Scientists have developed strains of modified bacteria that are intended to be used as sensors to detect environmental contaminants. The bacterium can be used to detect contaminants such as heavy metals. If implanted in the environment, the sensors can help scientists track how pollutant levels change over time over a wide geographic area. The caveat is that there was some concern that genetically modified bacteria could escape into nature by sharing their genes with other organisms.
MIT engineers believe they have created a way to make the implantation of bacterial sensors safer by wrapping them in a resistant hydrogel layer, preventing them from escaping into the environment. The project researchers found that they could incorporate E. coli into hydrogel spheres, allowing them to detect the contaminants they seek, but isolate them from other organisms.
Hydrogel balls also protect the sensors from environmental damage. The bacteria in question were designed to express genetic circuits that they don’t normally have, giving them the ability to detect multiple molecules. The circuits are designed so that the detection of the target triggers the production of green fluorescent protein or bioluminescence. The bacterium also recorded the event’s memory in the cell’s DNA.
The genetic circuits needed to do this in bacteria often include genes for antibiotic resistance. The researchers say that a particular gene allows them to ensure that the genetic circuit is inserted correctly into bacterial cells. It is easy to understand how bacteria genetically modified with a gene that makes them resistant to antibiotics can be harmful if released into the environment. Many bacteria can exchange genes between different species using a process called horizontal gene transfer.
To avoid this type of gene exchange, the researchers used in the past a strategy known as “chemical containment” involving the design of bacterial sensors so that they needed an artificial molecule that could not stay in nature. Another challenge is that, in a very large population of bacteria, there is a chance that a small number will acquire mutations that will allow them to survive without the molecule.
In this study, the researchers relied on physical restraint by encapsulating bacteria inside a device, preventing them from escaping. In the past, materials like plastic and glass were used, but they didn’t work because they form diffusion barriers that prevent bacteria from interacting with the molecules they need to detect. By encapsulating bacteria in hydrogels with pores large enough to allow molecules like sugar and heavy metals inside, the bacteria is protected and can still detect heavy metal molecules.