The research, published in New Scientist magazine,offers evidence that bacteria can be attacked by viruses as well as larger organisms.

John Martin, from the Center for Complex Infectious Diseases in California, says he has found more than 50 bacterial genes in a virus isolated from a woman suffering from ME.

Other virologists say that more work needs to be done before they can be convinced of his theory.

'Breaks boundary'

Mr Martin's centre was set up to look at diseases with no obvious cause, such as Chronic Fatigue Syndrome (CFS).

In one patient, he found a virus similar to one that is known to infect African Green Monkeys.

But within the virus' genetic make-up were scores of genes from a variety of bacterial species.

Most of them, however, had functions that were not useful to viruses, such as involvement in building bacteria cell walls. Mr Martin thinks some of the acquired genes will help them work more effeciently when it invades an animal.

Viruses are known to pick up DNA from animal hosts, but have never been found to be taking material from bacteria .

Mr Martin said: "It breaks the conventional boundary between bacteria and viruses."

However, Paul Griffiths, a virologist at the Royal Free Hospital Medical School in London, said that Mr Martin may have contaminated his sample with blood from the patient.