it is important to note that some zappers can and do reach and kill microbes inside of the intestines
>- Killing microbes in a petri dish is easy
Yes, but you fail to realize that the media may also have a very high resistance which limits the current.
Additionally, when doing this type of work it is typical to report the conditions, including measured current, current density, etc. so that proper comparisons can be made.
Regardless, it is still proof that, at least ParaZapper as well as some other zappers, can and do kill microbes.
>- it is much easier to kill a skin infection than one deep in the body because the electricity near the electrode is still concentrated before it spreads out.
You are correct on that point and it is important to note that some zappers can and do reach and kill microbes inside of the intestines. This is why many ParaZapper users regularly feel a need to replenish their intestinal flora after zapping, when they use it for that purpose.