December 10, 2012

Foodborne Illness Can Be Avoided Using Your Nose

There is a reason behind why people should avoid eating rotten food – and it isn’t just because it is gross to do so, either. Believe it or not, eating rotten food can actually be fatal because it could send unwanted bacterial pathogens into your digestive system.

Fortunately, we were born with an uncanny sense of smell that allows us to detect signs and symptoms of decay in order to avoid foodborne illness. If you have come to find that you aren’t great at detecting decay, though, then you can rely on fruit flies to do the job for you instead.

That’s right. A recent study by neurologists and behavioral scientists from the Max Planck Institute for Chemical Ecology has shown that fruit flies actually have a neural mechanism associated with a special escape reflex that activates itself to avoid food that is infected by harmful microorganisms. Fruit flies activate this reflex to avoid both laying eggs in rotten food and eating rotten food.

Basically, this is how it works: the minute a small piece from mold fungi or bacteria gets into the air, a dedicated and sensitive neural line from the fruit flies’ olfactory receptors will get activated and override all of their other odor signals, no matter how attractive they might be. This mold fungi or bacteria is called geosmin and works as a complete stop sign for fruit flies to stop them from approaching rotten food. It kind of works in the same way that we avoid forgotten dinners from the previous week until the minute we open our refrigerator doors and smell the rottenness.

This might not seem like such a big deal to most people, but this is probably because not a lot of people know that a lot of deaths in Germany last year were connected to fenugreek sprouts that were contaminated with bacteria. This just goes to show how important it actually is for people to know the difference between infected food and fresh food.

No matter what kind of food it is, it is colonized by various microbes, including bacteria – remember that. The amount of microorganisms in food will differ from one to the next in regards to storage conditions and freshness, though. Since the immune system generally doesn’t have any trouble dealing with microorganisms like these, it is alright to eat decaying food, as long as the decay isn’t too bad yet. However, when the pathogenic microbes get dangerously high, protection will be low for both humans and animals all around.

In a lot of cases, visual signs are enough for people to distinguish rotten food from fresh food. Certain odors that the hazardous microbes release are much easier to distinguish, though. What the team actually wanted to do in their study is figure out which functions in the brain underlie this flight behavior in fruit flies. To understand this reaction, they simply mapped the path from the flies’ odor molecules to their brains’ olfactory receptors.

Fortunately, since fruit flies are genetically characterized, they were the perfect objects to study to find out the answers for this. Usually, these flies eat yeast that grows on rotten food, so they can help distinguish good food from bad food with ease. The team eventually found out that food flies that ate fungi or bacteria died much faster, and the eggs that they deposited on this type of food never hatched.

As mentioned earlier, geosmin refers to an odor substance that several bacteria and fungi produce. Geosmin might trigger deterrent reactions from people, so we can detect it at 0.1 parts per billions of the concentration. It seems that fruit flies are even more sensitive than humans in that department, though.

Whenever geosmin gets into the air, even attractive food sources automatically become unattractive. This is quite interesting since it is very rare for single compounds to directly effect behavior, especially in low concentrations.

To start, they did electrophysiological experiments and then analyzed the fruit flies’ olfactory sensory neurons. Out of all of the neurons, only one actually ended up responding to geosmin. Thanks to this, the team successfully established the specificity within single neuron recordings connected to gas chromatography and in cell culture experiments where the cultures ectopically expressed the receptor.

Aside from that, the team also did optical imaging on the brain of the fruit flies and found that only one glomerulus got activated by geosmin. This was called DA2. All of the other glomeruli that have something to do with odor evoked aversion behavior could be found in the same area as DA2, as well. Whenever the team tried to stimulate DA2, a certain projection neuron got activated, showing geosmin presence in higher brain areas.

The scientists explained that the stimulus gets patched through directly from the antennae to the behavior in the circuit without any delays. Before, these patterns were only seen in sex pheromone responses, so this is actually the very first time that a dedicated odor neural pathway has been seen related to feeding behavior.

A new Flywalk system was put in place, as well, where single flies were placed in small tubes of glass and were given different odors to smell. This was done to quantify their overall behaviors to them. Through this, it has been proven that geosmin makes the fruit flies move away or stop before reaching the odor source. Plus, it tends to override all of the other attractive odors displayed at the same time, like fruit scents and vinegar. This helps fruit flies avoid eating unwanted pathogens by accident – a vital survival tool indeed!

Overall, this just goes to show how important it is for living organisms to avoid rotten food.

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