Science Buddies: "Ask an Expert"

Hello!

I'm working on a project trying out different methods to differentiate bacteria from each other. One of those methods involve differentiating the bacteria using their respiration. I found this study (https://link.springer.com/article/10.10 ... 3674315047) that uses methylene blue and resazurin as redox indicators. They state that their results show

Both indicators can be used for the distinction of strict aerobes from other bacterial representatives with definite respiration

Resazurin is able to differentiate strict anaerobes from microaerophiles and other anaerobes

However, how do the redox indicators work in terms of differentiating the bacteria from each other based on respiration? The biggest lead I've gotten so far is this video (https://www.youtube.com/watch?v=i0nTDjxUmSk) which explains that in the glycolysis, NAD is reduced and hydrogen is oxidised. Similarly, methylene blue can be reduced and change colour in the same process. However, as far as I'm aware, glycolysis occurs in both aerobic and anaerobic respiration, so I'm curious as to how that would be able to differentiate the bacteria based on their respiration?

Would love if anybody could help me out with this!

Hi,

That's an interesting project!

The study you mentioned uses redox indicators to differentiate between aerobic and anaerobic respiration in bacteria, while the video you referenced uses redox indicators in the context of fermentation. It's important to note that anaerobic respiration and fermentation are different processes, and thus redox indicators work differently in each case.

Fermentation: This process involves only glycolysis, where glucose is converted to pyruvate, and NADH is produced. The pyruvate is further processed to regenerate NAD+. Since fermentation doesn't involve the electron transport chain (ETC), there is no need for a final electron acceptor, which is oxygen in aerobic respiration. The redox indicators can be reduced by electrons from NADH.

Anaerobic Respiration: This process involves all three stages of respiration, including the ETC. However, instead of oxygen, other molecules serve as the final electron acceptor. In this process, the redox indicator acts as an alternative final electron acceptor in the ETC since there is no oxygen.

To differentiate between aerobic and anaerobic respiration, redox indicators can be used as the color observed will be different. For example, in aerobic respiration, methylene blue will remain unchanged in its oxidized (blue) state because oxygen is available to act as the final electron acceptor in the ETC. However, in anaerobic respiration, molecules in the methylene blue act as the final electron acceptor, reducing it and turning it colorless.

I apologize for the late response, but I hope this helps!