LAUSR

DOI: 10.1002/admi.202001807 consequences on human health[1,2] and industrial processes[3,4] as well as antibiotics resistance.[5] Various approaches to inhibition of biofilm formation and their eradication were developed.[6] Biofilm formation at an interface is determined by several pheno mena, e.g., the initial adhesion of single bacteria to the surface and the further biofilm growth and spreading due to cell proliferation and biopolymer production.[7] Therefore, surface energy and electrostatic interactions with the substrate are two key determinants of biofilm formation. On this basis, a common strategy to fight bacterial colonization is the functionalization of surfaces that are prone to biofilm fouling.[6,8] For instance, surface hydrophilization[9] and low surface energy strategies[10] were considered. Surface topology was also modified to influence bacterial colonization, as recently reviewed in detail.[11] Polyelectrolyte coatings allow changing the surface energy, charge, and mechanical characteristics of various substrates easily.[12,13] Therefore, polyelectrolyte assemblies are often considered for possible application as antibacterial coatings. However, understanding the fundamental principles of bacteria–surface interactions remains of high importance.[14–16] Due to the negative charges found on their external membrane, bacteria tend to tightly attach to positively charged surfaces.[17] It was demonstrated that biofilm spreading rate was decreasing with increasing strength of adhesion.[17] One possible explanation is that bacteria elongation preceding cell division is obstructed by strong electrostatic binding to the surface.[18] Besides this, it is also known that positively charged molecules (and poly cations to a greater extent) exhibit antibacterial properties due to their ability to disrupt membranes of bacterial cells.[19] In contrast, negatively charged surfaces provide less stable bacterial cell contact with the surface,[18] so that the initial adhesion step is difficult but further biofilm spreading meets less obstacle. In addition to help preventing healthcare and industryrelated issues, investigating biofilm formation is beneficial to understand the development of biological tissues,[20] cell adaptability,[21] and communication[22] since some morphogenesis principles are common with higher organisms’ tissues. Furthermore, colonies and biofilms of non-pathogenic microorganisms are considered to be promising to design hybrid living materials[23–27] challenging to get synthetically. Revealing Because bacteria–surface interactions play a decisive role in bacteria adhesion and biofilm spreading, it is essential to understand how biofilms respond to surface properties to develop effective strategies to combat them. Polyelectrolyte coating is a simple and efficient way of controlling surface charge and energy. Using polyelectrolytes of various types, with different molecular weights and polyelectrolyte solutions of various pH provides a unique approach to investigate the interactions between biofilms and their substrate. Here, the formation of Escherichia coli biofilms at a solid–air interface is explored, whereby charge and interfacial energy are tuned using polyelectrolyte coatings on the surface. Cationic coatings are observed to limit biofilm spreading, which remain more confined when using high molecular weight polycations. Interestingly, biofilm surface densities are higher on polycationic surfaces despite their well-studied bactericidal properties. Furthermore, the degree of polyelectrolyte protonation also appears to have an influence on biofilm spreading on polycation-coated substrates. Finally, altering the interplay between biomass production and surface forces with polyelectrolyte coatings is shown to affect biofilm 3D architecture. Thereby, it is demonstrated that biofilm growth and spreading on a hydrogel substrate can be tuned from confined to expanded, simply by coating the surface using available polyelectrolytes.