N EW DELHI metallo-beta-lactamase is a bacterial enzyme that poses a serious threat to people. It grants its host resistance to carbapenems and other beta-lactam antibiotics used by doctors around the world as a last line of defence against stubborn infections. And, as if that were not bad enough, it also confers protection against sunlight.

This second protection matters because, in many places, the idea that sunlight is the best disinfectant is no mere metaphor. Part of the treatment of sewage water in sunny climes is often to leave it out in the sun, permitting ultraviolet light to inflict damage to the complex molecules, such as DNA , that sustain bacterial life. Bacteria that survived such assault would be available to cause disease, or to pass their genes on to others that do so.

And, in 2015, one such was found in a treatment plant in Saudi Arabia. This was a strain of E. coli (a common bug that lives, usually harmlessly, in human guts) that was, indeed, in possession of New Delhi metallo-beta-lactamase. Peiying Hong at the King Abdullah University of Science and Technology, in Thuwal, has been working on this bug since its discovery and thinks she may have come up with a way to deal with its resistance to sunlight. As she reports in Environmental Science and Technology, she and her colleagues have found that phages (viruses which infect bacteria) sabotage this resistance.

Dr Hong knew from her earlier work that sunlight promotes the activity of some of the bug’s genes and suppresses that of others—a process she suspects is mediated by New Delhi metallo-beta-lactamase. In particular, the promoted genes relate to cell-wall synthesis, DNA repair and the production of compounds that mop up harmful oxidising agents produced by sunlight. The suppressed genes (downregulated presumably to avoid competition within a bacterial cell for scarce metabolic resources needed for more immediate tasks) include many related to fending off phages. She therefore speculated that letting phages loose on these now-undefended bacteria might kill them.

She and her colleagues collected naturally occurring phages from Saudi wastewater plants. They found seven types which, when unleashed on the photoresistant strain of E. coli, readily destroyed it. Three of the seven, moreover, looked particularly suitable for development as weapons against this strain. When offered various bacteria as potential prey, they attacked only it. And they were also tolerant of sunlight.

Experiment proved this notion correct. When suspensions of the three phages in question were mixed together as a cocktail and added to a suspension of photoresistant E. coli, the bacteria began to decay within two hours. In phage-free suspensions, by contrast, they held out for more than four hours.

Using phages to attack bacteria in wastewater plants is not a new idea. There is a long history of their being employed to breach the otherwise-impenetrable biofilms that many species form on bits of equipment used in such plants. Dr Hong’s approach, though, is completely novel—and may be of particular value in Saudi Arabia. In such a desert country, water is a precious commodity while sunlight is abundant and free. Wastewater facilities there are already under pressure to release treated water for agricultural and even domestic use. Making sure bugs of this sort are dead before such water is used is of the utmost importance. Using other, even smaller, bugs to do it has elegance.