Scientists engineer E. coli to monitor arsenic

Cornell scientists have engineered E. coli to act as a sensitive biosensor for monitoring environmental arsenic, a toxic pollutant most notably found in rice paddies in Southeast Asia.

A new study provides a proof of principle for a potentially cheap living sensor that can record even transient arsenic exposure under anaerobic conditions, preserve this information in the genome and allow delayed readout later in the open air of the lab.

The ability to detect and record arsenic exposures under both aerobic and anaerobic conditions has been a challenge for other types of arsenic biosensors.

At the same time, the mechanism within this sensor has the potential for use in other bacteria species and to detect other toxins.

"We've created what is called a whole cell biosensor, living machinery that tells us, if you let it grow in the environment, whether a target substance is present or absent," said first author Elisa Garabello, a doctoral student in the lab of Andrea Giometto, assistant professor in the School of Civil and Environmental Engineering in the Cornell Duffield College of Engineering.

Giometto is senior author of the study published March 23 in the journal Applied and Environmental Microbiology.

Rice commonly grows in paddies - flooded fields that are anaerobic (oxygen free) environments. Rice plants absorb naturally occurring arsenic from the flooded soil and water. In anaerobic conditions, arsenic is converted to a mobile form that is more easily absorbed by living organisms. Consuming contaminated rice can expose humans to arsenic, which is carcinogenic and can lead to developmental issues in children.

In the study, E. coli was engineered with an enzyme called Cre recombinase, which induces genetic changes when the bacteria come in contact with arsenic. The changes occur in DNA in plasmids (which are bacterial extrachromosomal DNA) and could stably be detected in E. colifor up to 12 generations from the removal of the target contaminant, providing a reliable record days later. Also, a fluorescent protein lit up when the bacteria's DNA was recombined.

In the study, the team used an anaerobic chamber in the lab of co-author Matthew Reid, associate professor in the School of Civil and Environmental Engineering in Duffield Engineering, to expose the engineered E. coli to arsenic for two days in a controlled environment. In order to measure the response, the bacteria samples were then regrown aerobically and passed through a flow cytometer, which can quickly detect fluorescence in individual bacteria that were exposed to arsenic.

"We can measure hundreds of thousands of cells and then detect as few as 100 that have recombined, which allows us to detect very low, nanomolar arsenic concentrations," Giometto said.

While biosensors that use an enzyme to alter DNA have been developed in health settings, they are rarely used for environmental applications.

Hyun Yoon, a former doctoral student in Reid's lab, is a co-author of the paper.

The study was funded by the National Institutes of Health, the Human Frontier Science Program and the National Science Foundation.