Bacterial glycocalyx integrity drives multicellular swarm biofilm dynamism

Abstract

The detection of a mass of intertwined polysaccharides surrounding bacterial cells, termed a "glycocalyx," remains a seminal discovery in bacterial physiology, giving rise to the biofilm concept for surface-attached microbial community growth within a polysaccharide matrix (Costerton et al., 1978). Within a biofilm, bacteria can physically interact, be protected from external stressors (e.g., antibiotics, reactive oxygen species, dehydration, etc.), replicate, communicate via secreted signals, and differentiate their functions (O'Toole et al., 2000;Zhang et al., 2012). While the importance of secreted polysaccharides for biofilm formation is widely appreciated, the mechanisms by which these polymers promote 3D matrix structuration and the cellular organization within are areas of intense study (Limoli et al., 2015). One such bacterium that displays robust biofilm existence is Myxococcus xanthus, a predatory Gram-negative δ-proteobacterium with a social multicellular lifecycle (Monds & O'Toole, 2009;O'Toole et al., 2000;Van Gestel et al., 2015). Groups of M. xanthus cells are encased within a secreted polysaccharide matrix, promoting intimate contacts. On surfaces, swarms of these cells

Bacterial outer-membrane polysaccharide export (OPX) proteins occupy three structural classes with selective β-barrel porin requirements for polymer secretion

“…However, as dedicated peptidoglycan-spanning polysaccharide export channels have yet to be identified in Gram-positive bacteria, any role for Class 3 OPX proteins in these systems with regards to interactions with a secretion pore of some sort would be unfounded speculation. In M. xanthus cells, WzaX/S/B Class 3 OPX protein deficiency does not lead to visible accumulations of polymeric material in the periplasm (Saïdi et al ., 2021) suggesting that EPS/MASC/BPS polymer assembly via Wzx/Wzy-dependent pathways does not indiscriminately continue in these mutant backgrounds; this is a similar observation to that for Wza Ec Class 1 OPX deficiency in E. coli cells (Nesper et al ., 2003). Such material from ABC transporter-dependent synthesis does however accumulate in the periplasm of KpsD Ec -deficient Class 2B OPX-mutant cells (Wunder et al ., 1994, Bliss & Silver, 1996).…”

“…Cells constitutively produce EPS, a specific surface-associated polymer that forms a glycocalyx surrounding the cell body (Saïdi et al, 2021) and which constitutes the main matrix component in biofilms of this bacterium (Hu et al, 2013, Smaldone et al, 2014. A biosurfactant polysaccharide (BPS) is also synthesized, but is instead secreted to the extracellular milieu (Islam et al, 2020), where it functionally destabilizes the EPS glycocalyx, leading to a range of fundamental behavioural and surface-property changes at the single-cell level (Saïdi et al, 2021). The synergy between EPS and BPS secretion as well as the spatiospecific production patterns of the two polymers (Islam et al, 2020) also impacts the internal architecture of M. xanthus swarm biofilms, as well as their Type IV pilus (T4P)-dependent expansion due to impacts on T4P production, stability, and positioning (Saïdi et al, 2021).…”

“…Capsule polysaccharide (CPS) chains are tightly bound to the cell surface and form hydrated exclusionary barriers to molecule entry, whereas exopolysaccharide (EPS) polymers form a more loosely surface-associated glycocalyx around cells that contributes to biofilm matrix formation in aggregates (Whitfield et al ., 2020). Certain HMW polysaccharides do not remain associated with the cell surface and are instead secreted to the extracellular milieu where they can influence bacterial physiology (Saïdi et al ., 2021, Islam et al ., 2020). In both monoderm (Gram-positive) and diderm (Gram-negative) bacteria, multiple secreted polymers often act in concert to modulate complex physiology (Pérez-Burgos & Søgaard-Andersen, 2020, Lavelle et al ., 2021, Franklin et al ., 2011).…”

“…This complex lifecycle is modulated by the secretion of three known polysaccharides (Pérez-Burgos & Søgaard-Andersen, 2020). Cells constitutively produce EPS, a specific surface-associated polymer that forms a glycocalyx surrounding the cell body (Saïdi et al ., 2021) and which constitutes the main matrix component in biofilms of this bacterium (Hu et al ., 2013, Smaldone et al ., 2014). A biosurfactant polysaccharide (BPS) is also synthesized, but is instead secreted to the extracellular milieu (Islam et al ., 2020), where it functionally destabilizes the EPS glycocalyx, leading to a range of fundamental behavioural and surface-property changes at the single-cell level (Saïdi et al ., 2021).…”

“…A biosurfactant polysaccharide (BPS) is also synthesized, but is instead secreted to the extracellular milieu (Islam et al ., 2020), where it functionally destabilizes the EPS glycocalyx, leading to a range of fundamental behavioural and surface-property changes at the single-cell level (Saïdi et al ., 2021). The synergy between EPS and BPS secretion as well as the spatiospecific production patterns of the two polymers (Islam et al ., 2020) also impacts the internal architecture of M. xanthus swarm biofilms, as well as their Type IV pilus (T4P)-dependent expansion due to impacts on T4P production, stability, and positioning (Saïdi et al ., 2021). Finally, the ma jor s pore c oat (MASC) polymer is produced by spore-forming cells undergoing development, providing a protective coat to cover nascent myxospores and provide resistance to environmental stresses (Holkenbrink et al ., 2014, Wartel et al ., 2013).…”