Yes, bread can be broken down by the microorganisms (specifically fungi). One easy way to check this is by leaving a moist bread slice out in the open. You will be able to see fungus growth on the bread. This growth would be visible in the form of grayish white patches. If you use a magnifying glass to look closely at these patches, you will be able to see black spots. This shows that bread can support microbial life. This breakdown is analogous to the breakdown of other food materials left out in the open by microorganisms.
It may be of interest to note that bread is made by using microorganisms. Thus, microorganisms contribute both to the manufacturing as well as the degradation of bread.
Hope this helps.