Zappers are probably less effective against microbes that have invaded cells than they are against ones that have not but that does not mean that they are completely ineffective.
I would be among the first to say the more studies are needed and there are improvements that can be made.
I have been dealing with a strep infection in my throat and sinuses for about a week now.
I find that if I zap, it seems to knock it out for a while but it does come back. Zapping every 12 hours seems to work better and 3 times a day is still better. This is the first strep infection that I recall having since my 30's. I am purely amazed that the zapper can keep it at bay without the help of Antibiotics because strep is so virulent.
Compared to this, stopping Staph Aureus is plain easy.