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This question originally appeared on Quora. Answer by Drew Smith

It is an overused and misleading term which lumps together bacteria that are resistant to some antibiotics but not particularly virulent with those that are both resistant and virulent.

To illustrate this point, consider common soil bacteria belonging to the genus Streptomyces. A 2006 study found that Streptomyces strains typically harbor resistance genes to a dozen different antibiotics[1] . Does this make them superbugs? No it does not. Although Streptomyces infections do occur, they are rare enough that they are reported as individual case studies. Streptomyces species are highly resistant to antibiotics (they are also producers of antibiotics), but are no threat to public health, at least not directly.

Most of the bugs that get labeled as superbugs are in more of a gray zone. Enterobacteriacaea like E. coli and Klebsiella are indeed serious human pathogens and a significant public health problem, especially when they acquire resistance genes. But their acquisition of multiple resistances is generally accompanied by a loss in virulence. The median age for death from infections by these multi-resistant Gram negative bugs is in the 60s, and victims nearly always have been weakened by underlying conditions: kidney failure, immune suppression by AIDS or chemotherapy or for organ transplantation. They rarely attack otherwise healthy adults.

But that is no cause for complacency. The first MRSA strains that emerged in the 1960s were fairly feeble and unable to infect any but the weakest patients. We kept overusing antibiotics, allowing these resistant strains to persist. Over time, they acquired new virulence factors and picked up compensatory mutations. In the 1990s these strains escaped the hospital and began infecting healthy young adults, often with devastating consequences. The USA300 MRSA strain is indeed a true superbug - it is resistant to many antibiotics, and is virulent enough to infect healthy people.

Multidrug-resistant TB strains also tend to be highly virulent and are grossly underappreciated  as a threat to human health

Superbugs exist, but they are still unusual. If we continue to maintain environments of high antibiotic use, such as hospitals and feedlots, we will see many more emerge in the coming decades.

Footnotes

[1] Sampling the Antibiotic Resistome

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