Scientists Tracked Evolution Of Antibiotic Resistance In Real Time

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Colorized scanning electron micrograph of Escherichia coli

CC BY 2.0: NIAID / https://flic.kr/p/rg1p9H

Many people don’t believe in evolution. Only one-third of Americans say it occurs through processes like natural selection and no involvement by God. But besides religion, there are several reasons why some reject the fact life evolves, such as not seeing adaptation happening with your own eyes.

Scientists have now provided further proof of evolution in action with a video of bacteria evolving resistance to an antibiotic drug. The short film shows the results of a study by microbiologists at the Université Claude Bernard Lyon 1 in France, which reveals how rapidly Escherichia coli can acquire genes that confer the ability to resist the effects of tetracycline.

Tetracycline molecules kill microbes by binding to the machinery cells use to translate genetic instructions, which stops them producing vital proteins. But some bacteria can expel tetracycline that enters their cells by pumping it back out through a special ‘efflux pump’ called TetA in their membranes. The gene for the TetA pump is carried on a DNA plasmid — a ring of genetic material that can be transferred from a donor to recipient bacterium via direct contact.

The French biologists tracked how a population of E. coli evolved in real time using live-cell microscopy and fluorescent labels. In the video, recipients are marked in red while donors are labelled with green fluorescence — microbes with the TetA gene which makes them resistant to tetracycline. Individuals turn from red to green after acquiring that antibiotic-resistance gene.

After the DNA plasmid carrying TetA is transferred from a donor, the recipient’s cellular machinery reads and translates the genetic instructions to produce both a TetA pump and green fluorescent protein, highlighting that the new genes have been acquired. The visualization reveals just how quickly a drug-resistance gene can be switched-on after transfer: the whole process happened in only 1-2 hours.

Importantly, although the experiments were performed with tetracycline present, microbes didn’t die immediately. This is thanks to a pump called AcrAB-TolC. Compared to TetA, which has a specific function that makes it efficient at expelling tetracycline, AcrAB-TolC can eject small amounts of multiple drugs, keeping concentrations below toxic levels to buy bacteria some time to develop a more robust defence through acquisition of TetA.

The general pump explains how resistance can evolve in hospitals despite the presence of drugs, which has major implications for public health. As the study’s lead researcher Christian Lesterlin suggests, the existence of AcrAB-TolC presents an alternative strategy to tackling drug-resistant microbes: “We could even consider a therapy combining an antibiotic and a molecule able to inhibit this generalist pump […] reducing antibiotic resistance and preventing its spread to the various bacterial species”.

Exposure to antibiotics drives microbes to adapt through natural selection, helped along by horizontal gene transfer between individuals — including members of different species — and illustrates the importance of believing in evolution.

As a doctor tells his patient — a creationist with tuberculosis — in the famous Doonesbury cartoon, “I need to know whether you want me to treat the TB bug as it was before antibiotics… or as the multiple-drug-resistant strain it has since evolved into.”

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