If we keep taking natural antibiotics from nature, mass manufacture them, won't we just train the world's bacterial populations to be immune to practically anything we can throw at them?
You are making a very good point. Currently antibiotic resistance is a serious problem, mostly because we are very slow in discovering new antibiotics. What is very exciting about this research is that it significantly shifts the odds in our favor by allowing very large scale screens for new antibiotics. It will allow us to outpace the rate of resistance development. The probability that a particular infection will be resistant to multiple different antibiotics drops exponentially with the number of antibiotics you have. If you have a tool chest of 5-6 antibiotics sooner or later you will have pathogens that are resistant to a significant proportion of these antibiotics. Make the tool chest 10 times larger, and you will have a lot less to worry about.
You didn't read the paywalled article, did you?
The antibiotic blocks the bacterial cell wall synthesis. Animals don't have this particular cellular component, so the drug is essentially inactive against humans. This was shown by doing tests on mice. There is the possibility that the drug may elicit allergic response in humans (penicillin often does), but this will be tested in clinical trials.
The more exciting part of the work that did not get any mention in the summaries is how they found the antibiotic. They developed an approach to grow on a large scale microorganisms that were previously impossible to culture in lab conditions. They capture the microorganisms on a chip and then put the chip back into the environment from which the samples was isolated. This means that they did not need to guess what kind of nutrients each microorganism will need (they tested ~10,000 different microbes). The approach allowed them to grow 50 fold more microorganisms compared to what was possible using the current state of the art. To me this is the big news, because antibiotic discovery has been limited by our ability to grow microorganisms in the lab.
Factorials were someone's attempt to make math LOOK exciting.