Imagine this: bacteria that need a metal to put electrons onto in order to respire. It’s not too strange, really; we need oxygen to put electrons onto when we respire so in that sense it’s a similar mechanism. These bacteria just use iron or molybdenum et cetera whereas we would use oxygen when we breathe. But now here’s where things get weird: if the bacteria just need something to dump electrons onto, why not just give them an electrode, and let them dump their electrons onto the anode? Indeed, that’s how many microbiologists have grown metal-respiring bacteria, and bacteria that can grow in these conditions are called “anode-respiring bacteria.”
Years ago when I first heard about bacterial nanowires in anode-respiring bacteria, I thought that my mind had been very thoroughly blown. Basically, microbiologists found that the mats of bacteria which grow on the anode are very thick, and indeed are so thick that you would think that the bacteria on top of the mat would not have sufficient access to the anode in order to respire. Further analysis suggested that the bacteria in these mats or “biofilms” actually had little “hairs” (since termed “bacterial nanowires”) which they used to pass electrons to their neighbours, and then they would pass the electrons on to their neighbours, and so on until it got to the bacteria living directly on top of the anode, who would then transfer the electrons onto it. It is an amazing network of electrical interactions between bacteria! This is also cool, because you can quantitatively measure the respiration rates by measuring how much of an electrical current your bacteria are putting into the anode:
Now, my mind has been totally blown all over again, with Lars Nielsen’s ASM 2012 talk on Bacterial “Microcables” and electrical currents in filamentous bacteria.
Lars noticed that his bacteria needed oxygen and iron sulfide in order to live and consume. First, both of these things were distributed evenly throughout the growth area. But as the bacteria grew, they consumed the nutrients where the environment would not replenish them. His bacteria produced large zones of depletion, wherein there was no oxygen and no sulfide. The top had lots of oxygen, but none of the iron sulfide it needed. All the iron sulfide was at the bottom, many centimetres away. The bacteria need both at the same time in order to survive. How were the bacteria getting access to both at the same time? Was it more nanowires? Were they using minerals in the anoxic zone to pass electrons down to the bottom? Were they using exofactors as shuttles to shuttle the electrons down?
Lars investigated this question, and found that they were actually filamentous bacteria acting as living cables themselves! A bacterium would divide incompletely, forming two cells attached by a shared outer membrane. Then those cells would divide incompletely, forming four cells attached the same way, et cetera. Soon there is a centimetres-long chain of the bacteria, all with one common outer membrane. Electron microscopy revealed fibres, which are presumably the conductive cables, running down the length of these chains. The cells were bridged together by these fibres and an outer membrane. A cross section gave a better look at the cables inside the periplasmic space of the membrane.
To test whether the bacterial chains were using these “microcables” to conduct electrons downwards through the anoxic zone, Lars’s lab did several tests. Here are two of my favourites:
They passed a thin wire through the anoxic zone, effectively “cutting” the bacterial chains in half. They found that growth stopped, and no more current was being produced in the electrode. After 24 hours, neither growth nor current had returned. So, cutting the chain of microcables correlated with the cessation of respiration. Respiration stopped instantly and the results were lasting.
They put a small film barrier in the middle of an anoxic zone, with holes large enough to let molecules and proteins through, but too small to let bacteria through. This was to test whether they could still use secreted shuttles and exofactors. The bacteria could not grow with this film blocking them in the middle.
I think the Nielsen lab has some very compelling evidence that they’ve found something totally new and awesome! Consider my mind blown.