Toxins go viral: phage‐encoded lysis releases group B colicins

The natural world is filled with instances of competition between organisms. One extreme form of competition— allelopathy—involves the production of chemicals that harm or kill competitors. Such chemical warfare is ubiquitous in the microbial world. Nearly every bacterial lineage studied to date contains strains that produce proteinaceous antimicrobials called bacteriocins (Riley and Wertz, 2002a). The best-understood types are the colicins, produced by and active against Escherichia coli and its relatives (Cascales, 2007). For one group of these colicins (group A), allelopathy is encoded in a three-gene operon housed on a plasmid. One gene encodes the toxin (colicin), a second gene encodes an immunity protein, which binds and neutralizes the toxin, and a third gene encodes a lysis protein. Under stressful conditions, a fraction of plasmid-bearing (colicinogenic) cells turn on the operon, produce toxin, and lyse, releasing the toxin to the external environment. Cells that lack the plasmid die upon exposure to the toxin. In cells that contain the plasmid, the immunity protein, which is constitutively expressed, prevents cell death (Riley and Wertz, 2002b; Cascales et al., 2007). Thus, active producers kill sensitive competitors, allowing their latent immune clones to utilize liberated resources (Riley and Gordon, 1999; Kerr, 2007). However, there exists another group of colicin systems (group B) that has only the toxin and immunity genes and appears to lack the lysis gene (van der Wal et al., 1995; Cascales et al., 2007). It is a mystery how these colicins get out of their host cells. In an exciting new study that appears in this issue, Nedialkova et al. (2016) show that prophages may offer a route of escape for the group B colicin ColIb. Prophages are bacterial viruses that have been incorporated into the bacterium’s genome (bacteria carrying prophages are called lysogens) (Campbell, 2003). Prophages are common in bacteria, comprising up to 20% of bacterial genomes (Bondy-Denomy and Davidson, 2014). Similar to colicin plasmids, prophages can be vertically transmitted within bacterial lineages without induction. However, under certain circumstances, the prophage can be induced, resulting in the production of progeny phage and lysis of the host cell. Interestingly, some of the same conditions (those activating the SOS response) that induce prophages also induce colicins. In this way, prophage systems may offer an escape route for group B colicins. Nedialkova et al. demonstrate the plausibility of this scenario through both additive and subtractive engineering. Specifically, the authors transformed the ColIb plasmid into a prophage-free E. coli strain. This transformant produced colicins, but these toxins were not released from the producing cell. The authors then transduced the ColIb E. coli with a temperate phage, 933W, and observed the release of the colicins. The focal bacterium of this study, Salmonella enterica serovar typhimurium, contains four prophages and the ColIb plasmid. With the prophages intact, colicin is released. However, when one of the phages, ST64B, is deleted, the release of colicin is dramatically reduced. For both E. coli and S. enterica, the lysis proteins of the phage appear to be critical for colicin release. Transduction with a lysis-deficient 933W phage is insufficient for colicin release from E. coli. Similarly, the loss of the lysis protein of ST64B can reduce colicin release from S. enterica. Using reporter assays, the authors additionally show that the timing of toxin production precedes the timing of phage-encoded lysis. Collectively, their experiments provide evidence that phage systems can complement the release deficiency of group B colicins. Further, the authors demonstrate that phage-mediated release is imperative for successful allelopathic effect in a community context: colicinogenic cells harbouring the prophage dramatically outcompete colicin-sensitive strains in co-cultures. However, colicinogenic cells without the prophage (or lacking phage lysis genes) have reduced Received 21 January, 2016; accepted 22 January, 2016. *For correspondence. E-mail: kerrb@u.washington.edu; Tel. 206-221-3996.