Conformation Change in a Self-recognizing Autotransporter Modulates Bacterial Cell-Cell Interaction

Abstract

Bacteria mostly live as multicellular communities, although they are unicellular organisms, yet the mechanisms that tie individual bacteria together are often poorly understood. The adhesin involved in diffuse adherence (AIDA-I) is an adhesin of diarrheagenic Escherichia coli strains. AIDA-I also mediates bacterial auto-aggregation and biofilm formation and thus could be important for the organization of communities of pathogens. Using purified protein and whole bacteria, we provide direct evidence that AIDA-I promotes auto-aggregation by interacting with itself. Using various biophysical and biochemical techniques, we observed a conformational change in the protein during AIDA-AIDA interactions, strengthening the notion that this is a highly specific interaction. The self-association of AIDA-I is of high affinity but can be modulated by sodium chloride. We observe that a bile salt, sodium deoxycholate, also prevents AIDA-I oligomerization and bacterial auto-aggregation. Thus, we propose that AIDA-I, and most likely other similar autotransporters such as antigen 43 (Ag43) and TibA, organize bacterial communities of pathogens through a self-recognition mechanism that is sensitive to the environment. This could permit bacteria to switch between multicellular and unicellular lifestyles to complete their infection.Many Gram-positive and Gram-negative bacterial species, including Escherichia coli (1), Bordetella pertussis (2), Staphylococcus aureus (3), or Streptococcus pyogenes (4), possess the ability to auto-aggregate. Auto-aggregation can be visualized microscopically by the clumping of bacterial cells and, in some cases, macroscopically by the settling of static cultures. It is thought that bacterial aggregates can enhance colonization, participate in biofilm formation, and confer resistance to host defenses (4 -7). In addition, cell-cell aggregation is a characteristic of many multicellular behaviors such as swarming (8). Auto-aggregation is usually conferred by surface-exposed adhesins like curli (9), pili/fimbriae (1, 10, 11), or autotransporters (12-14). How bacteria achieve and control their multicellular lifestyle has recently become a major research area. With a few notable exceptions (15), however, little is known on the mechanisms of action of the molecules that promote tight association between identical bacterial cells. A recently described family of E. coli outer membrane proteins called self-associating autotransporters (SAAT) 4 provides a desirable model system to study these interactions (6).SAAT are involved in adhesion to epithelial cells, biofilm formation, and bacterial auto-aggregation (6). To date, this family comprises three proteins: the adhesin involved in diffuse adherence (AIDA-I) (16, 17), the auto-aggregation factor Ag43 (18, 19), and the TibA adhesin/invasin (20, 21). AIDA-I was originally discovered in an E. coli strain isolated from a case of infantile diarrhea (16). Since then it has been associated with a high percentage of pathogenic strains of E. coli involved in neonatal and...

New Cell Surface Protein Involved in Biofilm Formation by Streptococcus parasanguinis

“…The intercellular TibA-TibA interaction is also responsible for bacterial autoaggregation and biofilm formation (40). AIDA-I (adhesin involved in diffuse adherence 1) and other similar autotransporter adhesins also utilize a selfrecognition mechanism to form biofilms which may allow bacteria to switch from free-living to sessile biofilm lifestyles in their native ecological niches (14,40). All these observations are consistent with our hypothesis that the BapA1-BapA1 interaction contributes to bacterial autoaggregation and biofilm formation.…”

“…Overnight cultures of different streptococcal strains were washed with PBS once, adjusted to the same optical density (OD 470 ϭ 1.9) in test tubes, and kept at 4°C for up to 30 h. The top half of the culture suspensions was sampled, the sample was transferred to a fresh test tube, and the OD value was measured at 470 nm for each sample (14). The experiments were repeated 3 times, and the data were analyzed using the TTEST program.…”

Structure-Function Analysis of the TibA Self-Associating Autotransporter Reveals a Modular Organization

“…All mutants affected in these functionalities were also affected in either autoaggregation or adhesion. Similar observations were made in previous studies with AIDA-I (8,14) and Ag43 (21,35). This suggests that adhesion and autoaggregation are the main functions of SAATs, whereas biofilm formation and invasion are secondary to these two functions.…”

ThetibAdherence Locus of EnterotoxigenicEscherichia coliIs Regulated by Cyclic AMP Receptor Protein

“…This low-salt medium was used for aggregation assays because we have observed that TibA-dependent aggregation, but not necessarily TibA expression, is progressively inhibited by increasing concentrations of NaCl. High salt concentrations have also been reported to disrupt self-interactions of AIDA-I, another autoaggregating adhesin of diarrheagenic E. coli (34). For qualitative comparisons, representative cultures of four or more independent replicates were photographed immediately after vigorous mixing and 3 h after standing at room temperature without agitation.…”