I was reading last week’s Nature and came across a fascinating article about the workings of the Type VI secretion system (T6SS) of Vibrio cholerae . This is a protein rod, related to a phage tail, that the bacterium assembles within itself and then uses to stab other bacteria or eukaryotic cells, often with lethal effect.
Looking at the pictures, I was suddenly reminded of Ensifer adhaerens, the totally neglected bacterium that L E Casida described back in 1982 . Casida describes the appearance of cells of Strain A, the type strain, after staining with uranyl acetate:
With this stain, darkly stained material was often observed as a bar along the side of the cell. This bar became wider near one or both poles of the cell. When cells of strain A attached to host cells, such as cells of M. luteus, this bar often extended between the strain A cell and the M. luteus cell.
Eventually, many strain A cells became bound to Micrococcus luteus cells, leading to the death and lysis of the M. luteus.
It certainly sounds like a T6SS, but can this secretion system be found in the Rhizobiaceae? Well, it turns out that T6SS are widely distributed in proteobacteria and, mirabile dictu, the very first example of a T6SS was described by Herman Spaink in Rhizobium leguminosarum .
Ensifer is named “the sword-bearer”, and it seems entirely possible that it owes its romantic name to an accessory gene system that it shares with other Rhizobiaceae. Other Rhizobiaceae? Well, yes, because unknown to Casida, who could only place it “in part 4, (budding and/ or appendaged bacteria) of division II (the bacteria) in Bergey’s Manual “, we now know that Ensifer is a close relative of Sinorhizobium. But that is a different story, and rather a long one that I have been meaning to tell.
I am sorry for the long silence, but will try to take up that story now that my teaching duties are nearly over for this term.
 Basler, M., Pilhofer, M., Henderson, G.P., Jensen, G.J., and Mekalanos, J.J. (2012) Type VI secretion requires a dynamic contractile phage tail-like structure. Nature 483: 182-186. DOI: 10.1038/nature10846
 Casida, L.E. (1982) Ensifer adhaerens gen. nov., sp. nov.: a bacterial predator of bacteria in soil International Journal of Systematic Bacteriology 32: 339-345. DOI: 10.1099/00207713-32-3-339
 Bladergroen, M.R., Badelt, K., and Spaink, H.P. (2003) Infection-blocking genes of a symbiotic Rhizobium leguminosarum strain that are involved in temperature-dependent protein secretion. Molecular Plant-Microbe Interactions 16: 53-64. DOI: 10.1094/mpmi.2003.16.1.53