Just like humans, monkeys in zoos are not eating enough fruits and vegetables. This may have led to them having "humanized" gut flora.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/monkeys-are-becoming-more-and-more-human-due-zoo-diets-study-shows
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/science-world-report
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/wildlife
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/monkey
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/zoos
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/animal-diseases
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vegetables