The architecture and behavioral phenotype of microbial cells housed in citadel-like biofilms

Understanding how biofilms came to be recognized as spatially organized, three-dimensional (3D) communities (rather than isolated planktonic cells) requires tracing the conceptual shift that redefined bacterial life as architecturally structured. Biofilms, defined as surface-attached communities of microbial cells, represent the dominant mode of life for microorganisms, creating favorable microenvironments for their constituent cells that promote antimicrobial tolerance and persistence, ultimately contributing to the emergence of antimicrobial resistance (AMR). This review examines how the structural organization and phenotypic plasticity of biofilms generate preconditions that enable antimicrobial tolerance in biofilm-incorporated cells, thereby contributing to the amplification of AMR. To integrate these dimensions, this review proposes a framework comprising five interconnected and overlapping categories of biofilm resilience mechanisms: physical, chemical, physiological, social, and genetic.Physical resilience arises from the 3D organization of the extracellular matrix, which creates a protective physico-chemical environment that limits the penetration of antimicrobials and immune clearance. Properties such as matrix viscoelasticity further enable biofilms to withstand and recover from mechanical stress. Chemical resilience is mediated by degradative enzymes that inactivate antimicrobial compounds, as well as by efflux pumps and oxidative-stress neutralization.Physiological resilience arises from community heterogeneity and metabolic latency, including dormant or persister cells. Social resilience involves metabolic cooperation, cross-feeding, cross-protection, and collective coordination through quorum sensing (QS).Genetic resilience involves adaptive gene-expression changes, stress-associated hypermutability, enhanced acquisition of antimicrobial resistance genes (ARGs) through horizontal gene transfer (HGT), which, together with biofilm-associated tolerance, create conditions that facilitate the emergence and dissemination of AMR. Collectively, these features support the conceptualization of biofilms as organized microbial systems, or “citadels”, capable of collective defense and long-term persistence. In this context, biofilms have emerged as programmed survival systems in which architecture drives selection, persistence, and resistance evolution. Key questions remain regarding how biofilm microenvironments modulate the balance between reversible antimicrobial tolerance and stable genetic resistance.