Antibiotic resistance means that an antibiotic no longer kills or inhibits the growth of a bacterium at the doses normally used.How resistance arises. Resistance starts with a random mutation in bacterial DNA. Mutations are not caused by the antibiotic; they occur by chance, for example as errors in DNA replication. Because bacteria divide so often and populations are huge, resistant mutants appear quite frequently.Mechanisms of resistance. A resistance gene can protect a bacterium in several ways:
• making an enzyme that breaks down the antibiotic, such as β-lactamase (penicillinase), which breaks the β-lactam ring of penicillin;
• altering the target site, such as the ribosome or a cell wall enzyme, so the antibiotic can no longer bind;
• efflux pumps in the membrane that pump the antibiotic out of the cell;
• changes to the cell surface that stop the antibiotic entering.How resistance genes spread.
• Vertical gene transmission: when a resistant bacterium divides by binary fission, both daughter cells inherit the gene.
• Horizontal gene transmission: resistance genes are often carried on plasmids. During conjugation, a copy of a plasmid passes from one bacterium to another through a thin tube (pilus). This can happen between different species, so resistance can spread quickly through a bacterial community, for example in the gut.
Some plasmids carry genes for resistance to several antibiotics, giving multiple resistance in a single step. The next lesson explains how the use of antibiotics selects for resistant bacteria.
• making an enzyme that breaks down the antibiotic, such as β-lactamase (penicillinase), which breaks the β-lactam ring of penicillin;
• altering the target site, such as the ribosome or a cell wall enzyme, so the antibiotic can no longer bind;
• efflux pumps in the membrane that pump the antibiotic out of the cell;
• changes to the cell surface that stop the antibiotic entering.How resistance genes spread.
• Vertical gene transmission: when a resistant bacterium divides by binary fission, both daughter cells inherit the gene.
• Horizontal gene transmission: resistance genes are often carried on plasmids. During conjugation, a copy of a plasmid passes from one bacterium to another through a thin tube (pilus). This can happen between different species, so resistance can spread quickly through a bacterial community, for example in the gut.
Some plasmids carry genes for resistance to several antibiotics, giving multiple resistance in a single step. The next lesson explains how the use of antibiotics selects for resistant bacteria.