Question: How could some Staphylococcus be resistant to antibiotics?

Jim Caryl answered on 20 Jun 2011:

Good question. Well, there are three main ways to be resistant to antibiotics: you can be intrinsically resistant, which means that something about the bacteria makes them not bothered about the antibiotic – so the bacteria may be surrounded by slime, or hide inside a human cell, or have a ‘skin’ (called a cell wall) that makes then tough.

Another way is mutation. Antibiotic medicines work by attacking something in the cell, such as the bacteria’s ability to make DNA, or proteins (the building blocks of the cell). If one bacteria has a mutation that makes DNA- or protein-making machinery inside the cell slightly different, the antibiotic may have reduced activity against that cell. So all the neighbouring cells without that particular mutation will die, and the one with the mutation will continue dividing until all the cells in the population have the mutation – as clearly in the presence if the antibiotic, it’s a good thing to have.

Another way is to basically acquire a fully evolved system for dealing with antibiotics. Bacteria can pick up rings of DNA called plasmids, and these plasmids make more if themselves inside the cell, and are separate from the cell’s main DNA. Plasmids can encode enzymes that break down, or modify, the antibiotic – making it useless. Or the enzyme might do what the ‘mutation’ above dies, and make the DNA- or protein-making machinery look slightly different. Or the plasmid could have a gene that makes a pump, which luterally pumps all the antibiotic medicine out of the cell before it does any damage.

I look at these plasmids, because they can turn a bacteria that is easy to treat into one that can’t be treated easily very quickly and effectively.