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Author: Admin | 2025-04-27
And the katE and katG genes involved in the subsequent transformation of hydrogen peroxide into oxygen [71] (Figure 2). 6. Defense Mechanisms of Biofilms against Metal and Metal Oxide Nanoparticles.In general, biofilms are a set of diverse bacteria within a matrix of extracellular polymers (ECP). The bacterial diversity presented in them results in the union of the various resistance mechanisms present in the different bacterial species to assemble a protective response against nanoparticles. Biofilms have already proven to be resistant to prolonged exposures to nanoparticles [72]. The very presence of the ECP serves as a barrier to the passage of the nanoparticles, avoiding their direct contact with the bacteria inside the biofilm [73]. The ECP of the biofilms is formed by extracellular DNA, lipids, proteins, and polysaccharides. These components interact with the nanoparticles by modifying their properties like surface charge, particle size, shape, and concentration. These modifications affect the antibacterial activity of the nanoparticles in contact with the biofilms [74], for example, electrostatic attraction between the negatively charged carboxyl groups in biofilms, and the positively charged nanoparticles like Ag NPs, ZnO NPs and SiO2 NPs [47,75]. Therefore, biofilms support higher concentrations of nanoparticles compared to planktonic cells. For example, Choi et al. reported the minimum bactericidal concentration of Ag NPs of 21 and 15 nm in planktonic bacteria and biofilms. The minimum bactericidal concentration was higher in the biofilm (38 mg/L) than in the planktonic bacteria (10 mg/L) [76]. The ECP of the biofilm not only modifies the nanoparticles but also has the property of binding them, preventing them from penetrating and making contact with the bacteria [47]. Peulen et al. reported that biofilms have a pore size (10–50 nm) that allows them to retain nanoparticles with sizes greater than 10 nm [76]. While, Jing et al. demonstrated that cerium oxide nanoparticles (CeO2 NPs) were trapped in the surface of Pseudomonas fluorescens and Mycobacterium smegmatis biofilms, while bacterial cells in the deepest part of the biofilms were not affected [77]. The degree of maturity of the biofilm also seems to influence the penetration capacity of the nanoparticles. The more mature
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