Why has there been a large increase in the number of antibiotic resistant strains of bacteria?

Answer 1

Like all organisms, bacteria vary from each other in many things. Let's say that you become infected, and must take an antibiotic like penicillin. A few of those bacteria are going to have a variation that makes them just a wee bit harder to kill than the others. In a few hours, many of the other bacteria have died from the antibiotic, but the ones that are a wee bit harder to kill continue to live and create many more bacteria like themselves that are harder to kill.

For the sake of argument, let's say that you quit taking the antibiotic before quite all of the bacteria died. What would that leave? Yep, the ones that were harder to kill are left. If you had been wise, you would have taken your medicine the full 10 days, and all the bacteria would have died. But, no . . . so you now have a new strain of bacteria that are a little resistant to penicillin. Let's say you pass the bacteria on to someone else, and they, in turn, do not take their medicine the full 10 days. Then, the harder to kill bacteria die off after a few days, but some of THEM have altered just a little to become even MORE resistant, and they don't die, but make zillions of more bacteria which are even harder to kill.

Over a period of time, after millions of cycles of the above, a small amount of bacteria have become so hard to kill that even after 10 days, they are still alive and making new, harder to kill bacteria (while the weaker ones die).

These tough bacteria now spread to other people, as is the way of such things, and before you know it, these new resistant bacteria are found across a continent, or even around the world. So doctors use stronger antibiotics for people infected with these new, tough bacteria. And enough people will not take their medicine for the full 10 days that an even newer strain of bacteria emerges that are resistant even to the stronger antibiotic. So what happens when these bacteria eventually grow resistant to even the strongest antibiotics? What happens is, there is no more magic medicine to make people well from their infections, and many people start dying from their infections from our new super-bacteria.

Answer 2

Prior to the past couple decades, worldwide indiscriminate use of antimicrobials has led to explosion of drug resistant microorganisms. Over time, many bacterial populations have become resistant via natural selection. Natural selection helps the organisms acquire new traits in order to survive to the changes in the environment. Those new traits help the populations evolve.
Bacteria, like most organisms, tend to build resistance and adapt to their problems. So the more the medicine is used on the strain, (Assuming it survives) it grows a tolerance. And it may actually adapt so well, it practically turns into a new virus.
an increasing threat is now Methicillin-Resistant Staph aureus (MRSA) which avoids methicillin and other β-lactams by an alteration in MRSA's penicillin target-binding site. Bacteria are constantly evolving such new defenses, so treatment of bacterial infections requires a constant search for antibiotics with novel modes of action.