Magnesium oxide nanoparticles (MgO NPs) were obtained by the calcination of precursor microparticles (PM) synthesized by a novel triethylamine-based precipitation method. Scanning electron microscopy (SEM) revealed a mean size of 120 nm for the MgO NPs. The results of the characterizations for MgO NPs support the suggestion that our material has the capacity to attack, and have an antibacterial effect against, Gram-negative and Gram-positive bacteria strains. The ability of the MgO NPs to produce reactive oxygen species (ROS), such as superoxide anion radicals (${\mathrm{O}}_2^{•-}$) or hydrogen peroxide (H₂O₂), was demonstrated by the corresponding quantitative assays. The MgO antibacterial activity was evaluated against Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacteria, with minimum inhibitory concentrations (MICs) of 250 and 500 ppm on the microdilution assays, respectively. Structural changes in the bacteria, such as membrane collapse; surface changes, such as vesicular formation; and changes in the longitudinal and horizontal sizes, as well as the circumference, were observed using atomic force microscopy (AFM). The lipidic peroxidation of the bacterial membranes was quantified, and finally, a bactericidal mechanism for the MgO NPs was also proposed.

Electron microscopy is one of the tools required to characterize cellular structures. However, the procedure is complicated and expensive due to the sample preparation for observation. Atomic force microscopy (AFM) is a very useful characterization technique due to its high resolution in three dimensions and because of the absence of any requirement for vacuum and sample conductivity. AFM can image a wide variety of samples with different topographies and different types of materials. AFM provides high-resolution 3D topography information from the angstrom level to the micron scale. Unlike traditional microscopy, AFM uses a probe to generate an image of the surface topography of a sample. In this protocol, the use of this type of microscopy is suggested for the morphological and cell damage characterization of bacteria fixed on a support. Strains of Staphylococcus aureus (ATCC 25923), Escherichia coli (ATCC 25922), and Pseudomonas hunanensis (isolated from garlic bulb samples) were used. In this work, bacterial cells were grown in specific culture media. To observe cell damage, Staphylococcus aureus and Escherichia coli were incubated with different concentrations of nanoparticles (NPs). A drop of bacterial suspension was fixed on a glass support, and images were taken with AFM at different scales. The images obtained showed the morphological characteristics of the bacteria. Further, employing AFM, it was possible to observe the damage to the cellular structure caused by the effect of NPs. Based on the images obtained, contact AFM can be used to characterize the morphology of bacterial cells fixed on a support. AFM is also a suitable tool for the investigation of the effects of NPs on bacteria. Compared to electron microscopy, AFM is an inexpensive and easy-to-use technique.

Geobacter sulfurreducens is a model bacterium widely used in microbial fuel cell (MFC) research due to its efficient extracellular electron transfer. However, the high cost of synthetic media limits the scalability of these systems, making agro-industrial byproducts like cheese whey a sustainable alternative. This study evaluated cheese whey as a growth medium for G. sulfurreducens and its influence on biofilm development on graphite bars electrodes. Bacterial growth kinetics and biofilm architecture were characterized using Atomic Force Microscopy (AFM) as the primary quantitative tool, supplemented by Scanning Electron Microscopy (SEM). Growth curves revealed a diauxic-like transition within the first 48 h, with high cell viability (94%). AFM analysis demonstrated a non-linear topographical evolution: an initial attachment phase was followed by a peak in structural heterogeneity at 14 days (S_q = 683.08 nm), eventually reaching a mature, confluent state at 21 days with a maximum thickness of ~8 μm. Energy-Dispersive Spectroscopy (EDS) confirmed an organic and mineral matrix consistent with bacterial biomass and whey components. These results demonstrate that cheese whey effectively supports the growth of G. sulfurreducens and the formation of structurally complex biofilms, highlighting its potential as a low-cost substrate for microbial cultivation and dairy waste valorization.