Animation showing drug-resistant bacteria reproducing, as antibiotic drugs kill non-resistant bacteria around them. These bacteria are on a surface in the human body. Antibiotic drugs (white flecks) are shown withering the yellow bacteria, while the blue ones are immune to its effects. Drug resistance is a serious problem in modern medicine, as widespread use of antibiotics has led to the natural selection of resistant strains, which are not killed by the most common antibiotics. Examples include MRSA (methicillin-resistant Staphylococcus aureus), Clostridium difficile, and carbapenem-resistant Enterobacteriaceae such as Klebsiella pneumoniae.