Micro-G and bacterial machinery 


Breaking down the microbiology world one bite at a time

Micro-G and bacterial machinery 

We know that one of the most amazing things about microbes is that they are everywhere, and so it is not truly surprising that they are found associated with space-faring vehicles despite precautions. This is fascinating to us as through their presence on these “life-defying” vehicles, we get to discover the secret behind the presence and survival of microorganisms in space- an environment so extreme that life hesitates to reveal itself.

Space factors influencing (drastic) microbial responses for survival can take the form of galactic cosmic radiation, solar UV radiation, space vacuum and microgravity. This article is a small attempt to portray the impacts of microgravity on bacterial metabolism, or simply the inner machinery of those resilient bacteria.

What is “microgravity”?

Often dubbed as the micro-g (μg) environment, this is the gravity range seen on the International Space Station (ISS). NASA defines microgravity as the condition in which people or objects appear to be weightless

Space-faring vehicles have the super ability to navigate through extraterrestrial conditions like low gravity, which could generally distress any form of life. Since bacteria are very “tiny”, we might presume that they perhaps do not experience gravity,  but this is actually not true; like all organisms, they too face the adverse effects of altered gravity (in the form of microgravity) from that of terrestrial gravity when moving to space. This often occurs in the form of cellular stress that mostly conveys itself by altering the metabolism, overexpressing genes associated with starvation, enhancing transmembrane influx, forming biofilm, and even changing their virulence accordingly, as observed over the past 60 years.

On earth, we (as its inhabitants) are invariably influenced by gravity, so much so that it is impossible to abolish even 1g of the gravitational force on the planet! This is why mimicking microgravity conditions is tough; and to imitate this microgravity, a “free fall” situation is created. 

Figure 1:  The image above depicts the concept of  “free fall” using the astronaut as the reference. Image source: By Jovana Andrejevic., 2018

What impact does microgravity have on bacteria?

But how can we mimic it? Well, Gayatri Sharma and her team (2022) propose performing experimental work on microgravity to be done in two ways: first on an actual spacecraft like the ISS, and/or second in Ground-Based Facilities (GBFs) that mimic microgravity. But beware- GBFs do not experience increased exposure to radiation unlike the actual spaceflights and therefore, both experimental designs do not always induce the same results. 

Microbes usually respond to changes in gravitational forces through mechano- transduction systems, or simply- the conversion of mechanical signals to cellular responses (gene expression, DNA supercoiling and so on). Incidentally, when stressed or in survival mode, each microbial species displays its specific responses, triggered by the induction of cryptic gene clusters. New molecules called secondary metabolites are then formed in unpredictable ways which help in homeostasis in cells. The effects of microgravity on secondary metabolism depend on the strain, growth condition, pathway utilized, or time course analyzed.

So far, the bacteria explored for space research are already those identified and explored as metabolite producers, and most of the secondary metabolites have been divulged as polyketides and non-ribosomal peptides.

Figure 2: Image source: microbiologynutsandbolts.co.uktt

Biofilm formation is a well-known response in space by the opportunistic Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa, influenced by amino acid metabolism. The amino acid arginine, in particular, plays a dual role during spaceflight; to regulate biofilm formation and prevent oxidative stress.

Bacterium Klebsiella pneumoniae grown on Shenzhou spacecraft 10 for instance, displays an increased adhering ability, increased biofilm formation, and survival under micro gravitational stress. Metabolic genes associated with amino acids, TCA, etc are also increasingly expressed irrespective of carbon sources available, and are downregulated during fatty acid metabolism.

Figure 3: A is the Shenzhou Spacecraft and B represents the colonies of Klebsiella pneumoniae. Source: China National Space Administration and universe84a.com 

Under the influence of microgravity, stressed bacteria also generate higher antibiotics (secondary metabolites) than normal. This is seen in Bacillus brevis (Gramicidin S antibiotic), Streptomyces hygroscopicus (rapamycin), Streptomyces ansochromogenus (nikkomycins).

Furthermore, under microgravity, bacterial cells accumulate toxins, during which diffusion and brownian movement (random movement of the molecules) limit the molecular movement due to lacking gravitational forces. This results in depleting nutrient- availability outside and accumulation of toxic cellular by-products near the cell. 

As it happens, the study on Microcystis aeruginosa is rather a tale of caution– especially to us and our fellow space- queriers. Under micro- g, the toxic cyanobacterium grew abnormally fast ( 2 days instead of the normal 6 days) actively releasing its toxin- microcystin into the media. Scientists traced the cause of this drastic toxin release to increased photosynthetic pigment concentration and higher nitrogen absorption under the stress caused by micro-g. They concluded that indeed simulated microgravity turned M.aeruginosa into far more dangerous.

Gayatri Sharma points out that the changes occurring in the primary metabolism are fairly obvious and in turn, influence the secondary metabolism inadvertently. However, there’s still a lot of gaping knowledge regarding the dynamic connection between the two (primary and secondary metabolisms) especially on spaceflights, that researchers can focus on.

The importance of the study:

Having a grasp on the metabolism of those tiny bacterial “free- rent” in space vehicles can discern the effects of the metabolites- specifically secondary metabolites (e.g. toxins)  that can further provide insights on mitigating possible damages to the space-faring infrastructure.

Data from transcriptomics and metabolomics of the bacterial pool could certainly bring forth novel microbial products and interesting gene clusters that would without a doubt, evolve bioprocessing and most microbial methodologies.

In modern bioprocessing technology, the changes(temperature, oxygen availability, and diffusion) sired by microgravity can be adequately harnessed for producing engineered microbes. Engineered microbes adapted from the findings could further be employed to grow plants aboard spaceships- how cool is this?! The image below (figure 4) shows similar experimentations (already at work!) with growing plants (radishes) on spaceflight (Expedition 64 Flight) in the year 2020.

Figure 4: Experiments on growing radish plants by NASA astronaut and Expedition 64 Flight Engineer Kate Rubins have already started by 2020. Image source: https://www.chron.com/news/space/article/Bacteria-growing-ISS-plant-growth-16032473.php

Space-vehicles sustaining (unwittingly) diverse microbiota are still understudied; findings from this study and further digging (research) would surely enable the next safe flight missions to be safe!

Link to the original post: Sharma G, Curtis PD. The Impacts of Microgravity on Bacterial Metabolism. Life. 2022; 12(6):774.

Featured image: Image source: https://www.quranmualim.com/explain-what-is-microgravity-quranmualim/