Poilj,
Irrelevant of your symptoms, root canaled tooth #18 is infected and it has been since it was root canaled, if not before that procedure. There are many reasons why all root canaled teeth are chronically infected. Perhaps the easiest for a lay person to understand has to do with the fact that each tooth contains hundreds upon hundreds of tiny microscopic canals called dentin tubules. These canals as well as some large canals contain nerve tissue. During a root canal procedure the only canals that are accessible are those few (depending on the tooth it is usually only 1-4) that are large enough to be seen and get an instrument into. This leaves hundreds ++ of canals untouched. During the procedure the vessels that feed the the tooth nutrition are cut rendering the tooth no blood, no nutrition, no circulation and non vital. The nerve tissue inside of all of the inaccessible tiny canals becomes necrotic and infected. The bacteria and infection will eventually spread beyond the tooth into the jaw bone and further. I will attach a diagram of the anatomy of a tooth to show you the dentin tubules.
Something that you may not be aware of is how the health of our mouth and teeth affect the rest of our body, our digestion, our intestines, etc. According to Ancient Chinese Medicine, our teeth and organs share the same pathways for blood and bacteria to travel through, these are called meridians. It simply means that bacteria is more likely to spread from certain teeth to certain areas of the body without hesitation. Tooth #18 is on the same meridian as the large intestine along with a few other areas. I will attach 2 meridian charts both will be on the same page. One shows the tooth and organ correlation and the other shows the tooth and emotional health correlation. It is not unusual for people to have infected teeth along with intestinal disorders.
Regarding the options you have with the bridge....
#1) Tooth #18 irrelevant of symptoms is infected and very unhealthy. Usually the lack of symptoms with an infected tooth is an indicator that the infection is draining either out from under the gum and into your mouth, down into your digestive system. Or draining into the jaw bone into your circulatory system. So it's important to know that symptoms are not always evident of the extent of the problem.
#2) The bone health surrounding #18 is not going to be healthy and not an ideal place for a dental implant. It is also best to have an oral surgeon not a general dentist remove this tooth as the surgery needs to be very thorough to help eliminate the infection. It is important that the tooth, the periodontal ligament and all diseased tissue and bone be removed at the time of the extraction. It is ideal if you convey to the oral surgeon at a consultation appointment that you want that done to assure yourself and him that he knows you want a thorough extraction. Never assume it will automatically be done.
#3) If #20 is not root canaled, is healthy and the crown fits well, then there is a good chance that the bridge can be cut off between 19 and 20 leaving 20 alone.
#4) If you leave #19 attached to 20 you are risking the health of #20 due to the forces of chewing on it with that fake tooth attached. It will also be difficult to clean underneath 19. However, in some cases this can be a temporary solution.
#5) If you remove 18 and 19 you are left with no molars in that area. If you are missing teeth on your lower right side, you may be a candidate for a removable partial. You would have to discuss that with your dentist.
Regarding your intestinal disorder .... what are you doing to treat or cure the colitis? Do you take probiotic supplements? What type of food do you eat? Yes, antibiotics could be troublesome for you. However, the infection from tooth #18 is also troublesome and needs to be eradicated. People who have intestinal disorders usually find it helpful to supplement with a probiotic called Saccharomyces Boulardii before, during and after taking the antibiotic. This strain of probiotics does not interfere with the absorption of an antibiotic. I can offer you further information on that if you want.
Did you have a bad experience with a tooth extraction in the past? Was the extraction done by an oral surgeon or your dentist? Also, how is the health of the rest of your teeth? Do you have other root canaled teeth or periodontal disease?
Tooth and organ charts link:
Tooth-Organ Acumeridian Relationships