BioEd Online

A new strain of bugs resistant to antibiotics may be eluding routine laboratory tests, microbiologists have warned. Their caution comes shortly after the superbug surfaced for the third time.

The bacterium is a strain of Staphylococcus aureus, which can cause grave hospital infections. Many strains have become resistant to common antibiotics such as penicillin.

Vancomycin is one of the last lines of defence against these bacteria, and health officials had long feared that the bugs would become resistant to this antibiotic by picking up resistance genes from other vancomycin-resistant bugs.

Their fears were realised in July 2002, when the first case of vancomycin-resistant S. aureus, or VRSA, was identified in a Michigan patient. Another case followed in Pennsylvania later that year.

Now a third case has been detected, in the urine of a patient in New York, in March¹.

Subsequent analysis at the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, has shown that automated testing machines often fail to identify that the new strain is immune to vancomycin. "It is clear that some systems did not pick it up," says Roberta Carey, chief of the CDC's epidemiology and laboratory branch.

Experts say that this means that other cases of VRSA may be going unnoticed by the widely used machines. "There is concern that this could happen," says Clifford McDonald, a medical epidemiologist also at the CDC.

Although overlooked cases are probably rare, because doctors normally ask for repeated laboratory tests on patients not responding to vancomycin, failure to detect the bug straight away could allow the bacteria time to spread to others.

Cloudy issues

There are two automated laboratory tests that are routinely used to screen bacteria for antibiotic resistance, and the CDC researchers found that neither of them reliably pick up VRSA. In one test, called Microscan, a tray of 96 wells containing increasing concentrations of vancomycin is inoculated with the strain of bug being tested.

If the bacteria grow in a well, the broth turns cloudy, and this change is automatically read by a machine. Bacteria able to flourish in concentrations above a threshold level of vancomycin are flagged as potentially drug resistant, and more thorough checks are carried out.

Carey is not sure why the automated tests are failing to pick up VRSA, and is now carrying out larger trials. One possibility is that the new strain grows relatively slowly, and so it appears to have been killed by the antibiotic. This could be resolved simply by programming the machine to allow a longer incubation time.

Meanwhile, the CDC is advising laboratories to use additional, manual tests for VRSA, such as growing samples on agar jelly soaked in vancomycin. "We have got to get the message out," says McDonald.

There are three drugs that can tackle VRSA, but none of them are ideal: S. aureus is already developing resistance to linezolid and quinupristin-dalfopristin, and is expected to develop immunity to a new drug called daptomycin. "We need new antibiotics, it is as simple as that," says Mark Enright, who studies S. aureus at the University of Bath, UK.