Capsular Polysaccharide Production in Bacteria of the Mycoplasma Genus: A Huge Diversity of Pathways and Synthases for So‐Called Minimal Bacteria

ABSTRACT Mycoplasmas are wall‐less bacteria with many species spread across various animal hosts in which they can be pathogenic. Despite their reduced anabolic capacity, some mycoplasmas are known to secrete hetero‐ and homopolysaccharides, which play a role in host colonization through biofilm formation or immune evasion, for instance. This study explores how widespread the phenomenon of capsular homopolysaccharide secretion is within mycoplasmas, and investigates the diversity of both the molecules produced and the synthase‐type glycosyltransferases responsible for their production. Fourteen strains representing 14 (sub)species from four types of hosts were tested in vitro for their polysaccharide secretion using both specific (immunodetection) and nonspecific (sugar dosage) assays. We evidenced a new, atypical homopolymer of β‐(1 → 6)‐glucofuranose (named glucofuranan) in the human pathogen Mycoplasma (M.) fermentans, as well as a β‐(1 → 6)‐glucopyranose polymer for the turkey pathogen M. iowae and galactan (β‐(1 → 6)‐galactofuranose) and β‐(1 → 2)‐glucopyranose for M. bovigenitalium infecting ruminants. Sequence and phylogenetic analyses revealed a huge diversity of synthases from varied Mycoplasma species. The clustering of these membrane‐embedded glycosyltransferases into three main groups was only partially correlated to the structure of the produced homopolysaccharides. Synthases are membrane‐embedded glycosyltransferases enabling the polymerization and secretion of homopolysaccharides that can form a capsule around mycoplasma cells. The nature of the capsular polymer is tightly linked to different structural features of the synthase. So far, at least four different types of capsular homopolymers have been evidenced in mycoplasmas, one of which—glucofuranan (β‐(1 → 6)‐glucofuranose)—has never been described in bacteria.