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New Naming System for S. pneumoniae Will Help Dispel Strain Confusion

What's in a name? That which we call Streptococcus pneumoniae serotype 9V by any other name would infect the same, right? But while multiple strains of drug-resistant pneumococci are spreading across the globe, the system of referring to strains by their serotypes has proved vulnerable to confusion and uncertainties. In addition, the traditional naming system makes it difficult to determine whether a strain that turns up in one area is truly new or the arrival of a clone that is established elsewhere.

In the July issue of the Journal of Clinical Microbiology (39:2565-2571), an international group of experts called the Pneumococcal Molecular Epidemiology Network (PMEN) proposes a nomenclature for ending such uncertainties, standardizing naming procedures, and otherwise facilitating studies of these increasingly ubiquitous pathogens. The PMEN also organized a reference set of antibiotic-resistant pneumococcal clones that the American Type Culture Collection (ATCC) in Manassas, Va., will maintain. The network and its products may serve as models for researchers working with other important, multistrain human pathogens.

"As the tools become available to characterize the clonality of pathogens, we're learning a lot more about the distribution of organisms that are causing disease globally," says Keith Klugman, the Emory University bacteriologist who cofounded PMEN with Helena Makela, former president of the International Union of Microbiological Societies. The network and its reference collection will help scientists and clinicians answer clinically important questions, such as which clones are showing up in a community, or whether a patient is having a relapse caused by the same organism or a different strain, Klugman says. The network will also help address more fundamental questions, such as what makes particular disease-causing clones so successful, he adds.

"The importance of all this is that we're appreciating for the first time that the distribution of organisms within a species is not homogenous," Klugman says. "Previously, we really didn't have the tools to identify these strains, so there was no way to tell just how important individual clones within a species may be. It's turning out to be extremely important. For instance, there is evidence from our network and people collaborating with it that the enormous burden of antibiotic resistance in pneumococci in the United States rests on just a handful of clones." The network will also help to track resistance traits within and among strains.

To develop the new nomenclature and organize their reference collection, PMEN members traced citations through the literature to the earliest report of each clone in a peer-reviewed journal and then requested isolates and information about them from the authors. "The evolution of resistance is important, so we wanted to get as early an example as possible for each strain," Klugman says. The genetic relatedness of candidate strains was determined using three molecular techniques: pulsed-field gel electrophoresis, BOX-PCR, and multilocus sequence typing. The use of these three techniques helped ensure the distinctiveness of each putative clone, he says. So far, 16 distinct drug-resistant clones circulating nationally and internationally have been identified.

Building on the old system of serotyping, the newly devised nomenclature classifies each strain by the first identified serotype and the country in which the strain most likely originated. "It's important to arrive at a nomenclature so that what somebody might call clone A or clone B in a paper can be identified immediately by all as one particular international clone," Klugman says. Of course, the new nomenclature is not an official mandate, he adds. "It's not like we're some sort of formally constituted body that has the final right to name these clones." However, he believes the new system will catch on among pneumococcal researchers, in part because so many of the world's experts belong to PMEN.

Klugman believes that the PMEN could serve as a model for researchers focused on other pathogens with widely disseminated strains. He points to Salmonella enteriditis, Campylobacter, and the gonococci as organisms that might lend themselves to this approach to classification.

PMEN members elected to centrally house the reference collection of pneumococci clones at ATCC out of practicality, despite the costs entailed, Klugman says. "We thought initially of one of our labs or maybe everybody keeping a set of strains, but in the end we decided it was best to have a professional organization do it."

PMEN will continue adding strains to its collection on an annual basis. Three more clones will likely be ratified during the PMEN's next meeting at the Interscience Conference on Antimicrobial Agents and Chemotherapy this month.

Christine Stencel
Christine Stencel is a communications manager and science writer at ASM.