When we think of ‘memory’ we typically think of the brain being able to recall facts and events, but ‘memory’ can take other forms: some plastics can ‘remember’ particular shapes, material shapes or magnetic alignment can be used for storing digital data and our immune systems also have a capacity to ‘remember’ past infections.
Bacteria are also believed to display a kind of ‘memory’ too, which helps them to digest nutrients they have encountered recently. A new study by researchers in the US explores this ‘memory’ in E. coli to see how it impacts on their ability to grow in environments where the food source changes.
So, what’s the point?
Bacteria are hardy survivors – the Mad Max of organisms. They are thought to be one of the very first forms of life to have evolved, and will certainly be around long after humans have gone (in fact, they’ll almost certainly have eaten our physical remains).
Central to their incredible capacity to survive is their ability to eat different things. Now while this might not seem particularly impressive to us – a species that invented the bacon ice-cream sundae or the cheeseburger in a can – species of bacteria have evolved to eat substances as unappetising as concrete, petroleum and nuclear waste (although no peer-reviewed tests have been conducted to see if any can eat the cheeseburger in a can).
When a bacterium is exposed to a new food source, it typically takes a while to become adjusted to it (the so-called ‘lag phase’). In this phase, the bacterium is generating the biochemical machinery (enzymes etc.) needed to digest the new nutrients it finds itself surrounded by.
During the ‘lag phase’ the bacteria are not dividing and so growth of the bacterial population is temporarily stunted. The researchers wanted to see if bacteria could, in effect, be trained to grow on more than one food source and reduce or eliminate the ‘lag phase’.
This idea is not completely crazy – the authors claim that ‘memory’ has been previously observed in bacteria, as some of the metabolic machinery for the original food source will still kick around inside the cell even after the bacterium has adjusted to the new food source.
Figuring out whether or not bacterial ‘memory’ helps them to quickly adapt to changing environments could be important in understanding how to control the growth and spread of bacteria. This is not just useful for killing harmful bacteria that we don’t want, but also for culturing or harvesting bacteria that we do want, for instance those used in waste treatment or for producing therapeutic molecules such as human insulin.
What did they do?
The researchers developed a microfluidic device, which fed a culture of bacteria (E. Coli) a stream of either glucose or lactose, with the feed changing every 4 hours for 3 complete glucose/lactose cycles (24 hours in total).
As the feed flows past the culture, individual bacteria are washed away, and are measured at a point downstream. The number of bacteria washed away in the stream is used as an indication of the total size of the population.
In a second experiment, they tested how long this ‘memory’ persists by growing bacteria on lactose for four hours, switching to glucose for a varying time (4, 5.5, 7, 9 and 12 hours) then measuring the ‘lag phase’ (if any) when the feed was switched back to lactose.
Did they prove anything?
The first time lactose was introduced (after 4 hours of glucose), there was a significant ‘lag phase’ as the bacteria adjusted to the new food (see Graph A below). However, the next time glucose was introduced, the ‘lag phase’ was much shorter and in subsequent changes of glucose/lactose there was no ‘lag phase’ at all, with the transitions described as ‘seamless’ (see Graph B below).
However, in the second experiment where the exposure time to glucose was varied, they found that when the bacteria were deprived of lactose for longer than four hours, the ‘lag phase’ reappeared. The ‘lag phase’ generally increased as the time away from lactose increased (see Graph C above).
In further tests, they looked at the lactose-digesting machinery (specifically proteins known as LacY and LacZ). They stated that LacY and LacZ are degraded extremely slowly, so this alone is unlikely to be the cause of the increase in ‘lag phase’.
However, they reckoned that LacY and LacZ are passed on from mother cell to daughter cell as each bacterium divides, causing ‘dilution’ of these proteins in each resulting cell (all of the residual LacY/LacZ of the mother cell would be shared between mother and daughter after division).
This can explain why there is little change in the length of the ‘lag phase’ at lactose-deprivation times below 4 hours (not much cell division occurs), but above this time there is a general increase in the ‘lag-phase’ as the number of cells dividing increases with time (sharing roughly the same amount of LacY and LacZ between a larger number of cells).
So, what does it mean?
There certainly appears to be a link between the length of time since the bacteria were exposed to lactose and the length of the ‘lag phase’, and the reasoning that cell-division is responsible appears to be consistent with the results.
This study could help scientists to better understand the mechanisms behind the growth behaviour of bacteria – important information in trying to control them. This is essential for harnessing their incredible capabilities technologically and curtailling their potential to harm us too.
Original article in PLoS Genetics Sep 2014
Text © thisscienceiscrazy. If you want to use any of the writing or images featured in this article, please credit and link back to the original source as described HERE.
Find more articles like this in:
Lambert, G., & Kussell, E. (2014). Memory and Fitness Optimization of Bacteria under Fluctuating Environments PLoS Genetics, 10 (9) DOI: 10.1371/journal.pgen.1004556