Antibiotic-resistance leaks outside hospital walls.

Rising Staph: Drug-resistant bacteria have emerged outside hospital walls in the US. Credit: CDC/Janice Carr

A new antibiotic-resistant and virulent form of Staphylococcus aureus has surfaced in significant numbers in cities across the US. Researchers are concerned that this community-based strain might become resistant to stronger antibiotics, limiting options for treatment.

Driven by overzealous antibiotic use, bacteria often evolve resistance to antibiotics in hospitals. Drug-resistant Staphylococcus, in particular, has been a pernicious presence in hospitals for more than two decades.

More than 60% of Staphylococcus infections acquired in hospitals are from S. aureus resistant to methicillin and the class of first-line antibiotics called beta-lactams, which includes penicillin. But unlike most of these drug-resistant strains, up to 20% of resistant strains the researchers encountered in the new study sprang from outside hospital walls (N. Engl. J. Med. 352, 1436–1444; 2005).

“The molecular biology of these seems to have changed,” says investigator Jeffery Hageman, an epidemiologist the US Centers for Disease Control and Prevention.

The new strain has a 'cassette' of genes that bestows resistance and which, at less than 24 kilobases, is more compact than those seen in resistant hospital strains. “What Staph has done is evolve a svelte, transmissible cassette,” says Robert Daum, professor of microbiology at the University of Chicago.

Whatever strategy we devise, [Staph] will come back with something. Viva la stalemate. , Robert Daum, University of Chicago

The cassette may be small enough to hop to other kinds of Staphylococcus, perhaps carried by a phage that infects the bacteria, and confer antibiotic resistance to those strains. It may already be in up to ten clonal lines so far, says Henry Chambers, chief of infectious diseases at San Francisco General Hospital.

The new strain, universally described as “nasty,” secretes more toxins and causes bigger lesions than some of its cousins. A second report says the strain caused “an alarming number” of cases of necrotizing fasciitis in a California hospital (N. Engl. J. Med. 352, 1445–1453; 2005). In that potentially fatal condition, the bacteria destroy the tissues underneath the skin at a rate of up to six inches an hour.

Clinicians may choose to treat patients infected with these strains with antibiotics such as vancomycin. But if that becomes standard practice, the bacteria might become resistant to these antibiotics as well, experts warn. Already a few scattered cases of vancomycin-resistant Staphylococcus infection have appeared in hospitals.

Staph is an ancient pathogen,” says Daum, who has evidence of yet another cassette of resistance genes—also small and promiscuous—in Australia and Japan. “Whatever strategy we devise, it will come back with something. Viva la stalemate.”