Trimeric autotransporter adhesins (TAAs) are a family of surface proteins from Gram-negative bacteria. As their name implies, these proteins mediate binding of the bacteria to a variety of surfaces, including host cells and matrix components, abiotic surfaces, and other bacteria. This latter ability is called autoaggregation, which is a widespread phenomenon among bacteria and confers protection against environmental threats. The autoaggregation mediated by TAAs is homotypic, i.e. the TAAs bind to themselves.
In a recent paper, Jack C. Leo and colleagues have investigated whether TAAs can also mediate heterotypic binding to other types of TAAs. This would lead to co-aggregation of bacteria producing different TAAs. To do this, Leo and colleagues genetically engineered laboratory Escherichia coli to produce a specific type of TAA as well as a fluorescent marker protein, either red or green. Using microscopy and software prepared for this study, the researchers quantified the interactions between two populations of bacteria producing the same (autoaggregation) or different TAAs (co-aggregation).
The results show that different TAAs can mediate co-aggregation, and generally the degree of co-aggregation correlated with the sequence similarity between the interacting TAAs. However, in some cases, two TAAs excluded each other, and aggregates of only one type of bacteria were formed. These findings have implications for the ecology of bacteria: co-aggregation is often a sign of co-operation between bacteria, whereas exclusion might indicate competition.
Khalil HS, Øgaard J, Leo JC. Coaggregation properties of trimeric autotransporter adhesins. MicrobiologyOpen. 2020;doi:10.1002/mbo3.1109.