Breaking down the microbiology world one bite at a time
Resisting cadmium like a champ
Did you know that cadmium (Cd) can be toxic at even the barest levels (0.001–0.1 mg L–1)? With a half-life of up to 25–30 years, this heavy metal is an environmental pollutant contaminating more than 5 million soil sites and costing more than 10 billion dollars per year, globally!
This is why, a group of scientists worked on the bacterium Gluconacetobacter diazotrophicus to understand how it resists the toxic effects of cadmium, and they found one main target: the proteins.
Gluconacetobacter diazotrophicus: our protagonist
The bacterium Gluconacetobacter diazotrophicus is well known as a plant growth-promoter, and for its ability to survive under adverse environmental conditions. It has been widely favored in agriculture due to its ability to fix nitrogen, produce hormones and siderophores, mobilize nutrients present in soil and control of phyto (plant) pathogens.
Dr Júlia R Moreira and her team (2022) chose this bacterium to unravel those proteins that ensure its resistance to cadmium toxicity. They employed proteomics analyses to divulge those regulated proteins found outside the cytoplasm.
In one of their experiments, the team evaluated the bacterium’s viability (ability to survive) using different concentrations of cadmium chloride (CdCl2.H2O). They found that at the lowest concentration (0.04 mM), bacterial growth was inhibited by about 34%, and at highest concentration (0.2 mM), the inhibition was around 70%. The team also noted that though the cells did not grow significantly, they did manage to more or less maintain their viability. These results suggest there are some underlying resistance mechanisms that maintain this cell viability under cadmium stress conditions.
The presence of the actors and subsequent findings
From their work, they identified 722 proteins which form 18% of the total coding capacity of the bacterial genome.
The scientists grouped the proteins involved into those associated with: nutrient uptake, oxidative stress response and protein control.
- Nutrient uptake:
In the bacterium, 16 proteins were associated with the uptake of nutrients; 9 of those were outer membrane proteins, one- an iron ABC transporter substrate-binding protein, and the remaining six were transporter proteins .
- In response to oxidative stress:
Six proteins related to the response to oxidative stress declined. They were linked to glutathione metabolism through the presence of Ggt protein, and let’s not forget- glutathione is a well known antioxidant popular for its ability to minimize oxidative stress.
- Protein control:
Nine proteins associated with protein quality control systems (to clear away misfolded proteins and their aggregates); three proteins prevented unfolded proteins from accumulating and six proteins were involved with chaperone activities.
To summarize these findings, a model was brought forth. This model proposes that G. diazotrophicus uses efflux pumps to stop Cd accumulation in the cytoplasm. Toxins are barred from entering by decreasing those proteins related to nutrients uptake that are found outside the cytoplasm (extracytoplasmic). Proteins linked with defense against oxidative stress are then regulated. Exposure to any form of stress- including cadmium toxicity, could encourage misfolded proteins. To regulate this, several control proteins start to play a role. Thus, proteins work in tandem and their regulation in each phase, enables the bacteria to not only survive, but actively overcome this heavy-metal stress.
The climatic reveal- Importance of this work for the future
The integrated response of the various proteins during Cd exposure provides an important insight. As stated by the researchers, extracytoplasmic proteins related to nutrient uptake, multidrug efflux pumps,oxidative stress response and protein control are vital to ensure Cd resistance for the bacterium. The molecular studies done have identified the key protein players and the arising responses that help in resisting Cd toxicity.
This research provides a substantial glimpse of the protection of the plants by the resilient endophytic bacterium (like our research protagonist) against heavy metal (Cd) contamination, as opposed to Cd stress in the crop plant projected through stunted growth due to resulting oxidative stress and/or low nutrient uptake.
Dr Moreira and her team hope that their study and further research will deepen the underlying resistance mechanisms in similar plant- associated- bacteria that interact closely with crop plants for their survival and better resistance to limiting factors (such as heavy metal toxicity), or even when grown in contaminated soils !
Link to the original post:
Júlia Rosa Moreira et al (2022). Essential role of extracytoplasmic proteins in the resistance of Gluconacetobacter diazotrophicus to cadmium. Research in Microbiology 173.4-5. DOI: https://doi.org/10.1016/j.resmic.2022.103922
Image source: Original image created using craiyon, biorender and vecteezy.com.