Friday, 29 December 2017

How bacteria survive in oxygen-poor environments


Biologists have revealed a mechanism by which bacterial cells in crowded, oxygen-deprived environments access oxygen for energy production, ensuring survival of the cell. The finding could explain how some bacteria, such as Pseudomonas aeruginosa, are able to thrive in oxygen-poor environments like biofilms and resist antibiotics. P. aeruginosa biofilm infections are a leading cause of death for people suffering from cystic fibrosis, a genetic condition that affects the lungs and the digestive system.

Bacteria rarely live by themselves as single-celled organisms. Most instead grow in communities, leveraging the strength of numbers to form a biofilm with tissue-like properties similar to a scaffold that serves to fortify the community, making it up to 1,000 times more resistant to most antibiotics.

Each individual cell must on its own extract electrons from food that are then transported along the cell's membrane until they reach an oxygen molecule. The energy released during this metabolic process is used to sustain life. As communities of bacteria continue to grow and form into a biofilm, however, they can become overcrowded, creating an environment where each cell has to compete for limited nutrients and oxygen to survive.

Research has shown that some bacteria, including P. aeruginosa, have evolved different strategies to respond to and cope with the low-oxygen conditions in biofilms. Communities of bacteria can, for example, change the overall structure of the biofilm so that its surface area-to-volume ratio is higher and a larger proportion of the cells inside are able to access the oxygen on the outside. P. aeruginosa can also make molecules called phenazines, which help to shuttle electrons from the inside to the outside of the cell and ultimately to oxygen available at a distance.

Another strategy is to make alternative versions of terminal oxidases, enzymes in the membrane that transfer electrons to oxygen, which use oxygen more efficiently or are better at scavenging oxygen when its concentration is low. While there have been numerous studies done to examine the importance of these enzymes and strategies for P. aeruginosa growth, they've largely been conducted in well-oxygenated liquid cultures in the lab. When P. aeruginosa infects an actual host, such as a human, it often grows as a biofilm and encounters vastly different conditions.

See:

Jeanyoung Jo, Krista L Cortez, William Cole Cornell, Alexa Price-Whelan, Lars EP Dietrich. An orphan cbb3-type cytochrome oxidase subunit supports Pseudomonas aeruginosa biofilm growth and virulenceeLife, 2017; 6 DOI: 10.7554/eLife.30205

Posted by Dr. Tim Sandle

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