After an issue of bonds has been authorized by a vote of the electors of any drainage district, the board of directors thereof shall advertise for bids and let contracts for the proposed improvements in such manner as they may deem advisable: Provided, That all contracts shall be let to the lowest responsible bidder, who shall give a good and sufficient bond, to be approved by the board of directors, conditioned that he will perform the work within the time specified in his contract and in accordance with the plans and specifications of the engineer, and that he will hold the district harmless for any damages that may arise from his negligence, malfeasance or failure in connection therewith. No bid shall be accepted that shall be higher than the engineer’s estimate of the cost of such improvement.