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The bacteria that help sheep eat grass

The views expressed are those of the author and are not necessarily those of Scientific American.


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There’s been a lot of focus on the human microbiome recently, and while I’m obviously thrilled at anything which makes people think more about bacteria it’s easy to forget that it isn’t just humans who provide internal living space for bugs. Bacteria are everywhere, inside and among every living creature, and some of them form important symbiotic relationships. The bacteria that live in the gut of ruminant mammals; sheep, cows, and other things that eat grass, are vital for the animals digestion. Next time you eat a steak, thank bacteria.

Unless you get food poisoning. In which case - curse bacteria! Photo by Jon Sullivan, credit link below.

Plant material is a hard thing to break down, and mammals in general aren’t very good at doing it. As I showed in an earlier post, complex plant carbohydrates are massive molecules made up of many repeating units. Plant cell walls are particularly difficult, as they contain tough fibrous materials such as cellulose, hemicellulose and pectin. In particular I’m going to look at pectin, which the more hippy/middle-class of my readers might recognise as the stuff you add to fruit when making jam.

Pectin makes up around 10-20% of the total carbohydrate content of grass, so it’s quite important to break it down for energy. Not only that, but breaking down pectin makes it easier to break down the cellulose and hemicellulose. It’s broken down by an enzyme called (appropriately enough) pectinase which breaks apart the molecule at various defined points in its structure.

The chemical structure of the subunit of pectin, which is a massive repeating molecule. The enzymes have a specific place in the repeating unit that they recognise and break apart.

By extracting fluids from sheep stomach, extracting the DNA and looking at the sequence of what they found, researchers (ref 1) were able to isolate DNA for bacterial pectinases forming a stored DNA library of all the pectinases they found. Sequencing the DNA allowed it to be analysed, using computer programs to look at how related all these genes were, and which bacteria they came from. Two of the DNA parts were also put into bacteria and expressed, so the actual proteins could be looked at and characterised. One of the most interesting things that (I think) they found was that both enzymes worked at very different acidity levels.

Graph from reference 1 below

The graph above show the activity of two different enzymes (the two curves) over a pH range. On the left is an enzyme that works well at low pH, so in very acidic environments (for example in the stomach) whereas the curve on the right shows an enzyme that only works at a higher pH, so in more alkaline conditions likely to be found further down the digestive tract.

This isn’t just interesting random research either; fascinating though the inside of a sheep is this research has further reaching implications. Enzymes are being used more and more in industrial processes as unlike inorganic catalysts (usually metals) they work perfectly at low temperatures, and a variety of pH and salt levels. Finding pectinases that work in different conditions allows them to be applied in a range of industries that require plant matter to be broken down, in the preparation of materials such as juices, biofuels and textiles.

Credit link for image 1.

Yuan P, Meng K, Wang Y, Luo H, Huang H, Shi P, Bai Y, Yang P, & Yao B (2012). Abundance and genetic diversity of microbial polygalacturonase and pectate lyase in the sheep rumen ecosystem. PloS one, 7 (7) PMID: 22815874

S.E. Gould About the Author: A biochemist with a love of microbiology, the Lab Rat enjoys exploring, reading about and writing about bacteria. Having finally managed to tear herself away from university, she now works for a small company in Cambridge where she turns data into manageable words and awesome graphs. Follow on Twitter @labratting.

The views expressed are those of the author and are not necessarily those of Scientific American.





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