maybe. it depends on the culture. most stool cultures are for bacteria but some check for other organisms. certainly gonorrhea may be detected but perhaps not chlamydia. if you are concerned that you have been exposed to an std from anal sex you need to specifically ask to be std tested so the right tests can be performed. as noted in the previous answer a stool culture is usually done to detect other bacterial infections not stds. if there has been anal sex the testing should include a swab of the anal area but not a stool sample--in some cases the presence of stool/feces leads to inaccurate results. when getting tested for stds you can request to be tested for oral and anal transmission in addition to regular testing of vagina/penis.
will an std show up on a stool culture