Biopolymers are a chemically diverse group of high-molecular-weight molecules composed of repeating building blocks, such as nucleotides, amino acids, or sugars and are produced by living organisms. Polynucleotides (such as DNA or RNA), polypeptides (i.e. proteins), or polysaccharides (polymeric carbohydrates) in many cases represent essential components of a cell. However, under certain conditions, bacteria can also synthesize supplementary biopolymers that can be intra- or extracellular. These are considered to have storage or structural properties, play a role as virulence factors, or be important for biofilm formation 1, 2. For a better distinction between the two classes of biopolymers, we suggest the term ‘specialized biopolymers’ for polymeric substances that are produced conditionally and have specialized functions and we will refer to this type of polymers throughout this review. Widespread examples for specialized biopolymers include intracellular compounds such as the polysaccharide glycogen (a polymer of α-1,4-linked and α-1,6-linked glucose), polyamides such as cyanophycin (a copolymer of L-aspartic acid and L-arginine), and polyesters, such as poly-(R)-3-hydroxybutyrate (PHB) (Figure 1). Secreted polymers of this class cover extracellular DNA (eDNA) and a broad spectrum of exopolysaccharides, including, for example, cellulose (β-(1,4)-linked D-glucose) or alginate (β-(1,4)-linked mannuronic acid and guluronic acid) [3].

Several of bacterial specialized biopolymers have useful chemical and material properties and have therefore been receiving a lot of attention as innovative and biocompatible materials in industry and medicine (reviewed in Moradali et al., 2020 [2]). But, despite their high value for biotechnology, we still lack comprehensive understanding of the mechanisms and triggers of biopolymer biosynthesis and degradation in bacteria and in microbial communities. In general, specialized biopolymers were long considered to fulfill storage functions, a view that particularly applies to intracellular polymers [4]. However, recent studies demonstrate that biopolymers have diverse physiological roles that clearly go beyond ‘simple’ storage and are tightly connected to bacterial stress and signaling biology. In this review, we discuss most recent developments in the field by focusing on the association of specialized biopolymers with bacterial fitness and resilience of microbial communities and highlighting novel links between biopolymer metabolism and nucleotide signaling networks.