Battles in the blood

Posted on November 4, 2013   by Benjamin Thompson

History is littered with examples of battles for the control of raw materials. If you want to win a war you need access to the things required to wage it, be they coal, oil or iron. Right now, inside many very sick hospital patients across the world there are microscopic battles going on, with bacteria struggling for control over the materials they need to survive, divide and cause disease.

Iron is essential for a number of microbial processes, notably metabolism, with bacteria needing to maintain internal concentrations of the metal at around 10-6 M (a tiny amount) in order to grow. Humans also require iron for survival, with around 70% locked away in red blood cells and much of the remainder tightly bound by iron-sequestering proteins like transferrin, a blood protein that has a very high affinity for the metal.

Having iron so securely controlled prevents it from falling into the wrong hands and is an effective way of inhibiting bacterial infection. Concentrations of free iron in human blood can be a trillion times lower than the levels bacteria require to survive. In response, they have evolved mechanisms for liberating their host’s iron. This acquisition allows bacteria to grow in the blood, causing life-threatening bacteraemia.

One of the ways that some bacteria scavenge iron from their environment is by producing large amounts of molecules called siderophores. These are cast by the bacteria into their environment and can wrestle iron away from transferrin (and similar iron storage molecules) thanks to their supremely high affinity for the metal. However, siderophores are not tethered to the bacterium that releases them, so it’s down to chance whether they get gathered back up – obviously not the most effective scavenging system.

Other, more targeted methods of iron scavenging exist. The best studied of these is the two-protein TbpA/TbpB system found in Neisseria meningitidis and Neisseria gonorrhoeae. One of the proteins is designed to grab onto transferrin and hold it close to the surface of the bacteria, while the other acquires the iron and internalises it into the cell. Having transferrin tethered nearby also increases the efficiency with which siderophores are recovered.

More than ten years ago, Dr Primrose Freestone, then a postdoc at the University of Leicester, discovered that enteropathogenic E. coli was able to bind transferrin to its surface much like Neisseria can, despite there being no evidence for the presence of a TbpA/TbpB system. Now, working with Dr Sara Sandrini (her former PhD student) and colleagues, Primrose has shown the mechanism that E. coli uses to cling on to transferrin, which may have important consequences for vaccine development and for hospital care. This work is published in the Society journal Microbiology.

The team showed that two porin proteins found on the surface of E. coli, OmpA and OmpC, are responsible for the transferrin capture. This result is a little strange, as these porin proteins are usually thought of as being simple transport tubes (literally pores) in bacterial membranes, which allow molecules to diffuse from the environment into the cell. However, Primrose describes OmpA as “something of a molecular Swiss Army knife” given its multitude of functions. Aside from being a pore, OmpA is also involved in structural integrity, conjugation, adhesion and biofilm formation, among others, while OmpC is similar, but with less functionality. How can one protein do so many things? Primrose explained to me that because they may have only a few thousand genes, bacteria, particularly free-living ones, have to be versatile and economical with the ones they have.

In addition to transferrin anchoring, this work shows that OmpA appears to have yet another function, acting as an entry point for catecholamine stress hormones such as dopamine and norepinephrine (also known as noradrenaline). Previous studies by Primrose and colleagues have shown that uptake of these hormones, released by the body in response to stress, can massively increase the growth and virulence of pathogenic organisms.

Serious infections can end up with bacteria growing in the blood, which is really bad news; the body’s response to such invasion is often to go into a whole-body inflammation, which can result in the release of a barrage of stress hormones that might actually help bacteria to survive. To compound matters, drugs used to treat acutely ill patients include norepinephrine and dopamine – shown in the new paper to promote bacterial growth.

This infection-related inflammation is known as sepsis and can lead to organ failure and death. I spoke to Dr Natalie Silvey, who works with the UK Sepsis Trust;  she explained to me how serious an issue it is.

“Sepsis has a ridiculously high mortality rate. In the UK, 37,000 people die as a result of it each year. It can be difficult to diagnose; sometimes it’s missed and patients can present quite late, as they’re unaware of the signs to look for.

“There have really been no new drug treatments that have been shown to work effectively against sepsis. Resuscitating the patient, giving lots of fluid and providing early antibiotic treatment, these are the things that work well.”

The importance of OmpA and OmpC to bacteria – and their location on the external face of the cell – makes them excellent vaccine targets. Primrose and her team showed that these proteins are involved in transferrin binding in a wide range of bacterial species, including those from the genus Salmonella and Shigella.

Several decades ago it was shown that mice innoculated with Salmonella porins were well protected against later infection, while recent work has shown the potential of using OmpA as a vaccine to prevent infection by drug resistant Acinetobacter baumannii.

Given the paucity of drugs we have to treat sepsis, preventing one if its major causes, bacteraemia, is a great idea. The knowledge that the molecular Swiss Army knife is so important to bacteria may help us win the war.

Sandrini S, Masania R, Zia F, Haigh R, & Freestone P (2013). The role of porin proteins in acquisition of transferrin iron by enteropathogens. Microbiology PMID: 24089578