Ask A Microbe To Tell The Microbe

Burden of Fungal Diseases

It has been estimated that 1.2 billion people suffer from fungal pathogens worldwide. Fungus-caused rice blast disease leads to 10-35% of rice harvest losses each year. The Irish potato famine was caused by the Phytophthora infestans fungus and it is now widely agreed that the women in the Salem witch trials had symptoms of ergot fungus poisoning. It is clear that fighting fungal diseases should be at the forefront of economic and global health agenda. But, before we can fight the microbes such as fungi, we need to be able to detect what microbes are causing the harm in the first place.

Yeast, a fungi themselves, could be used as biosensors to detect fungal pathogens
Yeast, a fungi themselves, could be used as biosensors to detect fungal pathogens

How to Detect Pathogens

Microbes are all around us. Some of them are good some of them are bad but distinguishing between the two is often not as trivial as one would like it to be. And, especially when it comes to harmful microbes, being able to quickly identify the microbe that is the cause of the damage is essential, so that the right treatment could be applied immediately.

But how do you find the invader if it is tiny and affects the host in a way that hundreds of other invaders do?

Well, you can begin by trying to look under a microscope. That’s assuming that the microbe in question has a distinct morphology and you have a spare microscope at hand. You do not? Ok, well then we could try to perhaps identify the unique genome sequence of the microbe or perhaps find a unique protein on its surface. That’s assuming you have the required tools and expertise to do it, and, I’m afraid to say, most places that need quick diagnostics have not. So what to do then? Well- ask a microbe to tell the microbe.

Microbes themselves have evolved many different ways to recognise their peers. In the micro-world, where the resources are often limited, it is essential to know who resides in your surroundings. And many microbes, whether that’s a bacterium, which recruits another bacterium to fight of environmental stresses, or a yeast cell that is looking for a mating partner, use signalling molecules to talk to one another. Perhaps, taking inspiration from the microbial crosstalk could also help us to figure out the pathogens in our surroundings.

How the yeast biosensor works
How the yeast biosensor works

Engineered Yeast Biosensor for Fungal Pathogens

Synthetic biology often uses natural systems in microbes to engineer new pathways that could be used as biosensors for an easy detection of pathogens. In one recent study scientists used a yeast mating system to detect different kinds of fungi from a diverse range of samples.

Yeast cells (yeast are fungi themselves) release pheromones to attract other yeast cells for mating. These pheromones are essentially small peptide molecules that bind to highly specific receptors on the yeast cells. The binding to the receptors triggers signalling pathways, which lead to behaviours required for mating. The key here is the specificity of the receptor, receptor A will only recognise pheromone A and not B, C or D. Inspired by this system scientists re-engineered the yeast cells to become biosensors for fungal pathogens (see the schematic in the image above).

The scientists took the typical yeast cells that we use in everyday baking and switched the mating receptors in them. Because the receptors are highly specific to the pheromones they bind, depending on the receptor they put into the yeast cells, different types of fungal pathogens could be detected.

Fungi that could be detected by the biosensor
Fungi that could be detected by the biosensor

Any biosensor has two parts to it: the detection and the readout. So even if the yeast cell can bind a pheromone released by a fungus, it still needs somehow to be able to show that a given pheromone was present and bound to the receptor. To make a readout, the scientists coupled the pathway that is activated by the binding of the pheromone to the receptor, to a pathway that produces a pigment called lycopene. Lycopene is a natural pigment found in bacteria and plants that has an orange tinge to it (tomatoes are full of lycopene). So now, every time the mating receptor was activated it induced a production of a pigment in the yeast cells that could be easily detected by a naked eye, simply by observing an appearance of an orange colour.

Easy-to-use system

Any effective diagnostic system should be cheap, stable and easy to use. The yeast-based biosensor checks off all of these criteria. Yeast are abundant and a relatively simple system to work with, so the production of these engineered yeast cells could be easily scaled up in bioreactors depending on the need. As long as we can figure out the specific receptors we need to detect the fungal pheromones, we can engineer new strains of yeast to detect them.

Obviously, carrying vials of yeast cells to some remote places is not the most convenient thing to do. So, in the same study, scientists made a dipstick to resolve this issue. Essentially, they have blotted some yeast cells on a piece of paper and all the user needs to do is to wet this paper and dip it into the sample, which may contain the pathogen. If the pathogen is present, then a spot on the paper where the yeast cells are will turn orange and if not- nothing happens. The dipstick was shown to work reasonably well with samples as diverse as soil or blood. The dipstick detection still needs around 9 hours to make a visible change in colour, which is perhaps the next step in the biosensor optimisation, but it already is as simple of a device that one could make and use.

Dipstick created for an easy-to-use biosensor system. The + circle turns orange when a pathogen is detected
Dipstick created for an easy-to-use biosensor system. The + circle turns orange when a pathogen is detected

So there you have it, another great example of how learning about the fundamental processes in life (not many perhaps care about how the yeast mate) can lead to great benefits for the humankind.

References:

N. Ostrov et al., “A modular yeast biosensor for low-cost point-of-care pathogen detection,” Science Advances (2017).

Fisher, Matthew C., et al. “Emerging fungal threats to animal, plant and ecosystem health.” Nature 484.7393 (2012): 186-194.

Denning, David W., and Michael J. Bromley. “How to bolster the antifungal pipeline.” Science 347.6229 (2015): 1414-1416.

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