I believe there is no guide to the exact information about it, but there must be some guide to control or prevention of diseases of wild animals that bring indirect information about how to avoid levels of bacteria that cause disease pose, until because it would depend on several factors.
You might want to clarify what bacterial loads - all, harmful, species-specific, zoonotic? Not my area and not in the zoo, but I know there is a large field re: disease, transmission, etc (including bacteria) in the primate tourism field. This includes transmission & regulations/suggestions for prevention.
Whitter, et al. Zoonotic disease concerns in primate field settings. This paper has a section on bacterial diseases (not sure about loads) (http://www.academia.edu/3491730/Zoonotic_disease_concerns_in_primate_field_settings)
You could check gray literature government regulations for various countries for regulations on importing or transporting (between facilities). I know health screenings have to be done to import/move captive primates in labs/zoos.
I work in the zoo sector and it would be interesting if you could survey zoos in the AZA and/or EAZA about their policy on this because from the many zoos I am familiar with, I have never heard of zoos having a maximum threshold that they monitor. In the UK, the governing body DEFRA sure doesn't. I expect there to be a difference between North American and European zoos which could prove very interesting to analyse.