Antibiotic Resistance Doesn’t Just Make Bacteria Harder to Kill – It Can Actually Make Them Stronger

Gerald Pier, Harvard Medical School and David Skurnik, Harvard Medical School

Antibiotics are wonderful drugs for treating bacterial infections. Unfortunately, disease-causing bacteria can become resistant to antibiotics that are meant to kill them. This is called selective pressure – the bacteria that are susceptible to the drug are killed, but the ones that withstand the antibiotic survive and proliferate. This process results in the emergence of antibiotic-resistant strains.

Once a bacterial strain is resistant to several different antibiotics, it has become a multi-drug-resistant (MDR) microbe. When there are virtually no antibiotics available to treat an infected patient, a microbe is said to be “pan-resistant.“ These strains are becoming more and more common in hospitals and in the community at large. You might have heard of some of them: for instance, methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococci (VRE) and carbapenem-resistant Enterobacteriaceae (CRE).

Bacteria can become drug-resistant in two ways – resistance can be natural, meaning that the genes conferring resistance are already present in the bacterial chromosome, or they can be acquired through mutation or by picking up antibiotic-resistance genes from other microbes.

It is now possible to use new DNA-sequencing technologies to take a closer look at how the antibiotic resistance can make some bacteria weaker or stronger. And in a new study, we found that – contrary to conventional wisdom around antibiotics – resistance can actually make some bacteria fitter and even more virulent.

MRSA (Methicillin-resistant Staphylococcus aureus) bacteria strain is seen in a Petri dish containing agar jelly for bacterial culture in a microbiological laboratory in Berlin.
Fabrizio Bensch/Reuters

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