World's Biggest Bacterium

Jean-Marie - When I opened them, they are leaf debris - pieces of dead leaves. Then, on the leaves, you can see white spaghetti-like filaments that are about one centimetre to two centimetres in length, swaying left, and right in the water attached, to the leaf. To give you an idea of the size, they are more or less the same size as a eyelash, most likely the biggest bacteria ever!

Chris - We call bacteria "microbes" because you need a microscope to see them. I mean, they are normally fractions of a millimetre across. This isn't a microbe: it's a "macrobe" I suppose you could say isn't it? Two centimetres!

Jean-Marie -Yeah, you could absolutely say that. They are, they are not fitting the definition of microbes very well. That is for sure. They are about 5,000 times bigger than most bacteria. It is the same thing as if you would discover tomorrow that some humans are five kilometres tall; like as tall as the Mount Everest, for instance. This is how gigantic they are.

Chris - Goodness! And what do these bacteria do?

Jean-Marie -They live happily in the mangrove waters. They feed on sulphur. They grow on the surface of the sediment at the bottom of the mangrove water.

Chris - How did they get that big and why?

Jean-Marie - That, that is a very good question that we have not answered yet . Two hypotheses: the, the first one is that if you are a small bacterium and you are being consumed by other microbes that are feeding on bacteria, if you grow to hundreds or even thousands of times bigger than your predator, then you don't have to worry about being eaten by it, right? And then the second idea is that they live in an environment that is unstable and they need to position themselves in a very specific place where they can extract the chemical energy they need for growing. And if they are bigger, they can probably better access this energy. So there is there are some hypothesis in, in that sense as well.

Chris - Are they definitely just one entity? So if you look at them, it's clearly just one bacterial cell - it's not a big assemblage of them that have all joined together and made a sort of "macro" cell?

Jean-Marie - That is a very good question. And the first part of the answer is yes, it is a single bacterial cell. We have done a lot of microscopic investigation to confirm that. Because there are some other large microbes that can make centimetre-long filaments, but they are composed of many cells. In our case, we prepared these giant cells in a way that we could observe them in their entirety. So we looked at entire cells with microscopes in three dimensions. We never detected any separation that would make the filament a multicellular filament. So we know it is a single cell.

Chris - Have you read its genetic code, because that can often give us clues about how something works and who its relatives are.

Jean-Marie - We have sequenced the genome. Yes. And that allowed us to place them on the tree of life. They belong to a group of bacteria called gamma proteobacteria. They are sitting on the tree of life in a branch that is completely away from the origin of eukaryotes for instance. So we know that they are not at the origin of the complex cells that make our body or, you know, other animals. And we have also some clues on the molecular basis for their extreme size. By reading the genome, we know that it has lost some of the genes that are necessary for bacteria to divide in the classical way. And we know that it has some of the genes that are necessary to increase the size, to elongate the cell, have been duplicated. So they are missing some division genes. And on the other hand, they have multiple copies of elongation genes. So that kind of connects very well with the unusual morphology that we observe.

Chris - And do you know how they have baby bacteria? How do they reproduce?

Jean-Marie - We do know, yes, we that is also a quite unusual way of making baby bacteria, like you said. They grow into these large filaments and at the tip of the filaments, the cell will produce a bud. So they will constrict really just the tip of the filaments. And this constriction will close off completely. And a small piece is gonna detach, find a good spot and grow into a new giant filament.

Chris - Does anything eat them? Have they got enemies?

Jean-Marie - We don't know that. I was discussing that with our collaborator in Guadeloupe just before on this interview. And what they said is that they have looked at other giant sulphur microphones and they, they have analysed their - we could call it - their chemical signature and they try to see if they find this same chemical signature in some of the predators and they could not find it. So they don't have any evidence that, you know fish or small animals in the mangrove are feeding on those giant bacteria. We know that they are not consumed by the classic predators of bacteria, because they are just too big to be eaten by other microbes that would normally feed on bacteria. So we don't know who is eating the giant bacteria!