Microbial limits test is frequently conducted to determine the presence or absence of specific objectionable organisms in a product for quality control purposes. In this case your product is an antibiotic. Azithromycin is a very good bacteriostatic antibiotic with a good bacterial and fungal coverage. However, there are many more bacteria especially of the Gram-negative group that azithromycin will not inhibit. So, microbial limits test goes for all products, pharmaceutical or not. You never know what might show up. It is a world of invisible life forms and the test is for quality control. You won't lose a thing for conducting it.