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It is a well-known fact that bacteria have the ability to gather or ‘swarm‘ in the event that they find a favorable ecological niche that is beneficial to their survival. Many scientists have even researched this swarming behavior to see what kind of benefits it offers. Recently, it was found that bacteria tend to swarm in groups in the event of danger, but what was even more surprising was that during the occurrence of such an event, they tend to scream or rather to be more accurate, signal their neighboring brethren to move away from the site of danger!
Swarming behavior in bacteria
Image credit: University of Austin, Texas
 

Altruism in bacteria?

This research was published by scientists at the Department of Molecular Biosciences at the University of Austin, Texas where they had observed this behavior in Escherichia coli cells, these are the good bacteria that live in your intestine and contribute to your microbiome that is responsible for your gut health. This is not to say that E. coli cannot cause infections but that can only happen in the case that there is an overload or disbalance in the gut microbiome which aggravates this issue. Coming back to the signaling behavior, when the bacteria were treated with an antibiotic solution, a portion of the whole population sacrifice themselves in order to save their brotherhood. Many studies have shown that bacteria have this tendency to swarm when they are faced with a threat to their survival. It was previously believed that when these bacterial populations are faced with such a scenario, it could pose many benefits to them such as allowing them to form a protective barrier against the antibiotics and even help distribute the limited nutrients to the rest of the population below the barrier. But in this one of a kind study, it was found that the bacteria rather emit a signal of sorts that warn the others in the swarm.
 
How do they accomplish this?
They manage to do this by releasing a biochemical signal known as AcrA, which is a protein component of an efflux pump known as Resistance nodulation division (RND) that helps bacteria transport material or substrates in an out of their cells. Though this is a characteristic of gram-negative bacteria, they can be found in other bacterial species as well. In this case, when the nearby bacterial cells sense the AcrA signal, the protein binds to a component on the outer membrane of the live cells and activate an efflux cascade. This not only tells the bacteria to move away from the problematic site but also drain out any antibiotic components they might have taken in. This type of altruistic behavior is not unique to the species studied but has been observed in other bacterial species as well. This leads to them acquiring an antimicrobial resistance (AMR) trait that they might not have had previously.

AcrA proteins (red) bind to the outer membranes of E.coli (green).
AcrA proteins (in red) are seen binding to the E. coli cells (in green)
Image credit: University of Austin, Texas
 
So, what are the implications of the study?
Well, first we’ll have to look at how does this benefit the entire population as such. This type of signaling cascade does confer antibiotic resistance to the population that survives which is similar to the phenomenon of persister cells that we see in bacterial biofilms. Even though a small population dies in the process, the others tend to be stronger than they were before because they have inherited a trait that they might not have had previously. This kind of upregulatory behavior can quickly spread through populations especially in the case where heterogeneous populations might be present in the swarm. The subpopulations present in the swarm had shown variable susceptibility to antibiotics as well showing the presence of a heterogeneous population and their ability to adapt to the necrosignal. 


This is where the discussion on how group traits are also successfully passed on between generations as opposed to individuals traits that are usually passed down. Previous conversations regarding AMR always associated it with individual traits that are species-specific but now it is important to look into how such signaling cascades have evolved over time and how AMR traits can cross different species. This gives a new perspective in the study of AMR by looking at how group traits can also confer resistance in phenotypes that were not present or minimally present before. 


It’s almost funny to see how a trait such as signaling in times of danger is not just limited to human beings but also present in prokaryotic organisms. This study in itself could provide leeway for scientists to look into which therapeutics target such behavior. For decades we have tried to thwart infectious diseases by developing new therapeutics but it always seems like these minuscule organisms are a step ahead of us. Such studies do give an insight into not only the pathogenic mechanisms that these organisms incorporate but also the parallels that can be drawn between their behavior and ours. 




References
Bhattacharyya, S., Walker, D. M., & Harshey, R. M. (2020). Dead cells release a ‘necrosignal’that activates antibiotic survival pathways in bacterial swarms. Nature communications11(1), 1-12.

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