**1. Introduction**

The widespread use of antibiotics has promoted the dissemination of multidrug resistance (MDR) genes in bacteria, whereas nanoparticles (NPs) have proven to be effective when dealing with this bacteria type [1]. Superbugs (i.e., bacteria which are resistant to nearly all antibiotics) have evolved due to the abuse, misuse, and negligence of antibiotics. They are capable of transferring their resistance by mechanisms as plasmid [2], transposon [3], and integrons [4]. This means that just one superbug can render an infection nearly untreatable. Antibiotics target mainly three areas inside bacteria: the cell wall, the translational machinery, and the DNA replication system. Superbugs can develop genes capable of resisting the effect of

antibiotics, ranging from the expression of enzymes and modification of cell components all the way to the creation of efflux pumps [5]. For example, enzymes such as β-lactamase [6] could alter the chemical structure of the antibiotics, leaving them inactive. Meanwhile, cell components such as ribosomes [7] offer resistance and efflux pump work by removing antibiotics from the inside of a cell and maintaining a safe environment for which the bacteria can grow normally.

Furthermore, NPs work differently when it comes to dealing with bacteria; this makes any antibiotic resistance ineffective against NPs. The NP antimicrobial mechanism falls into three categories: oxidative stress induction [8], metal ion release [9], and nonoxidative [10]. These mechanisms all work toward producing bacterial cell death.

Nanoparticles, as nanostructured materials, can be defined as three-dimensional materials with dimensions in the nanoscale (1–100 nm). As many studies suggest, they offer an array of antibacterial properties against Gram-negative and Grampositive bacteria, such as *E. coli* and *S. aureus*, respectively. For example, zinc oxide (ZnO) NPs are capable of inhibiting the growth of *S. aureus*, while silver NPs demonstrate antimicrobial activity against *E. coli* [11].

As mentioned above, there are three main mechanisms by which NPs produce apoptosis. One can summarize the entire process as follows: first, the disruption of bacterial cell membrane occurs, followed by (whenever possible) the production of reactive oxygen species (ROS), which induces the penetration of the bacterial cell to disrupt the intracellular mechanism needed by the bacteria to function properly. NPs can produce ROS by different mechanisms, but, in the scientific community, the photocatalytic hypothesis is most widely accepted.

MDR genes have proliferated to such an extent that they become a challenge for the existing resources to counteract them. Accordingly, investigations related to antibacterial materials have become mandatory to protect human lives.

Prior research demonstrated that separately chitosan and TiO2 (anatase polymorph) possess antibacterial properties under ultraviolet light irradiation. This characteristic can clean water sources, which represents one of the main vectors used by bacteria to infect the human body.

Water has become a precious resource, causing the access to fresh and clean water to become a critical matter at a global scale. Nowadays, innovative methods must offer clean, fresh, and purified water. As mentioned above, in this context, chitosan and TiO2 biocomposites could be an economically viable alternative to resolve this problem by removing oils, heavy metals, and pathogen colonies.
