Bacteria autoaggregation: how and why bacteria stick together

Abstract

Autoaggregation, adherence between identical bacterial cells, is important for colonization, kin and kind recognition, and survival of bacteria. It is directly mediated by specific interactions between proteins or organelles on the surfaces of interacting cells or indirectly by the presence of secreted macromolecules such as eDNA and exopolysaccharides. Some autoaggregation effectors are self-associating and present interesting paradigms for protein interaction. Autoaggregation can be beneficial or deleterious at specific times and niches. It is, therefore, typically regulated through transcriptional or post-transcriptional mechanisms or epigenetically by phase variation. Autoaggregation can contribute to bacterial adherence, biofilm formation or other higher-level functions. However, autoaggregation is only required for these phenotypes in some bacteria. Thus, autoaggregation should be detected, studied and measured independently using both qualitative and quantitative in vitro and ex vivo methods. If better understood, autoaggregation holds the potential for the discovery of new therapeutic targets that could be cost-effectively exploited.

Keywords: adherence; autoaggregation; biofilm; self-associating.

Figure 1.. Localized adherence of typical enteropathogenic Escherichia coli (EPEC E2348/69) on HT-29 cells is the formation of adherent autoaggregants mediated by bundle-forming pili and stabilized by other adhesins [8].

(A) The formation of adherent autoaggregants mediated by bundle-forming pili At 3 h postinfection of HT-29 cells with EPEC E2348/69) (B) this property allows for dense and efficient colonization of epithileal cells within 6 h. Saldana et al. 2009 [8].

Figure 2.. Velcro-like self-association between identical L-shaped passenger domains of adjacent Escherichia coli Ag43 molecules [26].

The interaction is held by nine hydrogen bonds [N29–T256 (two hydrogen bonds), N60–T256, N60–T237, D79–T237, N96–R200, T97–R200, T98–R200, G115–R200] and a salt bridge between the R59 and E216 side chains. Reproduced from Heras et al. 2014 [26] with permission.

Figure 3.. Self association between components of a Haemophilus influenzae multimer.

(A) Interactions among four Hap molecules shown in surface (coloured in yellow and by electrostatic surface potential) and cartoon (black and magenta) representations, respectively. (B) Slab view of the packing interface of the Hap–Hap multimer at a cross-section in the primary interaction site of D776–N777. The F1/F2/F3 faces are labelled. The F2 face and F1–F2 edge at the growing ends of the multimer are highlighted in red. Reproduced from Meng et al. 2011 [27] with permission.

Figure 4.. Visualizing autoaggregation as sedimentation.

Autoaggregation in liquid media mediated by (A) the heat-resistant agglutinin 1 gene expressed in Escherichia coli from an arabinose inducible promoter on the pBAD vector after induction with arabinose (B) vector control prepared with the same protocol. The tubes show settling patterns in broth cultures left statically for 6 h and the photomicrographs are crystal violet stained mounts of cells taken from just above the pellet. (C) Autoaggregation chain-forming Streptococcus salivarius in semi-lquid media. Chain-forming streptococci produce spherical suspended colonies (black arrow) while mutants unable to form chains produce ‘roots’ in the semi-solid medium (red arrow). Reproduced from Couvigny et al. 2018 [31] under a Creative Commons Attribution License.