The discovery of penicillin in 1928 began the dawn of a new medical era. From the 1940s to the 1970s, a golden age flourished, and more than 100 new antibiotics were discovered. For the first time in human history, diseases such as tuberculosis and pneumonia could be easily cured, and the average life expectancy around the world increased by 10 years.

For awhile, there was hope that many infectious diseases might be eliminated permanently. However, in a phenomenon known as antibiotic resistance, bacteria soon learned to survive our biochemical attacks. In the 1940s, shortly after penicillin came into widespread use, one strain of Staphylococcus aureus became resistant. Drug companies developed new versions of penicillin, such as methicillin and flucloxacillin, but the bacteria quickly became resistant to those as well. Fast-forward to the present, and antibiotic-resistant superbugs are making headlines around the world. Our early successes against infectious diseases were only the first battles in an ongoing war, and there is still no guarantee of who will prevail in the end.

Antibacterial resistance is an example of evolution in action. Whenever an antibiotic is used, there is always the chance that some of the bacteria will survive. Compared to the bacteria that were killed off, the survivors have genes that make them more resistant to the drug. Since bacteria reproduce asexually, all of the offspring produced by the surviving bacteria will be equally resistant to the drug, helping them to survive antibiotic treatment. Bacteria can also acquire resistance genes in a process called conjugation, where bits of DNA called plasmids are passed from one bacterium to another. This gene-swapping procedure can occur between bacteria of the same species or related species.

Bacteria use many strategies to escape death, such as making enzymes to inactivate antibiotics or changing the structure of their cell walls to make themselves less vulnerable to attack. Although there are more than 150 antibiotics, most of them belong to one of 15 different classes. Antibiotics that belong to the same class work in a similar way, so once a strain of bacteria becomes resistant to one antibiotic, it can also quickly become resistant to other antibiotics from the same class.

And since bacteria multiply rapidly and can produce a new generation in as little as 20 minutes, their resistance genes can spread quickly throughout the population. Eventually the drug-resistant bacteria become predominant, and can be killed only with more powerful antibiotics.

In addition to bacteria, other disease-causing agents (such as viruses, parasites, and fungi) can also become drug-resistant. The HIV virus and the Plasmodium protozoans that cause malaria are two examples of drug-resistant microbes.

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