Monday, July 06, 2015

Engineered Microbes- They are the sensors


In a recent blog post I had talked about the need to detect antibiotic sensitivity pattern for a better health care (Link). I was arguing about it with a friend of mine, I was pointed to examples where knowing the organism is important more than the antibiotic. The best case scenario is water quality testing. In such cases once again the classic microbiological techs are laborious and time consuming. Need another example? How about food testing for microbial contamination. Think about it. Had you ask this question a decade ago, PCR would be the option of choice. It still does in most cases, but is expensive and needs the equipment's in place. Say you need to do the test in a remote area and you want results in less than a day.

Fig 1
I can think of a lot of technologies that can give you answer in as short time as possible, at as less cost as possible. If you can catch it as fast as possible with as much as possible you are ahead of the pathogen in this game. In a nutshell we need 3 things- sensitivity, specificity and speed. Most of the technologies such as Immunoassays, PCR, Nanotechnology, chip array achieves the first 2 parts, but not the speed.

Welcome to the concept- "Bactero-detectors". The idea is simple. You engineer a bacteria to find some other bacteria. When it does its gonna give you a signal. That sounds crazy. But its so much doable. I have already talked about engineered bacteria that maybe deployed to combat pathogens (Link).  A fascinating idea of which I have heard is to use the quorum sensing system. If you build into a bacteria a genetic operon which responds to a quorum molecule specifically secreted by target bacteria linked to a response protein (which is a fluorescent protein) you basically have a Bactero-detector. See Fig 2 for Illustration. So lets imagine you have a water sample to be tested for Vibrio cholerae. Take about a 0.5ml culture of engineered bacteria, mix it with the water sample and wait. If vibrio is present it will secrete its specific quorum sensing molecules, picked up by the engineered bacterial promoter (supposed to respond only to that molecule). This will turn the fluorescent protein on and your mix which start giving a color. That's it. Since your detection system is a simple bacterial suspension you can have tons of this material easily maintained and such an assay will be cheap.

Fig 2: Illustration of bacterial gene engineering for
detection of pathogen of interest.
An extension of this idea is to use phage detectors. Instead of engineering a bacterial gene you put a luciferase system into a bacteriophage specific for the bacteria you want to detect. If the particular bacteria happens to be present in your sample phage will attack and the luciferase system will be activated. With the help of host ATP (Bacterial ATP), luminescence will be generated which can be easily detected. There are multiple smart modifications of this technique allowing for a range of detection.

It looks like there is more to the story now. In a recent paper, by a smart genetic engineering technique (Link), conditions such as diabetes and cancer could be diagnosed in the lab with remarkable sensitivity. These are the new generation technology sometimes referred as microbial biosensors. Their ability to be miniaturized and cheap has is too good to not see it.
Lim JW, Ha D, Lee J, Lee SK, & Kim T (2015). Review of micro/nanotechnologies for microbial biosensors. Frontiers in bioengineering and biotechnology, 3 PMID: 26029689

Courbet A, Endy D, Renard E, Molina F, & Bonnet J (2015). Detection of pathological biomarkers in human clinical samples via amplifying genetic switches and logic gates. Science translational medicine, 7 (289) PMID: 26019219

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