London

A bacterium that is spreading through hospitals because of its resistance to certain antibiotics is well equipped to develop yet more self-protection, according to a study of its genome.

Methicillin-resistant Staphylococcus aureus (MRSA) kills about 800 patients a year in Britain alone, and health officials have been watching its spread with alarm (see Nature 422, 791; 200310.1038/422791a ).

A genome sequence for one of its strains, published by a group at the Wellcome Trust Sanger Institute near Cambridge, UK, is significantly different from the six other resistant and non-resistant strains that have been sequenced (M. T. G. Holden et al. Proc. Natl Acad. Sci. USA 101, 9786–9791; 2004). And the areas that differ contain the genes that confer resistance to drugs.

The finding suggests that resistance genes can transfer very quickly between different strains, says Julian Parkhill, a geneticist in the Sanger team. “This is extremely bad news,” he adds. “Strains don't have to develop resistance independently.”

Other researchers now fear that many strains of the bacteria could acquire resistance to the antibiotic vancomycin, which is commonly used to treat patients infected with MRSA. Few other effective treatments are available.

In May, a study led by Mark Enright, a microbiologist at the University of Bath, UK, suggested that resistance to vancomycin was developing independently in several strains of MRSA (R. A. Howe et al. Emerg. Infect. Dis. 10, 855–857; 2004).

Vancomycin-resistant strains were first identified in 1997 and had been thought rare. But Enright looked at about 100 samples of MRSA that had shown some vancomycin resistance. He found that the microbes were related to the five major MRSA strains that cause problems in hospitals, and not to a single strain as some had originally thought.

The implication, says Enright, is that resistance is evolving in strains around the world. “Outbreaks are inevitable as long as we keep using vancomycin,” he says.

Researchers do not know when full vancomycin resistance will become widespread. Only three cases have been reported so far, and none led to fatalities. But Enright says that deaths from the bacteria can go unrecorded. “If people are ill anyway, you will not necessarily see MRSA on the death certificate.”