Thank you for your very complete information. I think it's great that you have helped these cats regain their health and are seeking good homes for them.
Regarding your health concern: I applaud your desire to protect the health of potential adopters. There is no reason to think these cats are secretly harboring campylobactyer after their successful treatment and negative cultures. In fact you could consider it a plus, because perfectly healthy, non-symptomatic cats can carry campylobacter which can be transmitted to humans. The vast majority of adopters don't do fecal cultures on the cats they adopt from a shelter, or anywhere else, so they would be totally unaware of non-symptomatic carriers.
In my experience, it's always best to be open and frank with adopters about medical history. I would simply say,"These cats had an intestinal infection, campylobacter fetus, when we got them, and we treated them with the appropriate antibiotic, and followed up with a repeat fecal culture, which was negative.This organism can cause disease in humans. . ." It would be best to give them a written handout so there's no question about informing them. If the potential adopters are very nervous about this issue, it probably makes sense for them to look at another cat to adopt, simply for their peace of mind. If you're talking about an immune-compromised adopter, I would probably steer them to a cat from an indoor home, who has the least chance of being an asymptomatic carrier of anything. The same could be said about homes with young children, since they are most at risk for any zoonosis, because of poor hygiene.
The following link has some important information about GI disease in cats and zoonotic potential. The paragraph thats starts "Salmonella spp., Campylobacter spp., E. coli, Yersinia enterocolitica,. . " refers to campylobacter.
What is important in discussing zoonosis with adopters is that any apparently healthy cat can carry organisms that infect humans. Good hygiene is the key, especially with children. Here is a good basic handout from the Cornell Feline Health Center:
Is it theoretically possible that one of these cats with a negative culture actually still harbors campylobacter? Yes, it is possible. Probable? No. If you want to be extra-safe, you could consider re-culturing 1 month after the negative tests. Another option is a PCR test, as detailed here:
But I know all this testing is very expensive. I don't think it's necessary.
We can't guarantee that any animal, outside of a controlled laboratory environment, is pathogen-free. Your adoption agreement should routinely state something along those lines. These cats are no different from any others in that regard.
I hope this has helped and and makes sense. Please feel free to ask any other questions. You are obviously doing a wonderful job caring for these cats. I wish you the best in finding them homes.