ANTIBIOTIC-RESISTANT superbugs are becoming a massive problem in hospitals worldwide. And if researchers in Britain are right, one reason is that the little devils can send signals through the air, telling other bacteria to turn on their resistance genes.
It鈥檚 known that bacteria exchange messages by releasing substances into the fluid in which they鈥檙e growing. These substances send a 鈥渨ake-up鈥� signal to neighbours or attract roaming bacteria towards the bacterial colonies called biofilms.
But Richard Heal and Alan Parsons of QinetiQ, formerly part of the British government鈥檚 Defence Evaluation and Research Agency, think that bugs can exchange signals through the air as well. 鈥淚t鈥檚 the first report of airborne communication, as far as we know,鈥� says Heal, who presented the team鈥檚 findings at the Warwick meeting.
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Heal and Parsons did their experiment in a Petri dish divided into two compartments. The only connection between them was a 5-millimetre air gap between the top of the wall and the lid. In one compartment they placed 100 or so blobs of the bacterium E. coli, together with various antibiotics.
When the other compartment was empty, the bugs simply died鈥攌illed by the antibiotics. But alarmingly, if thriving colonies of E. coli were placed in the other compartment, the first lot of bugs not only survived but they began to multiply.
If the gap between the compartments was sealed, the bacteria in the first compartment died. So the bugs in the second compartment must be sending some kind of airborne 鈥渟urvival鈥� signal, Heal and Parsons conclude. The signalling and recipient strains had been engineered so they could be easily distinguished, so the researchers are confident the signalling strain couldn鈥檛 simply have been getting over the wall and taking over.
Working at QinetiQ鈥檚 Winfreth Labs in Dorchester, Heal and Parsons showed that the signal made the recipient bacteria turn on genes that make them resistant to at least three common antibiotics鈥攁mpicillin, tetracycline and rifampicin. But they haven鈥檛 yet identified the signal. 鈥淲e鈥檝e tried without success to isolate the chemical signal from the air by dissolving it,鈥� says Heal. 鈥淣ext, we鈥檒l try gas chromatography.鈥�
Heal doubts whether it could be any of the known chemical messengers, or pheromones, that bacteria use. Nor is it likely to be any of the volatile substances discharged into the air by some soil microbes. Equally puzzling is why the unidenti-fied signal should switch on genes that make bacteria resistant to antibiotics. Perhaps it鈥檚 a way of arming relatives against rival species of bacteria or fungi that produce antibiotics, he speculates.
Whatever the signal is, it doesn鈥檛 seem to travel far鈥攂acterial growth tails off just centimetres from the dividing wall. But it might travel further on a breeze, if a fan is operating in a hospital, for example.
By identifying and neutralising the signal, it might be possible to stop new colonies of bacteria growing, or to stop them developing resistance to antibiotics, Heal says. He expects the discovery to be of most use preventing the growth of biofilms, which often clog surgical prostheses and catheters.
The results are 鈥渟triking鈥�, says Douglas Kell, who studies bacterial pheromones at the University of Wales at Aberystwyth. He knows of no other reports of airborne signalling between bacteria, but says there are parallels in plants. Leaves wounded by biting insects send out a gas called methyl jasmonate that warns other leaves to prepare for attack.
But Bill Costerton, director of the Center for Biofilm Engineering at Montana State University in Bozeman, is suspicious of the claims. 鈥淭he authors should be able to fractionate the gas and give us some idea of what types of molecules are involved,鈥� he says. 鈥淲ithout this, I鈥檇 be profoundly sceptical.鈥�