The World Health Organization (WHO) Monday identified 12 families of bacteria that are most in need of new antibiotic treatments, an issue that experts say is critically important as antibiotic-resistant superbugs become more common.

CDC has reported that antibiotic-resistant infections kill an estimated 23,000 Americans each year, while the European Food Safety Authority and European Center for Disease Prevention and Control estimated that superbugs kill 25,000 Europeans annually. Superbugs are "one of our most serious health threats," said Thomas Frieden, the recently retired director of CDC. And experts expect the threat to grow with time as antibiotic overuse and other factors reduce the effectiveness of existing treatments.

WHO list aims to prioritize response efforts

According to the Washington Post's "To Your Health," the list marks the first time WHO has released a list of antibiotic-resistant "priority pathogens," and the organization hopes it will help researchers and drugmakers prioritize their response efforts by focusing on a small range of particularly harmful bacteria.

The WHO list grouped the 12 bacterial threats into three groups: critical, high, and medium priority. According to Marie-Paule Kieny, WHO's assistant director-general for health systems and innovation, WHO selected the bacteria included on the list based on the level of drug resistance that already exists for each, how many deaths they cause, the frequency with which people get infected by them outside the hospital, and the burden that the infections impose on the health care system.

Many of the selected bacteria are so-called Gram negatives, which frequently "colonize intestinal reservoirs and can cause sepsis and severe urinary tract infections, especially among elderly patients," the Washington Post reports. In recent years, drugmakers have focused more on Gram-positive bacteria, WHO says, which tend to live on the skin of healthy individuals and are easier to develop treatments for.

Critical threats

WHO says bacteria classified as critical cause severe infections and have high mortality rates, especially for patients in hospitals and nursing homes, as well as those who are on ventilators, have blood catheters, are undergoing chemotherapy, or are transplant recipients.

The three most critical threats were:

• Acinetobacter baumannii, which can cause a range of infections such as blood infections and pneumonia;
• Pseudomonas aeruginosa, which can cause mild ailments in healthy people—but severe blood infections and pneumonia among sick people in the hospital; and
• Enterobacteriaceae, which live in the gut and include well-known pathogens such as E. coli and Salmonella.

Tuberculosis was not named as a critical threat because the need for new antibiotics to treat it has already been designated a high priority by WHO, Kieny said.

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High, medium priority

High priority bacteria represent pathogens that "cause a large number of infections in otherwise healthy people," STAT News reports. WHO identified six families of bacteria as high threat, including:

• Enterococcus faecium, vancomycin-resistant;
• Staphylococcus aureus, methicillin-resistant (more commonly known as MRSA), vancomycin-intermediate and resistant;
• Helicobacter pylori, clarithromycin-resistant;
• Campylobacter spp., fluoroquinolone-resistant;
• Salmonellae, fluoroquinolone-resistant; and
• Neisseria gonorrhoeae, cephalosporin-resistant, fluoroquinolone-resistant.

Medium priority bacteria are generally treatable today, "but doctors fear that resistant strains will push out weaker ones," the New York Times reports. WHO's medium priority list includes:

• Haemophilus influenza, penicillin-non-susceptible;
• Shigella, ampicillin-resistant; and
• Streptococcus pneumoniae, fluoroquinolone-resistant. 

According to Kieny, although many of the bacteria included in the high and medium threat categories don't have high mortality rates, they "have a dramatic health and economic impact, particularly in low-income countries."

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Experts say the fight against superbugs is at a critical moment

Developing new treatments for superbugs will be a long-term effort, "To Your Health" reports, and it is very difficult to bring new classes of antibiotics to market. According to the Pew Charitable Trust's antibiotic-resistance project, no new classes of antibiotics have made it to the market since 1984.

And Allan Coukell, senior director of health programs at Pew, said there aren't enough antibiotics in the pipeline to meet the projected need for them. He explained that "fewer than half [of the 40 antibiotics in clinical development in the United States] even have the potential to treat the pathogens identified by WHO." Moreover, he said that "based on history, most of those will fail to reach the clinic for reasons of efficacy or safety."

Separately, Kieny said, "Antibiotic resistance is growing and we are running out of treatment options. If we leave it to market forces alone, the new antibiotics we most urgently need are not going to be developed in time." She added that WHO hopes there will be more collaboration between doctors and veterinarians, as resistance that arises in animals can spread to humans.

Helen Boucher, an infectious diseases doctor at Tufts University and a spokesperson for the Infectious Diseases Society of America, said, "We cannot just discover and develop our way out of this crisis." She said prevention, the appropriate use of antibiotics, and surveillance are also important.

Regardless, experts say the fight against superbugs is at a critical moment. Jean Patel, CDC specialist in drug-resistant bacteria who consulted with WHO, said, "We're at a tipping point. We can take action and turn the tide—or lose the drugs we have" (Branswell, STAT News, 2/27; Sun, "To Your Health," Washington Post, 2/27; McNeil, New York Times, 2/27).

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