Antibiotic resistance in bacteria. Illustration showing a mechanism of development of antibiotic multi-resistance in bacteria. Bacteria pass on genes for antibiotic resistance to their offspring when they multiply (left). However horizontal gene transfer (HGT) is the primary mechanism for the spread of antibiotic resistance in bacteria. In this process bacteria receive resistance genes through horizontal plasmid transfer, where plasmids (fragments of DNA) are acquired from other bacterial cells, sometimes even from different species, such as Staphylococcus aureus and Escherichia coli. This mechanism can produce bacterial strains resistance to multiple antimicrobial drugs.