Section 3. County treasurers, except in Suffolk and Nantucket counties, shall be sworn before the county commissioners, who shall make a record thereof. Each treasurer shall give an annual bond to the county for the faithful performance of his official duties, in a form approved by the attorney general, with a surety company authorized to transact business in the commonwealth, as surety, and in such sum as the commissioners shall approve. An attested copy thereof and a certificate of the commissioners that the original is in their possession shall be filed with the state secretary within one week after its approval. Suit thereon may be brought in the name and for the benefit of the county by the commissioners or by the attorney general. The bond required herein shall cover the duties of the treasurer with respect to trust funds and funds of retirement systems which are in his custody by virtue of his office, and any such funds, for the purposes of said bond, shall be deemed to be public funds.