Flesh of the Fallen.


Breaking down the microbiology world one bite at a time

Flesh of the Fallen.

Consider this fun little experiment: we isolate people from 100 different families for almost three years, and give them no food at all. Well… William Shoemaker and his colleagues didn’t quite do that, but they did it with bacteria.

You might have noticed in your kitchen, bathroom or garden that if micro-organisms want to live, they will. Their tiny size does not stop them from ruthlessly getting what they want. In fact, those who read our blog more often may have seen that E. coli eats its own dead. Are there any more bacteria that do so?

Figure 1: Dinner is served! (image based on Stephen Nichols & Biorender)

Out of 21 bacterial taxa, Shoemaker found just one species civilized enough to die instead of feeding on the dead. The rest… Well, they just wouldn’t give up! Can you blame them? After the initial death of many individual cells, the surviving cells showed a remarkable recovery (Figure 2). The authors even correlated the amount of dead cells to the speed of recovering cell growth. Needless to say, the researchers found only traces of the deceased in samples of the bacterial cultures – the proteins, nucleic acids and membrane materials were all eaten.

Figure 2: amount of bacteria over 1000 days. A) the only bacterial species in this experiment which did not (efficiently) eat their own kind. B & C) two species which have roughly the same amount of cells after 1000 days, but have a different process. Image taken from the article by Shoemaker et al.

There is always an inevitable end to all life. Because of thermodynamics, the energy needed to create a cell is always more than you get from eating one. So how long can a bacterial culture survive? To combat the thermodynamic challenge, bacterial species slow down their metabolism. This can be seen from the doubling time: whereas the studied bacteria usually take a few hours to divide, during this starvation experiment the bacteria took 2-3 months. Because of a slowed metabolism the individual bacteria grew very old, up to 100 days (for comparison: your skin cells live around a week). Because of the slow metabolism and long life expectancy, Shoemaker predicts that the extinction of his bacterial cultures (depending on the species) takes 100 to 100.000 years!

Figure 3: Bon appetit! (image based on abcteach & Biorender)

Lastly, the researchers showed that enzyme-changing mutations were found in specific regions in the bacterial genome. Across all the studied species, mutations were often found in genes used for the biosynthesis of amino acid lysine and DNA-building block pyrimidine. Genes for transporter proteins for amino acids leucine, isoleucine and valine were also often found mutated. Such proteins seem to play a role in starvation regardless of bacterial species, but nobody knows why – for now.

So despite some genetic adaptation to the gruesome conditions created by Shoemaker et al., the scary truth remains this: most bacteria are merciless survivors. Their cultures will outlive most humans – because cannibalism is not an exception, but the rule for our micro-sized friends.

Link to the original post: William R. Shoemaker, Stuart E. Jones, Mario E. Muscarella, Megan G. Behringer, Brent K. Lehmkuhl, and Jay T. Lennon, Microbial population dynamics and evolutionary outcomes under extreme energy limitation, PNAS, August 2021

Featured image: https://www.abcteach.com/documents/clip-art-cauldron-bw-i-abcteachcom-33853 + Biorender