Bacteria and DNA, conceptual composite image. Scanning electron microscopy (SEM) and computer generated image (CGI) of DNA (deoxyribonucleic acid, yellow) and bacteria. Bacteria can become resistant to antibiotics that have previously harmed them. Bacteria can acquire resistance in two ways: either through a spontaneous genetic change that confers resistance, or by transfer of DNA from a bacterium that is already resistant. Even a simple DNA change can protect the bacteria if it affects the protein on which an antibiotic acts. Transfer of genes between bacteria is common, including the transfer of genes for antibiotic resistance.