On choosing the lesser of two evils
Salmonellae are cunning when it comes to triggering diarrhoea in their host. ETH Zurich researchers have succeeded in explaining a molecular mechanism that enables the bacteria to activate their host cell’s non-specific immune response, thus making the host ill. A single virulence factor is sufficient to allow the bacteria to trigger disease.
When Salmonellae gain entry into the gastro-intestinal tract, for example through contaminated egg-based foods such as mayonnaise or tiramisù, their victim’s culinary enjoyment is over. The infection is violent, lasts several days and weakens patients severely. The pathogens themselves are tenacious and can be detected in faeces up to 30 days after the initial infection.
Salmonellae have developed an ingenious mechanism to do this. Some of them penetrate into intestinal epithelium cells, which form the topmost layer of the intestinal tissue. Although the pathogens are killed in these cells, they nonetheless succeed in provoking inflammation that destroys the intestinal flora and nullifies their protective function. Their comrades of the same species that remained in the intestine exploit this and proliferate, and the affected person develops violent diarrhoea.
A completely different function
Scientists led by Wolf-Dietrich Hardt, Professor at the Institute of Microbiology (D-BIOL), have now shown for the first time which molecule is sufficient to trigger the diarrhoeal disease: the protein SopE, which Hardt has studied for a good part of the past 12 years and whose molecular function he determined during his post-doctoral research. However, the biological function of SopE is quite different to what the researchers expected at that time.
The bacteria inject the protein molecule into an intestinal epithelium cell, where it triggers a cascade of signals inside the cell. SopE tampers with two specific GTPases called Cdc42 and Rac1. These molecules are responsible for, among other things, building the cell’s skeleton and thus for the cell’s structure. When SopE binds to these two factors, the cell changes its surface and Salmonellae can penetrate into the cell. Previously, the researchers had assumed that this cell invasion brought about by SopE causes the diarrhoea.
Unexpected immune response
However, Hardt and his research group have now shown that Cdc42 and Rac1 are also a part of the cell’s early warning system. This system is indispensable for a rapid, non-specific defence against disease pathogens. What the two molecules do is to activate, via a route that is still unknown, the molecule Caspase-1, which is a cornerstone of inflammatory responses in the cell and also becomes active in sunburn, for example. Caspase-1 in turn activates chemical messengers that attract phagocytes such as macrophages. These finally put an end to the Salmonellae that penetrated into the intestinal tissue. On the other hand, earlier observations suggest that the Salmonellae that remain in the intestinal lumen can benefit from the resulting inflammation.
The perfidiousness of SopE is that the bacteria tamper with, of all things, a communication system which a cell cannot simply replace or switch off, because otherwise the non-specific immune response would fail to happen or the cell’s skeleton would be paralysed. Consequently, it is also not easy to find an active ingredient that disables Caspase-1, for example, to prevent the infection. Hardt stresses “That would impair the host’s general fitness more severely than the infection by Salmonellae.”
Diarrhoea is the lesser evil
Animals in which Caspase-1 did not function would quickly perish in the wild. This was shown in experiments on mice that lacked Caspase-1. Salmonellae and other disease pathogens infect the internal organs of these animals more severely. Thus the host is in a dilemma: although Caspase-1 allows Salmonellae to cause diarrhoea – allow them to colonise the host’s intestinal lumen more easily – without this protein, an animal or human being would be highly vulnerable to a large number of disease pathogens. Consequently diarrhoea is the lesser of the two evils.
Ironically, not all strains of Salmonella possess SopE. Only certain strains that have incorporated the genetic code for SopE into their own genome from a bacteria-specific virus, a bacteriophage, and can express it, can produce this substance. However, SopE is only one of 12 candidate chemical messengers that Salmonellae use to “crack” their host’s cell. That’s why Hardt’s researchers are now also studying how the other virulence factors operate.
Müller AJ et al. The S. Typhimurium Effector SopE Induces Caspase-1 Activation in Stromal Cells to Initiate Gut Inflammation. Cell Host&Microbe. 2009 Aug 20; 6:(2) 125-36.