To quote from a recent study of the belly button microbiome:
the frequent, abundant phylotypes encountered were dominated by well-known skin bacteria, specifically Staphylococci, Corynebacteria, and several genera of Actinobacteria (e.g., Micrococcus) and Clostridiales (e.g., Anaerococcus, Finegoldia, Peptidophilus), Bacilli, as well, to a lesser extent, Gammaproteobacteria (e.g., Acinetobacter). This composition corresponds to the previously reported composition of the skin microbiome in deep sequencing studies
See A Jungle in There: Bacteria in Belly Buttons are Highly Diverse, but Predictable