Microbiologists have expressed fears that hospitals are ill-equipped to cope with 'superbugs', after a bacterium resistant to one of the last lines of antibiotic defence was identified in the United States.

The US Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, reported last month that a strain of vancomycin-resistant Staphylococcus aureus had been isolated in June from the foot ulcer and catheter of a patient in Michigan. S. aureus often causes severe infections in patients' wounds, and outbreaks can lead to whole hospital wards being shut down.

Vancomycin became a vital weapon against S. aureus when strains resistant to the preceding antibiotic of choice, methicillin, emerged in the 1980s. But since 1987, when relatively harmless gut bacteria called enterococci evolved resistance to vancomycin, microbiologists have been warning that these genes could be transferred to S. aureus.

DNA sequence analysis revealed that the Michigan S. aureus strain did acquire its resistance genes from an enterococcus. A vancomycin-resistant Enterococcus faecalis isolated from the same patient could explain the jump, suggests the CDC's Fred Tenover, who led the analysis.

The outbreak is probably the first of many, as vancomycin-resistant enterococci are increasing in prevalence — almost a third of enterococci in US intensive-care units are now resistant to the antibiotic. Careful monitoring and rigorous control of outbreaks are the best response, says Gary French, a microbiologist at Guy's & St Thomas' Hospital in London.

Drug-resistant bacteria are usually detected in the lab by their ability to grow near paper discs soaked in antibiotic. But this method can fail with vancomycin, as it does not diffuse well, giving unusual patterns of bacterial growth that lab staff may not recognize. A back-up test, in which bacteria are grown on plates soaked in vancomycin, may not be used because of the costs involved.

If there is a repeat of the Michigan incident, French fears that some hospitals lack the staff and isolation rooms to control the infection. “We know what we ought to do but I'm concerned that there aren't enough resources to do it,” he says.

Vancomycin resistance is not as worrying as it would have been before the introduction of new antibiotics called linezolid and quinupristin/dalfopristin in 1999–2000, when vancomycin was often the last line of defence. But clinical biologists are keen to limit the use of these newer drugs, as some S. aureus strains are already resistant to them.