Not always pathogenic
Bacteria are omnipresent - in the water, the air and the soil, as well as in plants, animals and even people. We tend to think of bacteria as pathogenic, causing disease. We associate them with intestinal upsets and throat infections, pneumonia and blood poisoning. However, the great majority of bacteria are really useful - they play a role in our digestion, clean up waste water in sewage treatment plants, produce yoghurt and cheese from milk, and some are even used in the manufacture of drugs.
All the more reason then for getting to know bacteria really well and finding out how they grow and divide, interact with their surroundings and make us sick, or how we can put their properties to even better use. In spite of centuries of research, however, bacteria still hold many mysteries. A micro-sized mail coat
For fifty years now, bacteriologists have known that most bacteria develop an outside protein layer consisting of thousands of hooked together copies of a single protein.
The structure and function of this so-called S-layer can best be compared to an armor or mail coat. Until now scientists had a very limited understanding of the structure and function of this protective coat, which is rather remarkable, given that some bacteria invest up to a third of their total protein production in its construction.
With the publication of their findings in Nature, VIB researchers Han Remaut and Ekaterina Baranova at the Vrije Universiteit Brussel, together with French and British scientists, have pulled the hitherto unknown S layer out of obscurity. "We succeeded in imaging the structure of the protein coat for one specific bacterium (Geobacillus stearothermophilus) down to its individual atoms," says Han Remaut. "We were also able to determine how the individual proteins attached to each other to form a 2D structure similar to a kind of mail coat from the Middle Ages, but on a molecular scale, of course."
This tour de force required using a combination of technologies, including X-ray equipment and electronic microscopy. The most formidable challenge was converting the proteins into stable crystals. For that part of the research, the scientists used small antibodies, so-called nanobodies. These were able to stabilize the protein crystals so that their structure could be imaged in detail with X-ray diffraction. Protection from the outside world
"What we see confirms our earlier assumption that the S-layer functions as a protective coat against outside threats, such as viruses or proteins targeting the bacterial cell wall," continues Remaut, "because if the same bacteria are grown in a 'friendly' environment, free of extraneous threats, they do not develop an S-layer. We also saw that there are chinks in the armor which allow for the exchange of nutrients and other useful substances with the outside world."