Where will you get the media? Do you have access to good media? Do you have access to a good incubator?
I work in a microbiology lab at a hospital, and I do have that access, but I would be in trouble using it for purposes other than hospital-related work.
We do water tests at work (on lab water) from several labs. We use BHI media, and the procedure calls for using 1 ml of di H2O inoculated into the melted media, pouring it into the plate and allowing it to harden before incubating it at 36C for 48 hours. We count the colonies, although 99% of the time, the cultures show no growth (that’s what they’re supposed to show!).
I fear that testing the holy water fonts would only serve to frighten you and others because it would probably grow aerobic* Bacillus*, which looks disgusting on the agar, but is usually harmless to humans.
This experiment could lead to a lawsuit from someone who believes that they got sick from the fonts. I’m not sure it’s a good idea to do the experiment. If the fonts truly posed a threat to health, believe me, the Public Health Dept. would be in there shutting them down in two seconds flat!! Consider that over the centuries, there has been no known negative consequence from using these fonts, so maybe it’s best to just leave it and be at peace.
The only people who need to be careful are those who are immunocompromised, and they know who they are and what they have to do to stay healthy.