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Date: Fri, 15 Apr 2011 12:08:34 +0000

Subject: Small Things Considered

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Small Things Considered

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Microbial Embraces

Posted: 14 Apr 2011 10:00 AM PDT

by Elio

A Schistosoma mansoni pair, with the

thin female located in the male's so-called

gynaecophorical canal. Source.

There are embraces and there are embraces. Some may last for a lifetime, as was

thought to be the case with some schistosomes (though it turns out that a few

pairs do get a divorce). Other contacts, as with humans, may become quite

intimate and lead to the transfer of biological material. In the bacterial

world, embracing has been thought to be limited to conjugation, an iffy process

that can be interrupted by shaking. But herein lies the news.

Recently, a group of researchers reported that Borrelia, the agents of Lyme

disease and recurrent fever, undergo a very intimate sort of embrace. They

reported seeing Borrelia cells associating, sometimes for seconds or even

minutes, before again dissociating. Details of the process  as observed by

using cryotomographic electron microscopy revealed something novel:  the fusion

of the outer membranes of these cells (being Gram negative, they have an inner

and an outer membrane). Since the fused regions span several micrometers, the

two partners come to share a common periplasm. The flagella, which in

spirochetes are bundled and lie within the periplasmic space, here fused to make

a single bundle. And in one striking example (their words), the cytoplasms of

the two cells were also seen to have fused together.

Cryogenic electron tomography of closely apposed

cells of Borrelia garini. Blue arrowheads point to

the outer membrane, cyan arrowheads to the outer

surface layer, and magenta to the cytoplasmic

membrane. Source.

What’s this all about? Is this a form of conjugation, akin perhaps to the

fusion of gametes in eukaryotes? Care is needed. As the authors point out, it

remains to be seen if the fused bacteria reported here originate from two

separate bacteria undergoing close apposition or from bacteria that underwent

cell division but in which one of the daughter cells then changed its direction

of movement to push the not-yet-separated cytoplasmic cylinders towards each

other.

Reconstruction of a tomogram of two

closely apposed B. burgdorferi. Bright

and dark magenta correspond to the

different cytoplasmic cylinders, blue

to the outer-membrane sheath and yellow

to the periplasmic flagellar filaments.

Source.

The authors further state: Are the observed events, which we interpret as

fusion, biologically relevant? Electron microscopy has a long history of

revealing important findings, but it also has a record of preparation artifacts.

Nicely put. They also wonder about how fused bacteria separate again. But it is

evident that these spirochetes can house two genomes within a single outer

membrane. Fascinating!

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