Some drug-resistant bacteria were present at the outset of treatment and became more common as a result of natural selection.
Microbes may also swap genes, notably those that confer resistance to antibiotics on bacteria. Bacteria reproduce in billions. Bacteria that contain drug-resistant DNA have the ability to pass these genes on to other bacteria. Non-resistant bacteria absorb the additional DNA and develop antibiotic resistance.
Bacteria that are multidrug-resistant are those that are resistant to more than one antibiotic (MDROs for short). Hospitals and nursing homes are where the bulk of multidrug-resistant infections are found. They typically affect the elderly or the very ill, and they can result in severe infections.
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