Up until the Second World War, bacteria had had things very much their own way. Only the immune system stood between humans and thousands of disease causing bacteria. Then Howard Florey in England showed that penicillin, a compound produced by one of Sir Alexander Fleming's molds, could be sent into battle against these tiny invaders.
Use of penicillin marked a major turning point in therapeutic medicine, and antibiotics have become our first line of defense against many diseases.
Unfortunately, natural selection and evolution have prevented antibiotics from wiping out illness caused by bacteria and giving us a disease-free world. Despite their simplicity and apparent genetic homogeneity, many bacteria carry small circles of DNA on which genes can change rapidly.
These genes can mutate and alter their function without harming the ability of the bacteria to grow and reproduce. In this way, they provide a pool of variation somewhat similar to the variants of genes found in higher organisms.
When an antibiotic floods a person's body, millions and millions of sensitive bacteria die, taking their genes with them. Somewhere, however, in the vast population of bacteria in the human body, one cell has just the right mutation to change a protein in just the right way so it can fight off the killing action of the drug.
Once this protein in altered, the bacterium can live in the presence of the antibiotic. Resistance to the killing action of the chemical enables this one cell to grow, divide and reproduce under the new environmental conditions in which all its relatives have died.
The gene for antibiotic resistance, therefore, has a positive effect on the survival of a bacterium when the antibiotic is present.
Unharmed by the new environmental circumstances, the resistant bacteria take over and dominate the population by growing and dividing unchecked. Eventually, all the bacterial cells become resistant, rendering that particular antibiotic useless as a therapeutic agent for that person.
Some forms of syphilis, a worldwide killer, cannot now be controlled by penicillin any longer. All the cells causing this disease are now resistant. Medical authorities regard the steady increase in bacterial infections that cannot be treated with antibiotics as the most serious problem facing world health.
Although bacterial resistance to antibiotics sounds like a classic example of natural selection at work, the results could be explained in another way. The antibiotic itself could have acted on the bacterial population and caused a shift towards antibiotic resistance.