Most bacteria produce adhesion molecules to facilitate the interaction with host cells and establish successful infections. An important group of bacterial adhesins belong to the autotransporter (AT) superfamily, the largest group of secreted and outer membrane proteins in Gram-negative bacteria. AT adhesins possess diverse functions that facilitate bacterial colonisation, survival and persistence, and as such are often associated with increased bacterial fitness and pathogenic potential. In this review, we will describe AIDA-I type AT adhesins, which comprise the biggest and most diverse group in the AT family. We will focus on Escherichia coli proteins and define general aspects of their biogenesis, distribution, structural properties and key roles in infection.

DOI: https://doi.org/10.1002/pmic.201600431 Autotransporter proteins are the largest family of secreted proteins in Gram‐negative bacteria. As discussed in article number 1600431 by Julieanne L. Vo et al., this family includes outer membrane adhesins and secreted serine proteases, which promote virulence functions such as adhesion and invasion of host cells, biofilm formation and cytotoxicity. Autotransporters incorporate a membrane spanning beta‐barrel and an extracellular functional domain. This figure represents the architecture of a trimeric adhesin (YadA (1P9H, 3H7X, 2LME)); a beta‐helical adhesin (Antigen 43 functional domain and AIDA‐I beta‐barrel (4KH3, 4MEE)) and a serine protease autotransporter (Hemoglobin‐binding protease (1WXR, 3AEH)).

The formation of aggregates and biofilms enhances bacterial colonisation and infection progression by affording protection from antibiotics and host immune factors. Despite these advantages there is a trade-off, whereby bacterial dissemination is reduced. As such, biofilm development needs to be controlled to suit adaptation to different environments. Here we investigate members from one of largest groups of bacterial adhesins, the autotransporters, for their critical role in the assembly of bacterial aggregates and biofilms. We describe the structural and functional characterisation of autotransporter Ag43 variants from different Escherichia coli pathotypes. We show that specific interactions between amino acids on the contacting interfaces of adjacent Ag43 proteins drives a common mode of trans-association that leads to cell clumping. Furthermore, subtle variation of these interactions alters aggregation kinetics and the degree of compacting within cell clusters. Together, our structure–function investigation reveals an underlying molecular basis for variations in the density of bacterial communities.

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