Abstract
Biocides are widely used in water treatment for microbiological control. The rise of antimicrobial resistance and the need to assure properly managed water systems require a better understanding of the mechanisms of action of biocides and of their impact on cell’s viability as a function of dosage concentrations. The present work addresses these two aspects regarding the biocides benzalkonium chloride (BAC) and dibromonitrilopropionamide (DBNPA)—two biocides commonly found in the water treatment industry. For that, the following parameters were studied: culturability, membrane integrity, metabolic activity, cellular energy, and the structure and morphology of cells. Also, to assess cell’s death, a reliable positive control, consisting of cells killed by autoclave (dead cells), was introduced. The results confirmed that BAC is a lytic biocide and DBNPA a moderate electrophilic one. Furthermore, the comparison between cells exposed to the biocides’ minimum bactericidal concentrations (MBCs) and autoclaved cells revealed that other viability parameters should be taken into consideration as “death indicators.” The present work also shows that only for the concentrations above the MBC the viability indicators reached values statistically similar to the ones observed for the autoclaved cells (considered to be definitively dead). Finally, the importance of considering the biocide mechanism of action in the definition of the viability parameter to use in the viable but non-culturable (VBNC) determination is discussed.
Highlights
Biocides have been used in a wide range of applications, such as: water treatment, domestic environment, food preservation, healthcare sanitation, textile, and other industries to control and kill bacteria (Kahrilas et al, 2015; Jones and Joshi, 2021)
The minimum bactericidal concentrations (MBCs) corresponds to the lowest concentration of biocide that is able to kill 99.9% of the initial bacterial cells (Parvekar et al, 2020)
Fazlara and Ekhtelat (2012) obtained an MBC of 80 mg/L for Pseudomonas aeruginosa, which is lower than the value obtained in the present study; their bacterial concentration was lower (106 colony forming units (CFU)/mL)
Summary
Biocides have been used in a wide range of applications, such as: water treatment, domestic environment, food preservation, healthcare sanitation, textile, and other industries to control and kill bacteria (Kahrilas et al, 2015; Jones and Joshi, 2021). The same biocide can be used for several applications, sometimes by changing only the employed concentration. The final result can be the same (cell death), the mechanisms by which bacteria reach that state can be completely different. Many mechanisms of antimicrobial action were described and most times the antimicrobial action of a biocide is a combination of several mechanisms (Liwa and Jaka, 2015). Biocides can: (i) interfere with the replication of genetic information (nucleic acids); (ii) interfere with protein synthesis; (iii) alter structure and function of cell wall; (iv) increase permeability and disrupt the cytoplasmatic membrane; and (v) inhibit intermediate metabolic pathways (Liwa and Jaka, 2015)
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