Having new and stronger bacteria is bad. But if the resistant bacteria were to over-reproduce, or reproduce a lot, there would be more of the new and stronger bacteria. This would make it really hard to get rid of the bacteria with antibiotics. Not only would you need stronger antibiotics, but you would need to take it for a longer period.
Before Middle After
This doesn't effect us!
This process of the bacteria being selectively pressured, the fittest surviving and reproducing (maybe over reproducing), and genetic variation is called the evolution of bacteria. In this case the evolution was hurtful and not helpful. This is why the misuse of antibiotics is bad and evolution in bacteria is not always good.
After penicillin's mass production, when people took the antibiotics in 1945, they were able to survive bacterial infections because the antibiotics were new to their bodies and the bacteria. Therefore all of the bacteria was able to be killed by the antibiotics.
But when people took the antibiotics in 1965, most of them died because the bacteria in their bodies were already resistant to the penicillin. Since they didn't have any other type of antibiotics then or they didn't have stronger penicillin, the bacteria would just keep reproducing leading to death (most of the time).
See, we're still here!
Some people during that time period died because they were given penicillin for simple viruses which was ineffective in getting rid of the virus. Doctors back then didn't understand the right uses of penicillin so they gave it to everyone not knowing its actual uses.
These examples help show the misuse of antibiotics and its consequences including the evolution of bacteria.