ELI5: Listerine kills bacteria in my mouth, but I know it's not killing 100%. Aren't I genetically engineering superbug bad breath bacteria by using it?
A massive dose of alcohol isn’t like an antibiotic.
Antibiotics work by targeting very specific proteins that the bacterium needs to survive. If it can figure out a way to change that one protein so that the antibiotic doesn’t work anymore but the protein still gets its job done then it’s progeny will be highly resistant to the antibiotic.
Something like an alcohol solution just straight up tears the cells apart. Some organisms like yeast can deal with decently high alcohol concentrations, but it’s a lot more difficult for organisms to deal with and pretty much nothing’s surviving what’s in mouthwash (the amount of alcohol in mouthwash way outstrips even what something alcohol-tolerant like yeast can survive).
“99%” or whatever percentage they state that will be killed is due to the coverage of the alcohol. That is, it kills 100% of the bacteria it touches, and it touches about 99% of what you are trying to kill, NOT that 1% is touched but somehow impervious.