So this is the penultimate post in my SGM topic series: Bacterial vesicles.
The first thing to note about them is that they only form in Gram negative bacteria, which have an outer membrane covering a small glycopeptide layer (Gram positive bacteria have no outer membrane and a very large glycopeptide layer). The top layer simple peels off into a little vesicle, taking periplasmic proteins with it, as shown below (diagram from the reference):
The mechanism for vesicle formation is largely unknown, but it is found in both pathogenic and non-pathogenic strains of bacteria, and used for several different purposes. In pathogenic bacteria the vesicles often contain virulence factors, which can destroy or damage host cells. in the wild, they may also bind to or destroy other bacteria. In less-virulent strains they have been shown to act as a method of removing misformed or unwanted proteins from the periplasmic space (the space between the two membranes). They can also play a part in antibiotic resistance, it's not yet certain how but my guess is that they pump the antibiotic into the periplasmic space then vesicle it off to stop it just diffusing back in again.
When first discovered, the vesicles were thought to be a by-product of bacterial death, after all, why else would little bits of membrane with bacterial proteins inside be found floating around a large colony? However work done on pathogenic bacteria (which get more funding) and biopsies of infected tissues showed the vesicles playing an important part in infection. They are produced during the stationary phase of growth, the same period when bacteria start to produce most of their virulence factors and secondary metabolites. In an infected organism, this phase is after the bacteria has set off an inflammation reaction, and once it has multiplied in the site of infection.
Factors that affect the formation of vesicles include oxygen stress, the availability of iron (finding a regular iron source inside human bodies is a regular problem for bacteria) and the composition of the outer membrane (suggesting that at least some of the mechanism is mechanical). It is a ubiquitous process carried out across a range of Gram negative species.
As well as being used offensively, some vesicles were also shown to carry DNA between bacteria, although it's not at all clear how, or how the DNA gets into the periplasm in the first place. P. aeruginosa are also capible of transferring antibiotic resistant enzmyes between bacterial cell using the vesicles. This is not totally an act of complete altruism, as P. aeruginosa carries out much of it's infectious cycle as a biofilm, which requires lots of cells to form.
As I said, the actually mechanism for the formation of the vesicles is not yet established, so there's probably quite a lot of work to do with imaging their formation, genetics to find out any genes involved, and a mixture of genetics and protein work to discover more about what goes inside the vesicles. It looks like an interesting area of research, with the potential for some quite amazing imagery-work, and I look forward to reading more about it.
Kuehn MJ, & Kesty NC (2005). Bacterial outer membrane vesicles and the host-pathogen interaction. Genes & development, 19 (22), 2645-55 PMID: 16291643