Discovery and Therapeutic Targeting of Differentiated Biofilm Subpopulations

Abstract

The association of microorganisms into biofilms produces functionally organized microbial structures that promote community survival in a wide range of environments. Much like when individual cells within a multicellular organism express different genes from the same DNA blueprint, individual microbial cells located within different regions of a biofilm structure can exhibit distinct genetic programs. These spatially defined regions of physiologically differentiated cells are reminiscent of the role of tissues in multicellular organisms, with specific subpopulations in the microbial community serving defined roles to promote the overall health of the biofilm. The functions of these subpopulations are quite diverse and can range from dormant cells that can withstand antibiotic onslaughts to cells actively producing extracellular polymeric substances providing integrity to the entire community. The purpose of this review is to discuss the diverse roles of subpopulations in the stability and function of clonal biofilms, the methods for studying these subpopulations, and the ways these subpopulations can potentially be exploited for therapeutic intervention.

Keywords: antibiotic resistance; biofilm; heterogeneity; infectious disease; subpopulations; technology; therapeutic targeting.

FIGURE 1

Schematic representation of subpopulation development in a biofilm. The transition from a planktonic lifestyle to a surface-adhered biofilm lifestyle involves a number of factors. Stochastic events such as spontaneous mutation, external stimuli such as host immune factors and antibiotic treatment, intercellular interactions, and internal diffusion of oxygen, nutrient, and various stressors play a major role in influencing the formation of microenvironments and the physiology of the biofilm, resulting in the formation of distinct subpopulations within the biofilm strata (depicted as different colors within the biomass).

FIGURE 2

Different techniques used for detailed characterization of distinct biofilm components. Biofilm is a complex community encompassing different subpopulations (depicted in shades of purple and green) and therefore multiple techniques have been employed to characterize the different aspects of biofilm including its formation, development, and heterogeneity. Different approaches are used for evaluating particular aspects of biofilm such as surface and/or structural dynamics of biofilms (e.g., surface architecture, EPS composition, or diffusion rates), the metabolic heterogeneity within the entire population, or in-depth physiological profiling of select cells and/or subpopulations within the biomass.

FIGURE 3

Strategies for targeting distinct subpopulations in a biofilm. (A) Traditional therapeutics are known to target active subpopulations but not dormant ones. (B) The metabolically dormant subpopulations that reside in the center can be reactivated to restore antibiotic susceptibility. (C) The subpopulations producing shared resources could result in the collapse of the community. (D) A combinatorial therapeutic approach could be used that could target both the active and dormant subpopulations. The populations potentially targeted by each strategy have been highlighted in red shading. Additionally, it is unknown whether or not uncharacterized hypothetical subpopulations would be targeted by these strategies and have therefore been marked with a question mark.