Most antibiotic-resistant lung infections are caused by a handful of bacterial strains, microbiologists now believe, contrary to previous opinion. This homogeneity raises hopes for vaccines, the American Society for Microbiology heard last week in Salt Lake City.

Bacteria can share antibiotic-resistance genes through the transfer of loops of DNA called plasmids. Although the staphylococci that cause hospital-acquired and childhood pneumonia do not carry plasmids, they can incorporate plasmid DNA into their genomes. This was thought to explain the worldwide spread of scourges such as methicillin-resistant Staphylococcus aureus (MRSA).

It now seems that resistance is spreading not because diverse strains are acquiring plasmids, but through clonal reproduction of a few 'superbugs'. Microbiologist Keith Klugman of Emory University in Atlanta, Georgia, co-founder of the Pneumococcal Molecular Epidemiology Network, presented evidence that about 75% of drug-resistant childhood pneumonia cases are caused by 10 strains of pneumococcus. Almost half are caused by one strain, Spain 23-F.

Herminia de Lencastre, a molecular biologist at Rockefeller University in New York, said her laboratory tested 3,000 MRSA isolates from 14 countries and found that 70% of infections belonged to five strains.

Low natural variability makes resistant bacteria good targets for vaccines. Vaccines against MRSA are still under development, but the pneumococcal vaccine Prevnar is showing signs of success against antibiotic-resistant strains in South Africa.