqueous matrices. The duration of the experiments was 180 minutes for A. baumannii and 45 minutes for C. albicans, indicative of the different sensitivity of the two microorganisms to chlorination conditions.Subsequently, UVC radiation from a 15W lamp was applied to disinfect the aqueous matrices (0.8% NaCl, Lake Agia, DEYAX). Each microbial sample was subjected to a triplicate experiment, lasting 20 minutes.According to the results of the study, both disinfection methods proved to be extremely effective in inactivating the microorganisms A. baumannii and C. albicans. Chlorination achieves complete inactivation of bacteria within 60-180 minutes and fungi within 4-45 minutes, depending on the concentration of the disinfectant and the aqueous matrix. In contrast, disinfection with ultraviolet radiation (UVC) leads to faster inactivation of microorganisms, with A. baumannii being inactivated within 4-10 minutes and C. albicans within 4-6 minutes depending on the aqueous matrix. However, it was observed that after about 24 hours, microbial organisms exposed to UVC radiation often exhibited repair mechanisms, as a result of which complete elimination was not always achieved. The aqueous matrix mainly affects the time required for inactivation of microorganisms and not the effectiveness of the process. Specifically, the 0.8% NaCl solution (without organic matter) resulted in faster inactivation compared to samples from a lake or wastewater, suggesting that the presence of organic matter may protect microorganisms from disinfectant effects.In summary, bacterial inactivation is significantly affected by the composition of the aqueous matrix. Experimental data show that wastewater and the pond reduce the effectiveness of disinfectants, allowing a significant percentage of pathogens to survive. In contrast, the use of a NaCl solution (0.8%), which is devoid of organic matter and other substances that may interfere with the process, leads to complete elimination of bacterial populations. The above observations highlight the critical importance of water quality in disinfection processes and point out the need for further research in order to determine the precise mechanisms governing the interactions between pathogens, disinfectants and various water components.

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