Nice or nasty: Protein translocation between bacteria and the different forms of response.

Bacterial cells are now understood to live in close-knit communities where they can experience complete interdependence, as well as fierce competition with their neighbors. In extreme cases, cells can commit suicide for the benefit of the surrounding bacteria (1). In contrast, cells have also evolved a complex array of different ways to kill one another (2). Evolutionary theory can successfully make predictions about when cells should be nice to their neighbors and when they should be nasty. A key prediction is that cooperative behavior is more strongly favored when it is directed at cells that share genes in common, and that antagonistic behavior should be directed toward nonrelatives. An article in PNAS (3), however, shows how the same behavior can be nice or nasty, depending on the identity of the cell toward which it is targeted. Garcia et al. (3) study a mechanism of protein translocation between bacteria known to result in growth inhibition (Fig. 1A) (4). Two bacteria need to be in direct contact for one bacterium to deliver a protein to its neighbor. Upon protein uptake by the recipient bacterium, the protein acts toxic and inhibits its growth. Bacteria that make the toxic protein themselves are immune and not inhibited in growth even if they receive the toxic protein from a neighboring bacterium (Fig. 1B). The advantage of toxin translocation results from targeting bacteria sensitive to the toxin and inhibiting their growth. Translocation of toxin to immune bacteria might seem like an inefficient mechanism and even a waste of toxin: that is, unless immune bacteria respond to the receipt of a toxic protein in a way that is beneficial for the community, such as by the formation of biofilms. Contact-dependent protein translocation is known to also play a crucial role in the ability of microbial communities to form biofilms. Biofilms are composed of extracellular polysaccharides, nucleic acids, proteins, and lipids (5). Bacteria embedded in biofilms benefit from adhesion, resistance to antimicrobial compounds, and better access to nutrients (5). In their study, Garcia et al. (3) describe the formation of bacterial aggregates and the production of extracellular polymers in dependency on the translocation of catalytically active toxin between immune bacteria of the species Burkholderia thailandensis. These observations are in support of previous findings describing the requirement of toxin translocation for the formation of biofilms (6, 7). The requirement of the contact-dependent protein delivery system for the formation of biofilms has also been shown in Escherichia coli (8), although by a different mechanism than that described by Garcia et al. (3). B A