Bacteria are everywhere (despite what Dettol may tell you), and our own bodies are thought to have evolved side-by-side with the bacteria that call us home and help our bodies to function properly.
But some bacteria can of course cause disease in humans, and there are growing concerns about some of these becoming resistant to the antibiotics we use to fight them off.
Now scientists from Switzerland and France have discovered how a single mutant gene can render Salmonella 250 times less virulent in mice – and how it works is pretty mind-boggling.
So, what’s the point?
Salmonella is a genus of bacteria that is thought to cause tens of millions of food poisoning cases per year. In the developed world Salmonella infection rarely results in anything worse than a stomach bug, but in parts of the developing world it can be a different story:
Salmonella can be more widespread in these regions due to many reasons, including a lack of health education and poor water quality and sanitation. Salmonella typhus, which can cause typhoid, is particularly prevalent in parts of Asia, and is estimated to be responsible for several hundred-thousand deaths per year.
In Sub-Saharan Africa, non-typhoidal strains of Salmonella can cause potentially fatal blood infections. It is thought that people with weakened immune systems (e.g. sufferers of HIV, malaria or malnutrition) are particularly vulnerable.
Salmonella can generally be treated with a course of antibiotics, but some strains have developed resistance to some common antibiotics. Discovering new ways of killing the bacteria, or reducing their ability to infect people is therefore crucial to developing new treatments that can fight them off.
What did they do?
The study focused on a protein called EIIAGLC, which helps with the uptake of certain sugars and to control a number of metabolic proteins inside the bacterium (Salmonella enterica).
They first wanted to see what difference it made to the growth and virulence of the bacteria, so they injected two types of bacteria into mice: one set was normal (wild-type) Salmonella, and the other was a mutant strain which could not produce EIIAGLC.
The wild-type Salmonella ‘outcompeted the… mutant 250-fold in 3 days’, but the mutant was still able to survive in the mice. When they grew the mutant in a cell-culture dish, they also grew as normal. This suggests that not having EIIAGLC has not greatly impacted the Salmonella’s ability to grow and survive, but it has affected its ability to infect host organisms.
The next thing was to figure out why. The scientists tested more mutant strains which, rather than lacking EIIAGLC itself, were unable to do some of its jobs, such as transporting sugars and controlling the production of certain proteins.
They also wanted to see if EIIAGLC had any other roles that had not yet been discovered. They used a method known as co-immunoprecipitation, which allows you to see if certain proteins bind strongly to one-another. Strong-binding can indicate that the proteins interact with one-another in the organism.
Did they prove anything?
They found that all of the mutants which could not perform selected functions of EIIAGLC, but still produced it, still outcompeted the mutant without EIIAGLC, and in some cases were pretty much as virulent as the wild-type.
This suggests that none of the EIIAGLC functions they tested explained the lack of virulence by themselves.
But the co-immunoprecipitation revealed that EIIAGLC binds to a needle-like protein complex called TTSS-2 (Type Three Secretion System-2). TTSS-2 sits across the outer shell of the bacterium and can recognise host cells and inject them with proteins which then help the bacterium to infect them – like a syringe.
Because TTSS-2 is a complex of proteins (a structure made from numerous protein building-blocks), it needs all of the proteins that comprise it to not only be produced, but also to assemble together in the right way in order to function properly.
But when the scientists compared the TTSS-2 structures in the wild-type vs. the EIIAGLC-lacking mutant, they found that the mutant had lower levels of some of these building blocks. This was despite the fact that the overall levels of these proteins in each type of bacterium was similar.
This suggests that EIIAGLC plays a role in helping some of these building block proteins assemble together. Without it, the full TTSS-2 complex cannot form properly and the bacterium loses one of its most important tools for invading host cells.
So, what does it mean?
The research seems very systematic – they observed that the mutant strain was less virulent than the wild-type and methodically followed steps to figure out why. They began by looking at the mechanisms that EIIAGLC is known to participate in, and when that yielded no obvious answer, they looked at what else it could be doing – eventually discovering its involvement with TTSS-2.
Identifying this role for EIIAGLC means that future treatments could be designed around blocking its function, for instance by using a drug molecule or antibody that binds strongly to it. It also suggests that disrupting the formation of TTSS-2 can make the bacteria less virulent – another potential drug target.
Figuring out how pathogenic bacteria grow and infect uncovers new ways to fight them. This kind of research is key to finding particular proteins and mechanisms inside a bacterium to target new treatments towards.
Original article in Cell Reports May 2014
All images are open-source/Creative Commons licence.
Credit: Rocky Mountain Laboratories, NIAID, NIH (First);
B Metcalf (Second); Y tambe (Third); A Mazé et al. (Annotated by TSIC) (Fourth); O Scrhaidt et al. (Fifth)
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