Mich. Comp. Laws § 570.101
When public buildings or other public works are about to be built, repaired, or ornamented under contract at the expense of the state, or of any county, city, village, township, or school district thereof, it shall be the duty of the board of officers or agents, contracting on behalf of the state, county, city, village, township, or school district, to require sufficient security by bond for the payment by the contractor of all subcontractors and for the payment for all labor performed and materials and certain supplies furnished and used in the erection, repairing, or ornamenting of the public buildings or works. However, if the contractor is a common carrier as defined in section 3 of Act No. 300 of the Public Acts of 1909, as amended, being section 462.3 of the Michigan Compiled Laws, or the designated operator of a state subsidized railroad, the contractor may provide an irrevocable letter of credit from a state or national bank or a state or federally chartered savings and loan association instead of the bond.
MCL 570.101