Ok, time for a quick lesson on fecal coliforms.
E.coli is what is known as an indicator microorganism. E. coli is a normal and natural inhabitant in mammals other animals. When you test water or food for the "possibility" of having other not so nice bacteria present (pathogens) which can and do make you sick. Some strains of E. coli can also make you sick but most are harmless normally.
E. coli is an indicator that human or other animal poop has contaminated the item being tested. The theory is that if E. coli is there then the other nasty bugs might also be there. Sewage is normally treated to kill off E. coli and the other nasty bugs in both sewage systems and drinking water. But, if somebody does doody and touches their pooper and does not wash their hands well (or at all
) then what ever they touch after that can be contaminated with said poop.
Anything that grows in the environment (fruit, vegis, cows, etc.) can become contaminated with just about anything that is in the environment. Most of this in limited to the outside of the item (except cows which create their own interior contamination). Processing plants generally treat the outside of these items with chlorinated water to reduce the contamination on the outside. But, if the forementioned non-handwashing doody dude uses untreated water to wash the fruit or if it gets contaminated in the food distribution system they the bugs will get back on the outside of the item...in this case the lemon peel.
To avoid all bacteria is impossible. If you are concerned your lemon is tainted then either refuse it and hope they actually washed your glass in hot soapy water or carry you own lemons which you maintained in a hermetically sealed container after thorough sanitization in a 7 % bleach-water solution for 10 minutes.
Or, don't worry about it. More than likely you have ingested lots and lots of E. coli over the years and never knew it. Being micro-phobic can make you sick.