I had ass surgery last year, the doctors went in through my asshole and removed a tumor (about 5cm long) from my colon about 15cm in from the hole. They used a super cool tool and except for the discomfort of having people dig around in my ass, with multiple people watching, and a sting as they applied the local anesthetic it was perfectly fine. The whole thing took about 1 hour from getting into the hospital, to being back in the car again. The tool they used allowed them to get into the gut, without a full surgery. This tool allowed the doctor to see what she was doing and cut away the tumor with a sort of heat wire cutter. It also had grabbers to pull on the tumor to get it out cleanly. She showed me the tumor when she got it out, proud to show how big of a thing she was able to remove. As the tool used heat to burn the flesh, there was minimal bleeding. I did however bleed from my ass for over two weeks.

The doctor told me to use a bandage on my asshole, to prevent the blood from leaking. Aside from switching out this bandage a couple of times a day, I had to do nothing. It hurt bad for a couple of days and after about a week it was just discomfort. She told me I could poop just as normal and to not force or hold in, just follow the regular schedule and it would be just fine.

As it was explained to me the colon is touching poop all the time and knows how to deal with it. A small burn isn't a big deal for the gut, it won't get infected or be a problem at all. She told me the signs to look for and alert them if any of them showed. Then they could use some medication to deal with things like infections. But luckily it all went fine, pooping didn't hurt more than just existing did and there were no complications.

So for small operations through the asshole, the colon can handle it and it won't get infected. The doctors also use tools to for example burn instead of cut to prevent an fully open wound.

Patients are asked to clean their guts before major rectum and colon surgery,similar to what you do when you get a colonoscopy. Ideally that removes most if not all fecal matter and a significant portion of the bacteria.

Furthermore of course the colon is rinsed before the actual surgery and often during the surgery as well if necessary, if necessary with disinfecting solutions (but far less than one would think - it's actually important to do so to the least amount possible,see below). Additionally wound closures are done with techniques that allow extremely easy healing in the most problematic parts and often multiple sutures are made to adapt tissue borders on multiple layers and in multiple ways. And during and after the surgery strong iV antibiotics are given to prevent infection. But it's actually not a good idea to totally get rid of all bacteria. Especially the guts need a healthy bacterial flora to function and,and this is important here, to avoid "bad bacteria" to take over the space. There is more and more focus to make the right bacteria grow back on the colon/and to some extent the rectum, so there are some procedures that are now done with direct faecal transplants afterwards. Nevertheless often patients will not be allowed to eat for quite some time after an operation and are fed with intravascular solution during that time. Not a pleasant experience but sadly necessary. For more external operations (rectum) patients are also given stool softeners (medication that makes the passing of faecal matter easier) and advised to do disinfecting baths often, sometimes three times a day.

And of course the body is quite good at fighting bacteria and the colon and rectum - it is built to do so,the end of the whole "waste producing" system is outside the actual abdomen inside the pelvic sack,separated by a barrier. And the whole area is heavily supplied with blood (which is actually a good thing for infection control).

And last but not least for major operations there is always the option to create an enterostoma - an artificial opening/shortcut for people to get rid of fecal matter through it. These are usually done through the abdominal wall. After everything has healed up (usually after 6 months+x) the now healed colon and the small intestine/unaffected large intestine are connected back together and the artificial opening is closed.

(Sadly this is not always possible - then patients are getting a "Barbie Butt" - a behind without an opening. Mostly for cancer.)

Not an answer, but I have to say you have a way with words.

TLDR your immune system is heavily present in the area since there’s lots of bacteria and stuff up there.

You get antibiotics and they wash you quite thoroughly during surgery. I think it’s a combo of 3 antibiotics? You’re also not going to eat for a day or two and you’ll drink something to empty your colon. It’s not exactly a pleasant lead up

I don't know the answer, but you definitely deserve a round of applause for that hilarious question lmao

A similar question was recently asked on asklemmy. Caveat: there were many probably more informative answers there as there likely are here. However, I'd like to reiterate my answer:

I am not qualified to answer this, but I did once see a similar question asked on Reddit. The best response I saw was from a commenter whose name I can't remember, else I would credit them.

That commenter said that his infant daughter had required an operation on her rectum. The commenter asked the surgeon how the surgery site could possibly not become infected and was told "the asshole knows how to handle shit."

That answer seemed reasonable to me and I probably will never forget it.

I just googled for this, because I vaguely recall a relative having something like this?

Check out this article specifically

With some colorectal surgeries, you may have an ostomy—an opening created surgically to allow for stool or other wastes to exit—temporarily or permanently placed to address your condition.

I think part of your question related to how its kept clean after surgery? I think this addresses it

I don't know, but I do know docs can rebuild a damn sphincter, which is fucking magic to me.

Not an answer, but a related song

Think about this. There is an issue called an anal fistula. It's essentially a hole near the rectum that goes inside to the intestine, usually caused by an infection. The surgery to correct it can involve just filleting it open and leaving a big open wound.