How do bacteria evolve and become resistant to antibiotics? Why is antibiotic resistance such an important topic in medicine?
Bacteria are living organisms. Population of a particular bacteria would comprise of millions/billions of individual organisms. Within a population individuals show genetic variation, which means all individuals of a bacterial population are not genetically absolutely similar. This means some antibiotic resistant bacteria are already present in the population, though their number could be very small.
Thus some bacteria may get easily killed by a group of antibiotic chemicals while the same chemical may show no negative impact on few other individuals. So when antibiotic medicine is administered into our body to treat bacterial disease, the medicine helps in killing most of the disease causing organisms but not all.
There are always some naturally resistant organisms present in the population, which will remain alive even after the disease symptoms subside. These powerful, naturally resistant organisms may multiply at faster rate in absence of competitors (as most of the bacteria have perished due to antibiotic treatment) and the disease will relapse if antibiotic is not continued.
This time the body is invaded by more powerful offspring of drug resistant bacterial organisms. Diseased person would spread a deadlier form of the disease which could kill many.
Esther Lederberg and Joshua Lederberg, in their Replica Plating Experiment, demonstrated this for the first time that naturally occurring variation of antibiotic resistance gets selected in presence of antibiotic, and a new population of antibiotic resistant bacteria may emerge.
Doctors fear these antibiotic resistant organisms because many a times they fail to find suitable new antibiotic to fight these 'superbugs'.