A very special bacterium

 A very special bacterium

 Bacteria are unicellular procaryotic organisms. They are very small in size, too small to be seen with the naked eye, and have a very simple structure. In general the word “bacteria” is evocative of diseases and dangers. The fact that not all bacteria are dangerous and that, on the contrary, we could not leave without bacteria will be the subject of  an additional post. Here I want only to introduce a very special bacterium we discovered in our laboratory at Pisa University.

Like the majority of bacteria it is small (oval in shape 2 µm, that is two thousandths of millimiter in length and 1 µm in width) but possesses a complex structure and is far for being dangerous! We called this bacterium “Epixenosome”  (from the ancient Greek external alien body) because it lives on the dorsal surface of ciliated protozoa of Euplotidium genus.

Why epixenosomes are so special?

First of all because, as you can see in the picture, their structure is more complicated than that of the majority of known bacteria. Some characteristcs are more eukaryotic than prokaryotic and a  functional compartmentalization corresponds to the structural complexity,

Longitudinal section of epixenosomes (transmission electron microscope)

  Their DNA is localized in the upper region, is bound to basic proteins and,

at the electron microscope, it assumes a chromatin-like appearance. Moreover in the cytoplasm there are bundles of tubules in all  likelihood consisting of tubulin (a protein that up to now was considered only eukaryotic).  But the more astonishing structure is, in my opinion, the extrusive apparatus.

 The extrusive apparatus is a very well engineered structure.  It consists of a ribbon rolled up around a central core. In resting position it is compact, with shape and size well adapted to that of the intact epixenosome. The detection of external signals through membrane receptors starts up the extrusive process.

During the ejection  the ribbon unrolls from the inside by the slipping of the layers one into the other (like when we make a nose with streamers). Thus a tube forms which passes through the opening of the cell membrane (the first step of the ejecting process), and takes away the apical portion of the epixenosome containing  the genetic material.  At the end of the process the tube is 40 mm long,  that is 20 times the length of the organism, with a head 2mm long. The wall of the tube consists of the layers that are rolled up in the resting state but which are now extended, one after the other, with oblique overlapping thus ensuring the continuity of the tube.  At present we do not know the meaning of the ejection, which causes the dispersion of the epixenosomal DNA, for the bacterium itself.

 The tube at the end of the ejection

Euplotidium with some ejected epixenosomes

We know however that the ejection of epixenosomes is useful for the ciliate host. Indeed we experimentally demonstrated that predator ciliates do not eat Euplotidium with epixenosomes while easily eat those without. Note that Euplodidium without epixenosomes have never been found in nature: they can only been obtained in the laboratory. This means that  the presence of epixenosomes and their ejection is not vital for the host but represents an efficient defensive tool in nature. 
Other ciliates are able to escape predators by means of their own extrusomes:  ciliates of Euplotidium genus grow epixenosomes on their surface for their defence. For this reason in an american blog they have been compared to James Bond, the 007 agent able to invent different special weapons!