We know now exactly which drug-resistant “superbugs” are most likely to kill us.

This week, the World Health Organization (WHO) released a “global priority list,” of bacteria intended to guide research and discovery of new drugs and treatments. It is a rogue’s gallery of bacteria that are already immune to most drugs and well on their way to resisting all drugs.

Taken as a whole, they paint a terrifying picture of bacteria moving up an evolutionary ladder towards total resistance to drugs. In that world, hospitals are incubators for ever more virulent patient infections, minor illnesses kill children, and people with severe injuries or compromised immune systems are at unrelenting risk for deadly bacterial infections.

Twelve kinds of bacteria are on the superbug list. Let’s take a look.

Three types of bacteria are listed as critical

These are the ones we desperately need new tools to fight. All three are Carbapenem-resistant, which means that the antibiotics of last resort are already losing effectiveness against these bacteria. All three thrive in hospital and health care settings, making even routine medical care increasingly dangerous.

Acinetobacter baumannii. No one knows where Acinetobacter baumannii originally lived, but its current habitat is hospitals. It can survive for a long time on artificial surfaces, and it forms sticky, difficult to remove biofilms that cause infection. It kills people recovering from injuries or surgery, and immunocompromised patients. It is almost perfectly structured to resist antibiotics, with a range of biological defense mechanisms against them. Carbapenem-resistant A. Baumannii is resistant to almost every antibacterial drug currently in resistance, and its presence in hospitals is growing.

Pseudomonas aeruginosa. Pseudomonas aeruginosa is found everywhere. Soil, water, on human skin, and in buildings, especially hospitals. It can survive low-oxygen environments. It can infect every part of the human body, from blood to bones, and it can cause organ failure and death. It’s so aggressive that treatment generally requires removal of infected tissue as well as drugs, and the drugs are losing effectiveness.

Enterobacteriaceae. Enterobacteriaceae is actually a whole category of biologically related disease-causing bacterial, including some familiar ones like e. coli. The bacteria of concern to WHO are the ones that are resistant to both Carbapenem antibiotics and most forms of cephalosporin antibiotics. This makes them nearly impossible to kill. Like the other two critical types of bacteria, Enterobacteriaceae are becoming a constant, dangerous presence in hospitals.

Six types of bacteria are high priority.

By Piotr Smuszkiewicz, Iwona Trojanowska and Hanna Tomczak - <a rel="nofollow" class="external text" href="http://www.casesjournal.com/content/1/1/125">Late diagnosed necrotizing fasciitis as a cause of multiorgan dysfunction syndrome: A case report</a>. Cases Journal 2008, 1:125. <a href="https://en.wikipedia.org/wiki/Digital_object_identifier" class="extiw" title="w:Digital object identifier">doi</a>:<a rel="nofollow" class="external text" href="http://dx.doi.org/10.1186%2F1757-1626-1-125">10.1186/1757-1626-1-125</a>, <a href="http://creativecommons.org/licenses/by/2.0" title="Creative Commons Attribution 2.0">CC BY 2.0</a>, <a href="https://commons.wikimedia.org/w/index.php?curid=5639655">Link</a>

Flesh eating bacteria. By Piotr Smuszkiewicz, Iwona Trojanowska and Hanna Tomczak – Late diagnosed necrotizing fasciitis as a cause of multiorgan dysfunction syndrome: A case report. Cases Journal 2008, 1:125. CC BY 2.0, Link

They cover a broader range of infections and environments than the critical bacteria. Twenty years ago, they were mild annoyances. However, all six have evolved resistance to strong, second-line antibiotics like methicillin, vancomycin, clarithromycin, and fluoroquinolone. These bugs are not yet resistant to carbapenems, the last resort antibiotics, but that time will come. Whenever we use an antibiotic, bacteria evolve resistance. If we don’t find new ways to stop these critical bacteria, they’ll evolve resistance to carbapenems too and become as dangerous as the critical bugs.

The high priority bacteria are vancomycin-resistant forms of Enterococcus faecium, a bacteria that lives in most people and occasionally causes serious illness; methicillin and vancomycin-resistant Staphylococcus aureus, which is also known as flesh-eating bacteria; clarithromycin-resistant Helicobacter pylori, which causes ulcers, fluoroquinolone-resistant Campylobacter, a major cause of bacteria, fluoroquinolone-resistant Salmonella spp., a familiar food-borne illness, and 3rd generation cephalosporin-resistant and fluoroquinolone-resistant Neisseria gonorrhoeae, which causes the sexually transmitted infection gonorrhea.

Finally, the WHO lists three medium-priority bacteria.

All three are child killers, and they infect children everywhere – not just in hospitals. They’re minor infections, but in some children they get worse – bad enough to be fatal. Right now, we stop them with antibiotics, but these infections are a priority because they are already evolving resistance to our most frequently used antibiotics.

The three medium priority bacteria are penicillin-non-susceptible Streptococcus pneumoniae, which causes most common pneumonia infections; ampicillin-resistant Haemophilus influenzae, commonly known as Hib, which causes ear and blood infections; and fluoroquinolone-resistant Shigella spp., which causes diarrhea.

This drug list looks like a ladder.

At the bottom there are bacteria we still have tools to fight. Every time we use those tools, however, the bacteria evolve. Over time, they become immune. That’s the difference between a medium and a high priority bacteria; high priority bacteria have evolved to resist more types of drugs. At the top of the latter, the critical bacteria have evolved resistance to almost every kind of drug we have. We can’t treat them without testing to find out what drugs work on them; sometimes no drugs work.

Bacteria are constantly moving up that ladder. They evolve every time we use antibiotics. And antibiotics aren’t just used for urgent medical care. Most antibiotics that are manufactured are actually used to make farm animals slightly bigger, and that use contributes to resistance, too.

So far, we can’t move bacteria down the ladder. Once they’ve evolved immunity, they stay immune. What the WHO wants is another ladder – a whole new set of tools to use on these bacteria once we’ve exhausted the ones we have. So far, there are no obvious tools out there. Getting everyone to focus on these 12 bacteria, however, should make the search a little more effective.

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