## 1 Introduction  
Biofilms are bacterial communities that colonies different surfaces, both abiotic and biotic [1; 2; 3]. They produce extracellular polymeric substances (EPS) to adhere to interfaces and to create a chemically and physically protective barrier. The EPS, also referred to as the biofilm's matrix, protects bacteria from desiccation and chemical agents (antispetic, antibiotics) and it controls water retention [1; 4]. The matrix is mainly composed of different biopolymers, proteins and lipids [4; 5; 6]. It contributes to the sorption and storage of nutrients and it is the place where many extracellular enzymatic reactions take place. Moreover, it keeps the microorganisms in tight contact with each other to facilitate quorum sensing (QS), and it contains genetic information as well, e.g., the extracellular DNA [5; 7]. Pathogenic bacteria organised in biofilms are considered to be a major health concern [8; 9; 10; 11; 12]. Bacteria living in a biofilm are much more resistant to antibiotics than their planktonic form, leading to persistent infections [13; 14]. They have been found in hospital settings, for instance on medical devices or implants (valves, stents, catheters), leading to infections and death [15]. Within the food chain and industry, they are responsible for contamination and cross-contamination of food products and therefore lead to outbreaks of foodborne diseases [11]. Moreover, their ubiquity on common surfaces in, e.g., kitchens (kitchen worktops, refrigerators), bathrooms (sinks, toilets) and offices (desks, keyboards), requires devising new disinfection strategies. The study of biofilms is becoming more and more multidisciplinary. To define the different aspects of biofilms (structure, morphology, genetics), several methods/techniques are available in the literature. Even though many complex techniques are currently available to study biofilms (e.g., polymerase chain reaction, chromatography and advanced microscopy), the conventional techniques, e.g., viable cell counting and crystal violet (CV) assay, are still most commonly used. Viable cell counting is a microbiological technique used to quantify the number of colony-forming units (CFU) within the biofilm, while the CV assay is used to quantify the biomass using a dye that stains both the EPS and cells [16]. These techniques are easy-to-use and rapid, and they do not require expensive equipment, they are accessible in every microbiological lab. Since recent decades have witnessed a huge growth in the number of publications concerning biofilms, having a reproducible method to grow biofilms is of the utmost importance. The current methods for growing biofilms are based on the usage of CDC reactors [12], flow cell systems [17], microwell plates [11; 12], tubes and petri dishes [18]. CDC reactors and flow cells, such as microfluidic devices, are systems in which the nutrients and bacteria are always replenished through a feed flow; these devices allow the control of the feed rate, the pH and the concentration of nutrients. In contrast, biofilms grown in microwell plates, tubes, or petri dishes are batch systems. In the latter, a certain amount of nutrients and initial bacterial population are provided and only controlled at the beginning of the experiments. Thus, the medium in which the biofilms develop naturally undergoes changes in pH and nutrient concentration. All methods for growing biofilms have advantages and drawbacks, but the choice of a method also depends on the natural biofilm growth conditions that are mimicked. The existing methods are characterised by different static or hydrodynamic conditions influencing the conditioning, attachment, colonization and detachment of the biofilm [19]. For instance, flow cells can mimic the condition of a piping system, while a tube can be representative of a temporary storage tanks in industries [20; 21; 22]. Literature is lacking biofilm growth protocols that are specifically designed to mimic growth on flat horizontal surfaces.