Back to Microbes


Antibiotics are substances that kill bacteria or stop them from growing. They only work on bacteria and it is important to note that antibiotics have no effect at all on viruses. Many antibiotics, such as penicillin, are actually produced by fungi.
The white disks in the image above are antibiotic discs. They have been placed on a Petrie dish full of agar. The light brown colour you see is all bacteria. Notice how near most of the disks there is an area of space where there are no bacteria growing. This is because the antibiotic is either killing the bacteria or stopping them from growing.

Antibiotic resistance

Antibiotics are not as effective as they used to be, because many bacteria are developing resistance to them. Let's have a look at how this resistance occurs.
Let's start by recalling how bacteria reproduce. They reproduce by binary fission. The first step of binary fission is for the bacterial cell to copy all of its genetic material. (It's DNA.) Most of the time, the bacteria do this without making a mistake. But sometimes, when the bacteria tries to copy its DNA, there is an accidental change. This means that the new version is different to the old version in some way. Such a change in the DNA is called a mutation.
Most of the time, mutations don't make that much difference, but very very rarely, one of these changes may make a particular bacterial cell resistant to an antibiotic. This resistant bacterial cell reproduces by binary fission and copy all of its DNA. Now we have two resistant bacteria. Then they reproduce and we have 4, then 8, then 16 etc. And pretty soon there are millions of bacteria that are all resistant to antibiotics.

Antibiotic resistance video