## 1 Introduction  
Our understanding of the growth of bacteria in the environment has changed over the years. It is now known that biofilm formation is the preferred lifestyle for most bacterial species, and approximately 40-80% of the bacterial cells on Earth can form these communities [1]. Biofilm could be defined as a structured community of microbes that adhere to each other and/or to the abiotic surface, and that is often embedded in a self-produced matrix of extracellular polymeric material [2]. It is believed that biofilms develop on all surfaces when in contact with nonsertile water and appear to be self-sustaining once formed. Due to serious medical and economic risks, biofilm formation is a problem in different sectors, ranging from public health to the food industry [1]. Stainless steel device surfaces (furniture, sensor probes, washing sinks, hand washing sinks, waste trolleys, instruments, etc.) are supposed to be the main sites for microbial adhesion and biofilm formation [3]. It is also common for a bacterial biofilm to form on glass surfaces since glass products are used in many areas of human life. On a daily basis, we can have contact with glass table tops, glass cabinets and walls, and glass displays (e.g., mobile phones). The structure of the biofilm allows bacteria to resist many types of environmental stress, including UV radiation, lack of nutrients, and the presence of antimicrobial agents [4]. Previously, it was found that biofilm bacteria were 10-1000 times more resistant to chemical treatments than planktonic forms [5; 6; 7]. A simple strategy to effectively remove biofilms from a variety of surfaces may appear to be to increase the concentration of the antimicrobial agent significantly. It was previously shown that this approach was not only costly but also environmentally unfriendly and did not guarantee the elimination of pathogens [8]. Therefore, the elimination of biofilms can be achieved through the combined use of treatments characterized by different mechanisms of action. For example, in the food industry, cleaning pipelines and production equipment is performed using a cleaning-in-place (CIP) technique. This method is based on the sequential use of sodium hydroxide and nitric acid and combining this protocol with disinfectants. Thus, removing biofilms from a variety of surfaces remains difficult and can be costly. Consequently, the development of new biofilm elimination protocols (Figure 1) has become the main area of research in this field [9; 10; 11; 12]. One technique that appears to be a promising procedure to control biofilm growth effectively is antimicrobial photodynamic inactivation (aPDI), which is based on the production of reactive oxygen species using a light-activated photosensitizer (PS). It is considered that the antibiofilm activity involves two steps [13]. In the first step, the photosensitizer binds to the biofilm matrix and can be fully sequestered by EPS, while in other biofilms, PS may partially pass through the EPS and contact bacterial cells. Some photosensitizers bind to the cell surface, while other types of PS can pass through the cytoplasmic membrane and get into the cytoplasm. After irradiation of the photosensitizer, regardless of its location, ROS are generated to initiate the second step, i.e., multitarget oxidative damage. ROS attacks a variety of molecules, including targets in the biofilm matrix (e.g., polysaccharides), on the cell surface (e.g., lipids), and inside cells (e.g., proteins and DNA), destroying the structure of the biofilm matrix and killing pathogenic cells [14]. It should be emphasized that many cellular components (proteins, fatty acids, DNA, etc.) are the target of aPDT-induced oxidative stress after light activation of the photosensitizer; hence, the lack of sensitivity of microorganisms to aPDT (as it is observed in the case of bacterial resistance to antibiotics) is unlikely. Figure 1: The strategies for controlling biofilm formation.