Enter the age of antibiotics. You are a bacterium. You can divide and reproduce every 20-30 min. You can easily exchange genetic material with other bacteria. You are invincible! This principle is the basis for my belief in the "you use it you lose it" theory of antibiotics. If you introduce a new antibiotic into use in people or animals or for crops or any combination of these, you immediately apply pressure on the bacterial populations. All living things live in some kind of relationship with bacteria. Humans (and animals and plants) have enormous populations of bacteria living on their surface and in their guts. These bacteria serve a useful purpose - they crowd out the bad actors, they help digest some foods, etc. As soon as you add large quantities of antibiotic to the mix, everything is turned upside down. We kill the bad guys (we hope) but we also kill a lot of the good guys. And everybody is trying to survive. If, in that population of bacteria, there are just a tiny number that are resistant to the antibiotic in question (and there frequently are), when you apply the antibiotic those few will survive. With time, they will multiply and may come to dominate the niche during antibiotic therapy and even afterwards in some cases. These resistant bacteria may be able to share the mutation or the gene causing resistance with other bacteria. They may have acquired some gene coding for resistance to the antibiotic in question eons ago, but this may be the first time it has actually been needed for a major assault. This foreign gene may also be capable of transfer to other bacteria. How fast all this happens and how quickly it spreads is very variable. So some antibiotics were on the market for decades before we discovered significant resistance to them while resistance plagued others before they were ever manufactured in large scale.
D.M. Shlaes,Antibiotics.@ Springer Science+Business Media B.V. 2010
