Field of Science

Living without a cell wall...

ResearchBlogging.orgA cell wall is one of the most important features bacterial cells possess. They provide a barrier against the harsh conditions of the outside world, as well as helping the cell maintain its shape and integrity. They are vital for nutrition uptake, and for cell and chromosomal division.

They are also, however, the main point of attack for other competing organisms, and for the human body when under attack. There are numerous antibiotics that direct against the cell wall. It is thought therefore that some cells have adapted to live in the body without a cell wall, their innards kept inside by merely a small lipid membrane.

But how do they survive? How do they replicate? And, most importantly, how on earth do you study them in a lab. If you take the cell wall off a bacteria under laboratory conditions it turns inside out. And then explodes. It certainly doesn't stay in any kind of workable state.

Recently though (very recently) the Center for Bacterial Cell Biology in Newcastle have found a way to grow bacteria (Bacillus subtilis to be exact) without a surrounding cell wall. The mutation is quite simple to make, and by adjusting the outside conditions to prevent the cells being damaged, they managed to grow colonies of cells with no cell wall at all, and keep them alive to study.

One of the most interesting things about these cells was their division mechanism. In normal bacterial cells, division depends on the cell wall as an anchoring point to hold the chromosomal DNA while it divides, and then control the lengthening and splitting of the cell, as shown in the diagram below (from here):

How do cells without a cell wall manage to divide? In order to find out, the group at Newcastle took little movies of their cells, following them as they grew and developed. The movie isn't in the paper, but there are a series of stills from it, showing a single cell growing and dividing, and following a very different pattern of division than usually seen in bacterial cells, or in any cells:

Image taken from reference one: link

Instead of splitting into two in an organised manner, the cell blobs out to form a long strand, which then breaks up into many little pieces, each containing a copy of the cell DNA. The usual proteins needed for organised division (in particular FtsZ) are not required, the cell is using a totally different system.

What is even more interesting, is that this looks very similar to a system proposed by Ting F. Zhu and Jack W. Szostak for how the very first forms of proto-life might divide, back when life consisted of not much but a small membrane with a twisted DNA coil inside. Working totally indepentantly, their work was examining the growth and division of simple loops of lipid membrane. They would form one, and make it grow by adding micelles, little circles of membrane. They found that as they added them, the cell would eventually start elongating and, when agitated, split up into little blobs, which could then grow and divide in a very similar manner:

Image taken from reference two: link

This looks strikingly similar too the images of the dividing bacteria shown above. In both cases the membrane stretches out and then splits up again into little circles. The only change the proto-life would have to make to the physical behaviour of the membrane would be to make sure that copies of the DNA got packaged inside each little circle.

This makes the work done at the centre at Newcastle even more exciting. Not only are they developing systems to study and explore bacteria that are immune to a wide variety of antibiotics, they are also helping to explore how the earliest forms of life might have survived and propagated. This provides a glimpse into a world before even bacteria had evolved, and does being to light just how highly sophisticated and complex bacteria are, compared to their membranous blob-like ancestors.

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Leaver, M., Domínguez-Cuevas, P., Coxhead, J., Daniel, R., & Errington, J. (2009). Life without a wall or division machine in Bacillus subtilis Nature, 460 (7254), 538-538 DOI: 10.1038/nature08232

Zhu TF, & Szostak JW (2009). Coupled Growth and Division of Model Protocell Membranes. Journal of the American Chemical Society PMID: 19323552

2 comments:

cuco3 said...

Nice! The moment I saw those pictures I thought of lipid membranes: they're just like them. I watched a BBC documentary recently which covered that. ("The Cell", presented by Dr Adam Rutherford. Very good, I thought.)

Captain Skellett said...

Very cool, a naked cell. Just combine that with what we already suspect about heat and atmospheric molecules creating amino acids, sugars and other building blocks (which can even happen extraterrestrially as shown by the Murchison meteorite, which I blogged about here) and we're getting pretty close to filling in all the blanks on how life began.

A few years ago the explanations of how early cell reproduction began were impossible to believe, but I'm becoming increasingly convinced.